A method for treating wastewater during the recycling of NdFeB waste.

Abstract

This invention relates to the field of wastewater treatment technology, specifically to a method for treating wastewater generated during the recycling of NdFeB waste, comprising the following steps: (1) Wastewater collection and homogenization: the wastewater generated during the recycling of NdFeB waste is collected in a wastewater collection tank for homogenization; (2) Solid-liquid separation: the homogenized wastewater is subjected to solid-liquid separation to remove suspended solids and particulate impurities; (3) Acid recovery: the wastewater after solid-liquid separation is subjected to diffusion dialysis treatment to recover free acid from the wastewater; (4) Stepwise precipitation recovery of valuable metals: the acidic residue after acid recovery is subjected to iron precipitation and rare earth precipitation in sequence, and the ferric hydroxide precipitate and oxalic acid rare earth precipitate are recovered by filtration; (5) Deep treatment; (6) Concentrate treatment. This invention achieves efficient recovery of valuable resources such as iron and rare earths from wastewater, while ensuring that the treated effluent meets discharge standards or reuse standards.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a method for treating wastewater generated during the recycling of neodymium iron boron waste. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets are currently the most powerful permanent magnet materials, widely used in new energy vehicles, information storage, wind power generation, aerospace, and other fields. my country is a major producer of NdFeB materials, generating approximately 30% waste during the production and processing of NdFeB. This waste contains 20%–30% rare earth elements (mainly neodymium, with the remainder being terbium, dysprosium, etc.), with the remainder primarily consisting of valuable elements such as iron and boron, possessing extremely high economic value for recycling.

[0003] Currently, the recycling of NdFeB waste mainly employs a wet process. The basic procedure involves first dissolving the waste, then separating the rare earth elements from the iron, purifying the solution with a rare earth acid, and finally precipitating the rare earth elements and then calcining them to obtain rare earth oxides. This process generates a large amount of industrial wastewater containing high concentrations of salt, heavy metal ions (iron ions, rare earth ions, etc.), residual acid, and recalcitrant organic matter, making it a typical high-salt, recalcitrant wastewater.

[0004] Currently, there are some technical solutions for treating wastewater from NdFeB waste recycling, but all of them have varying degrees of shortcomings: (1) Traditional neutralization and precipitation method: The wastewater is neutralized and then directly discharged or simply precipitated. This method requires a large amount of alkaline neutralizing agent, and valuable resources such as iron ions and rare earth ions in the wastewater cannot be effectively recovered, resulting in resource waste. At the same time, the treated wastewater is often difficult to consistently meet the discharge standards.

[0005] (2) Oxalic acid precipitation method for rare earth recovery: For example, the method of extracting neodymium from pickling wastewater in the pretreatment of neodymium iron boron electroplating involves adding oxalic acid to form neodymium oxalate precipitate to recover neodymium. However, this method is only applicable to wastewater from a specific stage, and the supernatant after rare earth recovery still contains a large amount of pollutants such as iron ions, requiring further alkalization treatment. The overall process is incomplete and does not achieve comprehensive wastewater treatment and resource utilization.

[0006] (3) Membrane separation technology: such as using a combination of bipolar membrane electrodialysis and reverse osmosis membrane technology to treat wastewater from NdFeB waste recycling, or using electrodialysis technology to desalinate high-salt and difficult-to-degrade wastewater. Although membrane technology has a good desalination effect (the desalination rate can reach more than 93%), it has problems such as serious membrane fouling, high operating costs, and difficulty in secondary treatment of concentrated water, and its ability to recover valuable metal ions from wastewater is limited.

[0007] (4) Zero-emission treatment system: such as a treatment system that uses a bar screen, magnetic powder adsorber, sand filter, carbon filter, pH adjustment reaction tank, heavy metal capture device, microfilter, ultrafilter, reverse osmosis device, spray evaporator, etc. in series. This system requires a large investment in equipment, has high operating costs, and has a long process flow and complex operation, which is difficult for small and medium-sized enterprises to afford.

[0008] (5) Fenton's reagent oxidation method: Fenton's reagent is used to treat wastewater from the recycling of neodymium iron boron waste. Although this method can effectively remove COD, it requires a large amount of chemical reagents, generates a large amount of iron-containing sludge, and has a high treatment cost.

[0009] In summary, existing wastewater treatment technologies for NdFeB waste recycling mainly have the following problems: Insufficient resource recovery: Valuable elements such as iron and rare earth elements in wastewater are not effectively recovered, resulting in resource waste; High treatment costs: Requires large amounts of chemical reagents or expensive membrane modules and equipment; Complex process flow: Multiple treatment stages and high operational difficulty hinder widespread adoption in the industry; Difficulty in achieving emission standards: Wastewater still struggles to consistently meet emission standards even after treatment by some processes; Risk of secondary pollution: The treatment process generates large amounts of sludge containing heavy metals, which can cause secondary pollution if improperly disposed of. Therefore, developing a wastewater treatment method for NdFeB waste that can efficiently recover valuable resources such as iron and rare earth elements from wastewater while simultaneously achieving wastewater treatment standards or reuse is of significant practical importance. Summary of the Invention

[0010] This invention addresses the shortcomings of existing technologies by providing a method for treating wastewater during the recycling of NdFeB waste. This method achieves efficient recovery of valuable resources such as iron and rare earth elements from the wastewater through wastewater classification and treatment, graded recovery of valuable metals, and advanced treatment of residual wastewater. Simultaneously, the treated effluent meets discharge or reuse standards.

[0011] The technical solution of the present invention: A method for treating wastewater during the recycling of NdFeB waste includes the following steps: (1) Wastewater collection and homogenization: Wastewater generated during the recycling of NdFeB waste is collected in a wastewater collection tank for homogenization; various types of wastewater, including pickling wastewater, extraction residue, precipitate mother liquor, washing wastewater, etc., are homogenized to stabilize the wastewater quality.

[0012] (2) Solid-liquid separation: The homogenized wastewater is subjected to solid-liquid separation to remove suspended solids, particulate impurities, and insoluble solids. The wastewater is passed through a magnetic separator, which utilizes the magnetic properties of the neodymium iron boron waste to recover magnetic particles from the wastewater, reducing the amount of magnetic seeds required. The magnetic separator is either a permanent magnet drum type magnetic separator or an electromagnetic magnetic separator.

[0013] (3) Acid recovery: The wastewater after solid-liquid separation is subjected to diffusion dialysis treatment to recover the free acid in the wastewater; the recovered acid can be reused in the acid dissolution process of NdFeB waste. (4) Stepwise precipitation and recovery of valuable metals: The acidic residue after acid recovery is subjected to iron precipitation and rare earth precipitation in sequence, and the ferric hydroxide precipitate and rare earth oxalate precipitate are recovered by filtration respectively; the iron precipitation causes the iron ions in the wastewater to precipitate out in the form of ferric hydroxide, and the ferric hydroxide precipitate and rare earth-containing supernatant are obtained after solid-liquid separation; oxalic acid is added to the rare earth-containing supernatant and oxalate rare earth precipitate is formed under stirring conditions, and the oxalate rare earth precipitate and precipitate mother liquor are obtained after solid-liquid separation.

[0014] (5) Advanced treatment: The mother liquor is subjected to pH adjustment, coagulation and sedimentation, filtration and membrane separation in sequence to obtain qualified permeate and concentrate; (6) Concentrate treatment: The concentrate is evaporated and concentrated to obtain crystalline salt and condensate. The condensate is reused.

[0015] Preferably, in step (1), aeration is performed during homogenization, with an aeration intensity of 2~3 m³ / (m·h), a mixer linear velocity of 0.15~0.3 m / s, and a homogenization time of 8~24 hours.

[0016] Preferably, the specific steps of solid-liquid separation in step (2) include: S21, the homogenized wastewater enters the magnetic flocculation reactor, and magnetic seeds and flocculants are added; under the action of stirring, the non-magnetic or weakly magnetic suspended matter in the wastewater forms magnetic flocs with the magnetic seeds as the core; at the same time, a weak magnetic field is applied outside the magnetic flocculation reactor to accelerate the growth of magnetic flocs and make their structure more compact. S22. Wastewater containing magnetic flocs enters the hydrocyclone tangentially. In the centrifugal force field, the denser magnetic flocs are thrown against the wall of the hydrocyclone and spiral down, and are discharged from the bottom outlet of the hydrocyclone; the less dense clean water rises to the center and is discharged from the overflow outlet of the hydrocyclone, thus achieving solid-liquid separation.

[0017] Preferably, the magnetic seed in step (2) is Fe3O4 powder, the amount of the magnetic seed added is 50-300 mg / L, the Fe3O4 powder is added under high-speed stirring at 200-400 r / min, and the stirring time is 1-3 min; The flocculant is polyaluminum chloride (PAC) or polyacrylamide (PAM), and the flocculant dosage is 15–60 mg / L. The stirring speed is reduced to 100–200 r / min, the flocculant is added, and the reaction time is 3–5 min. The weak magnetic field is a permanent magnet or an electromagnetic coil, and the magnetic field strength is 200–500 mT. The tangential pressure entering the hydrocyclone is 0.15 ~ 0.40 MPa; The density of denser magnetic flocs is at least 1.6 g / cm³; In step S22, the magnetic flocs discharged from the underflow outlet of the hydrocyclone enter the magnetic drum separator, where the magnetic seeds are separated from the sludge under the action of a high-gradient magnetic field. The recovered magnetic seeds are returned to the magnetic flocculation reactor in step S21 for recycling, while the sludge enters subsequent dewatering treatment. The magnetic field gradient inside the magnetic drum separator is 10. 6 Order of magnitude.

[0018] Preferably, the diffusion dialysis in step (3) uses an anion exchange membrane, the ratio of wastewater flow rate to receiving liquid volume flow rate is 1:0.8 to 1.2, the diffusion dialysis treatment temperature is 20 to 40°C, and the wastewater flow rate is 50 to 200 L / h.

[0019] Preferably, the anion exchange membrane in step (3) is selected from at least one of S-203 anion membrane, CJMADD-2 quaternary ammonium anion membrane, TWDDA3 anion membrane or Neosepta-AFN anion membrane; In step (4), an excess of 30% hydrogen peroxide was added to the iron precipitate, and the pH was adjusted to 3.5-4 with ammonia. The reaction temperature was from room temperature to 60-80°C, the reaction time was 30-120 minutes, and the stirring speed was 50-200 rpm. In step (4), oxalic acid is used as the precipitant for rare earth precipitation, the pH is controlled at 1.5-2.0, the amount of oxalic acid or soluble oxalate added is 1.0-1.5 times the mass of rare earth, the reaction time is 60-180 minutes, and the stirring speed is 100-300 rpm. Preferably, in step (5), concentrated ammonia solution with a mass fraction of 25%-30% is used to adjust the pH to 7.0-7.5; The coagulant used in step (5) for coagulation and sedimentation is polyaluminum chloride, polyferric sulfate or a combination of both, with an addition amount of 2.5 to 3.5 mg / L. The stirring speed during coagulation and sedimentation is 30 to 100 r / min, the stirring time is 5 to 10 min, and the sedimentation is allowed to stand for 15 to 20 min.

[0020] Preferably, the membrane separation in step (5) is a combination of ultrafiltration and reverse osmosis, with the ultrafiltration membrane having a molecular weight cutoff of 5000 to 100000 Da, the reverse osmosis operating pressure being 1.0 to 4.0 MPa, and the pH being adjusted to 6.5 to 6.8 before ultrafiltration.

[0021] Preferably, the evaporation and concentration in step (6) is performed using an MVR evaporator or a multi-effect evaporator. The concentrate is evaporated and concentrated to obtain crystalline salt and condensate. The condensate is reused in the production process, and the crystalline salt is sold as a by-product or disposed of properly.

[0022] Preferably, the treated wastewater is reused in the NdFeB waste recycling process.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1. High comprehensive recovery rate of valuable resources This invention utilizes a tiered resource recovery process of "diffusion dialysis to recover acid, stepwise precipitation to recover iron and rare earth elements, and membrane separation to recover water," achieving comprehensive recovery and utilization of free acid, iron, rare earth elements, and water resources in wastewater, thus avoiding the resource waste problem in traditional processes.

[0024] 2. Processing costs are significantly reduced. Acid recovered through diffusion dialysis can be reused in production, reducing the cost of purchasing new acid; iron and rare earth elements recovered through stepwise precipitation have economic value and can offset some of the treatment costs; and the recovered water can be reused in production, reducing the amount of fresh water required.

[0025] 3. The process flow is reasonable and the operation is simple. The processing units of this invention are closely connected and logically clear. They can be modularly combined according to the wastewater quality, making them highly adaptable and easy to promote and apply in industrial applications.

[0026] 4. No secondary pollution In this invention, the precipitates (ferric hydroxide, rare earth oxalate) generated during the processing are all recycled as products, and the salt produced by evaporation and crystallization can be used as a by-product. The entire processing process does not generate secondary pollution.

[0027] 5. It can achieve zero or near-zero wastewater discharge. By combining membrane separation and evaporation concentration processes, zero or near-zero wastewater discharge can be achieved, meeting increasingly stringent environmental protection requirements.

[0028] 6. High degree of automation Each processing unit can be automated to reduce manual labor intensity and improve operational stability. Detailed Implementation Example 1

[0029] 1000 L of wastewater from a NdFeB waste recycling company was collected and deposited into a wastewater collection tank for homogenization. Aeration was carried out during the homogenization process at an aeration rate of 2.5 m³ / (m·h), the linear velocity of the mixer was 0.2 m / s, and the homogenization time was 16 hours.

[0030] Homogenized wastewater enters a magnetic flocculation reactor, where 100 mg / L of Fe3O4 magnetic seed is added and the reaction is carried out for 2 min with high-speed stirring at 300 r / min. Subsequently, 30 mg / L of polyaluminum chloride is added, the stirring speed is reduced to 150 r / min, and the reaction is carried out for 4 min. Then, 2 mg / L of polyacrylamide is added, the stirring speed is reduced to 60 r / min, and the reaction is carried out for 8 min. A permanent magnet with a magnetic field strength of 300 mT is applied externally to the magnetic flocculation reactor. Wastewater containing magnetic flocs enters tangentially into a hydrocyclone with a diameter of 100 mm, a cone angle of 15°, and a magnetic floc density of approximately 2.65 g / cm³ at a pressure of 0.25 MPa. After exiting from the underflow outlet, the wastewater enters a magnetic field gradient of 10... 6 The magnetic drum separator recovers the magnetic seeds for recycling; clean water is discharged from the overflow port. The wastewater after solid-liquid separation enters the diffusion dialysis system, which uses a TWDDA3 anion exchange membrane. The ratio of wastewater flow rate to receiving liquid flow rate is 1:1, the temperature is 30℃, and the wastewater flow rate is 100 L / h to recover free acid.

[0031] The acidic residue after acid recovery undergoes sequential iron precipitation and rare earth precipitation. Iron precipitation: Excess hydrogen peroxide (30% by mass) is added to oxidize Fe. 2+ The pH was adjusted to 3.8 with ammonia, the reaction temperature was 70℃, and the reaction time was 60 min. The precipitate of ferric hydroxide was obtained by filtration. Rare earth precipitation: Oxalic acid was used as the precipitant in the filtrate, with the amount of oxalic acid added being 1.2 times the mass of the rare earth. The pH was controlled at 1.8, the stirring speed was 200 r / min, and the reaction time was 120 min. The precipitate of rare earth oxalate was obtained by filtration.

[0032] The pH of the precipitate mother liquor was adjusted to 7.2 with 28% concentrated ammonia solution. Polyaluminum chloride (PAC) at 3.0 mg / L was added as a coagulant. The stirring speed was 60 r / min for 8 min, followed by settling for 18 min. The supernatant after sedimentation was filtered through a plate and frame filter press, and the influent pH was adjusted to 6.6 before entering a membrane separation system. It first underwent ultrafiltration (with a molecular weight cutoff of 50,000 Da), followed by reverse osmosis at an operating pressure of 2.5 MPa, yielding qualified permeate and concentrate. The concentrate was then evaporated and concentrated in an MVR evaporator to obtain crystalline salt and condensate. The condensate was reused in the production process. The treated permeate was reused in the NdFeB waste recycling process. Example 2

[0033] 2000 L of wastewater from a NdFeB waste recycling company was collected and deposited into a wastewater collection tank for homogenization. Aeration was carried out during the homogenization process at an aeration intensity of 2.0 m³ / (m·h), the linear velocity of the mixer was 0.15 m / s, and the homogenization time was 24 hours.

[0034] Homogenized wastewater enters a magnetic flocculation reactor, where 200 mg / L of Fe3O4 magnetic seed is added and the reaction is carried out for 1 min with high-speed stirring at 400 r / min. Subsequently, 50 mg / L of polyaluminum chloride is added, the stirring speed is reduced to 200 r / min, and the reaction is carried out for 3 min. Then, 4 mg / L of polyacrylamide is added, the stirring speed is reduced to 80 r / min, and the reaction is carried out for 10 min. An electromagnetic coil with a magnetic field strength of 500 mT is applied externally to the magnetic flocculation reactor. Wastewater containing magnetic flocs enters tangentially into a hydrocyclone with a diameter of 150 mm, a cone angle of 20°, and a magnetic floc density of approximately 2.0 g / cm³ at a pressure of 0.40 MPa. After exiting from the underflow outlet, the wastewater enters a magnetic field gradient of 10... 6 The magnetic drum separator recovers the magnetic seeds for recycling; clean water is discharged from the overflow port. The wastewater after solid-liquid separation enters the diffusion dialysis system, which uses an S-203 anion exchange membrane. The ratio of wastewater flow rate to receiving liquid flow rate is 1:0.8, the temperature is 40℃, and the wastewater flow rate is 200 L / h, to recover free acid.

[0035] The acidic residue after acid recovery undergoes sequential iron precipitation and rare earth precipitation. Iron precipitation: Excess hydrogen peroxide (30% by mass) is added to oxidize Fe. 2+ The pH was adjusted to 4.0 with ammonia, the reaction temperature was 80℃, and the reaction time was 120 min. The precipitate of ferric hydroxide was obtained by filtration. Rare earth precipitation: Oxalic acid was used as the precipitant in the filtrate, with the amount of oxalic acid added being 1.5 times the mass of the rare earth. The pH was controlled at 2.0, the stirring speed was 300 r / min, and the reaction time was 180 min. The precipitate of rare earth oxalate was obtained by filtration.

[0036] The pH of the precipitate mother liquor was adjusted to 7.5 with 25% concentrated ammonia solution. 3.5 mg / L of polyferric sulfate was added as a coagulant. The stirring speed was 100 r / min for 10 min, followed by settling for 20 min. The supernatant after sedimentation was filtered through a plate and frame filter press, and the influent pH was adjusted to 6.8 before entering a membrane separation system. It first underwent ultrafiltration (with a molecular weight cutoff of 100,000 Da), then reverse osmosis at an operating pressure of 4.0 MPa, yielding qualified permeate and concentrate. The concentrate was then evaporated and concentrated in an MVR evaporator to obtain crystalline salt and condensate. The condensate was reused in the production process. The treated permeate was reused in the NdFeB waste recycling process. Example 3

[0037] 500 L of wastewater from a NdFeB waste recycling company was collected and deposited into a wastewater collection tank for homogenization. Aeration was performed during homogenization at an intensity of 3.0 m³ / (m·h), the mixer linear velocity was 0.30 m / s, and the homogenization time was 8 hours.

[0038] Homogenized wastewater enters a magnetic flocculation reactor, where 50 mg / L of Fe3O4 magnetic seeds is added and the reaction is carried out for 3 min with high-speed stirring at 200 r / min. Subsequently, 15 mg / L of polyaluminum chloride is added, the stirring speed is reduced to 100 r / min, and the reaction is carried out for 5 min. Then, 1 mg / L of polyacrylamide is added, the stirring speed is reduced to 40 r / min, and the reaction is carried out for 5 min. A permanent magnet with a magnetic field strength of 200 mT is applied externally to the magnetic flocculation reactor. Wastewater containing magnetic flocs enters tangentially into a hydrocyclone with a diameter of 50 mm, a cone angle of 10°, and a magnetic floc density of approximately 1.6 g / cm³ at a pressure of 0.15 MPa. After exiting from the underflow outlet, the wastewater enters a magnetic field gradient of 10... 6 The magnetic drum separator recovers the magnetic seeds for recycling; clean water is discharged from the overflow port. The wastewater after solid-liquid separation enters the diffusion dialysis system, which uses a CJMADD-2 quaternary ammonium anion exchange membrane. The ratio of wastewater flow rate to receiving liquid flow rate is 1:1.2, the temperature is 20℃, and the wastewater flow rate is 50 L / h, to recover free acid.

[0039] The acidic residue after acid recovery undergoes sequential iron precipitation and rare earth precipitation. Iron precipitation: Excess hydrogen peroxide (30% by mass) is added to oxidize Fe. 2+ The pH was adjusted to 3.5 with ammonia, the reaction temperature was 25℃, the reaction time was 30 min, and the precipitate of ferric hydroxide was obtained by filtration. Rare earth precipitation: Oxalic acid was used as a precipitant in the filtrate, with the amount of oxalic acid added being 1.0 times the mass of rare earth. The pH was controlled at 1.5, the stirring speed was 100 r / min, the reaction time was 60 min, and the precipitate of rare earth oxalate was obtained by filtration.

[0040] The pH of the precipitate mother liquor was adjusted to 7.0 with 30% concentrated ammonia solution. A coagulant of 2.5 mg / L (1:1 mass ratio of polyaluminum chloride to polyferric sulfate) was added as a coagulant. The stirring speed was 30 r / min for 5 min, followed by settling for 15 min. The supernatant after sedimentation was filtered through a plate and frame filter press, and the pH of the influent was adjusted to 6.5 before entering a membrane separation system. It first underwent ultrafiltration (with a molecular weight cutoff of 5000 Da), followed by reverse osmosis at an operating pressure of 1.0 MPa, yielding qualified permeate and concentrate. The concentrate was then evaporated and concentrated in a multi-effect evaporator to obtain crystalline salt and condensate. The condensate was recycled for use in the production process. The treated permeate was recycled for the NdFeB waste recycling process.

[0041] Comparative Example

[0042] Solid-liquid separation was performed using a conventional "flocculation sedimentation + plate and frame filtration" process, and acidic residual liquid was treated by neutralization precipitation.

[0043] (1) Wastewater collection and homogenization: Similar to Example 1, 1000 L of wastewater generated by the NdFeB waste recycling company was collected in the wastewater collection tank for homogenization and adjustment. The homogenization time was 16 hours.

[0044] (2) Solid-liquid separation: The homogenized wastewater directly enters the flocculation reaction tank, where 30 mg / L of polyaluminum chloride and 2 mg / L of polyacrylamide are added. The stirring speed is 60 r / min, and the reaction time is 15 min. The flocculated wastewater enters the inclined plate sedimentation tank, where it is allowed to settle for 120 min. The supernatant after sedimentation is filtered through a sand filter and then enters the next process. The bottom sludge is discharged into the sludge tank, dewatered by a plate and frame filter press, and then transported off-site for disposal.

[0045] (3) Acid recovery: The wastewater after solid-liquid separation enters the diffusion dialysis system, using a TWDDA3 anion exchange membrane. The ratio of wastewater flow rate to receiving liquid flow rate is 1:1, the temperature is 30℃, and the wastewater flow rate is 100 L / h to recover free acid.

[0046] (4) Recovery of valuable metals: The acidic residue after acid recovery is directly added to ammonia water to adjust the pH to 9.0, so that iron ions and rare earth ions co-precipitate. After filtration, a mixed iron and rare earth hydroxide precipitate is obtained.

[0047] (5) Advanced treatment: The pH of the precipitate mother liquor was adjusted to 7.2 with concentrated ammonia solution with a mass fraction of 28%, and polyaluminum chloride (PAC) 3.0 mg / L was added as a coagulant. The stirring speed was 60 r / min and the stirring time was 8 min. The mixture was then allowed to stand for 18 min to settle. The supernatant after sedimentation was filtered through a plate and frame filter press and then discharged.

[0048] Technical defects of the comparison ratio 1. The solid-liquid separation process is inefficient and time-consuming. A comparative example using conventional flocculation and sedimentation processes resulted in flocs with a density close to water (1.05 g / cm³), slow settling speed, and a sedimentation time as long as 120 minutes. In contrast, this invention utilizes magnetic seed loading to form high-density magnetic flocs (density ≥ 1.6 g / cm³), combined with the efficient separation of a hydrocyclone, requiring only 3–5 minutes of hydraulic residence time, significantly improving separation efficiency.

[0049] 2. Incomplete removal of suspended solids. Conventional flocculation and sedimentation have limited effectiveness in removing fine suspended solids and colloidal particles. Residual suspended solids in the effluent can cause pollution and clogging of subsequent diffusion dialysis membrane modules, affecting membrane lifespan and acid recovery efficiency.

[0050] 3. The added value of recycled valuable metal products is low. A comparative study showed that co-precipitating iron and rare earth elements yields a mixed iron-rare earth hydroxide, which cannot be directly utilized and is difficult to separate further. This invention employs a stepwise precipitation strategy, first oxidizing Fe... 2+ For Fe3+ Ferric hydroxide was selectively precipitated at pH 3.5–4.0, and rare earth elements were selectively precipitated with oxalic acid at pH 1.5–2.0, respectively, to obtain high-purity ferric hydroxide and rare earth oxalate products, which greatly improved the economic value of the products.

[0051] 4. Without magnetic seed recycling, operating costs are high. The comparative example did not employ magnetic seed loading technology, resulting in high flocculant consumption. This invention utilizes a magnetic field gradient of 10... 6 The order-of-magnitude high gradient magnetic field separates magnetic seeds from sludge and allows for recycling, with a magnetic seed recovery rate of ≥95%, significantly reducing operating costs.

[0052] 5. Wastewater is not utilized as a resource. In contrast, the mother liquor is simply coagulated and precipitated before being directly discharged, resulting in water waste. This invention treats the mother liquor using a combination of ultrafiltration and reverse osmosis membrane separation. The treated water is reused in production processes, and the concentrated liquid is evaporated and crystallized to obtain crystalline salt, achieving zero wastewater discharge and full resource utilization.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for treating wastewater during the recycling of NdFeB waste, characterized in that, Includes the following steps: (1) Wastewater collection and homogenization: Wastewater generated during the recycling of NdFeB waste is collected in a wastewater collection tank for homogenization; (2) Solid-liquid separation: The homogenized wastewater is subjected to solid-liquid separation to remove suspended solids and particulate impurities; (3) Acid recovery: The wastewater after solid-liquid separation is subjected to diffusion dialysis treatment to recover the free acid in the wastewater; (4) Stepwise precipitation and recovery of valuable metals: The acidic residue after acid recovery is subjected to iron precipitation and rare earth precipitation in sequence, and the ferric hydroxide precipitate and oxalic acid rare earth precipitate are recovered by filtration respectively. (5) Advanced treatment: The mother liquor is subjected to pH adjustment, coagulation and sedimentation, filtration and membrane separation in sequence to obtain qualified permeate and concentrate; (6) Concentrate treatment: The concentrate is evaporated and concentrated to obtain crystalline salt and condensate. The condensate is reused.

2. The method according to claim 1, characterized in that, In step (1), aeration is carried out during homogenization. The aeration intensity is 2~3 m³ / (m·h), the linear velocity of the mixer is 0.15~0.3 m / s, and the homogenization time is 8~24 hours.

3. The method according to claim 1, characterized in that, The specific steps of solid-liquid separation in step (2) include: S21, the homogenized wastewater enters the magnetic flocculation reactor, and magnetic seeds and flocculants are added; under the action of stirring, the non-magnetic or weakly magnetic suspended matter in the wastewater forms magnetic flocs with the magnetic seeds as the core; at the same time, a weak magnetic field is applied outside the magnetic flocculation reactor to accelerate the growth of magnetic flocs and make their structure more compact. S22. Wastewater containing magnetic flocs enters the hydrocyclone tangentially. In the centrifugal force field, the denser magnetic flocs are thrown against the wall of the hydrocyclone and spiral down, and are discharged from the bottom outlet of the hydrocyclone; the less dense clean water rises to the center and is discharged from the overflow outlet of the hydrocyclone, thus achieving solid-liquid separation.

4. The method according to claim 3, characterized in that, The magnetic seed in step (2) is Fe3O4 powder. The amount of the magnetic seed added is 50-300 mg / L. The Fe3O4 powder is added under high-speed stirring at 200-400 r / min for 1-3 min. The flocculant is polyaluminum chloride (PAC) or polyacrylamide (PAM), and the flocculant dosage is 15–60 mg / L. The stirring speed is reduced to 100–200 r / min, the flocculant is added, and the reaction time is 3–5 min. The weak magnetic field is a permanent magnet or an electromagnetic coil, and the magnetic field strength is 200–500 mT. The tangential pressure entering the hydrocyclone is 0.15 ~ 0.40 MPa; The density of denser magnetic flocs is at least 1.6 g / cm³; In step S22, the magnetic flocs discharged from the underflow outlet of the hydrocyclone enter the magnetic drum separator, where the magnetic seeds are separated from the sludge under the action of a high-gradient magnetic field. The recovered magnetic seeds are returned to the magnetic flocculation reactor in step S21 for recycling, while the sludge enters subsequent dewatering treatment. The magnetic field gradient inside the magnetic drum separator is 10. 6 Order of magnitude.

5. The method according to claim 1, characterized in that, In step (3), the diffusion dialysis uses an anion exchange membrane, the ratio of wastewater flow rate to receiving liquid volume flow rate is 1:0.8 to 1.2, the diffusion dialysis treatment temperature is 20 to 40°C, and the wastewater flow rate is 50 to 200 L / h.

6. The method according to claim 5, characterized in that, The anion exchange membrane mentioned in step (3) is selected from at least one of S-203 anion membrane, CJMADD-2 quaternary ammonium anion membrane, TWDDA3 anion membrane or Neosepta-AFN anion membrane; In step (4), an excess of 30% hydrogen peroxide is added to the iron precipitate, and then the pH is adjusted to 3.5-4 with ammonia. The reaction temperature is from room temperature to 60-80℃, and the reaction time is 30-120 minutes. In step (4), oxalic acid is used as the precipitant for rare earth precipitation. The pH is controlled at 1.5 to 2.

0. The amount of oxalic acid or soluble oxalate added is 1.0 to 1.5 times the mass of rare earth. The reaction time is 60 to 180 minutes and the stirring speed is 100 to 300 rpm.

7. The method according to claim 1, characterized in that, In step (5), the pH is adjusted to 7.0-7.5 using concentrated ammonia solution with a mass fraction of 25%-30%; The coagulant used in step (5) for coagulation and sedimentation is polyaluminum chloride, polyferric sulfate or a combination of both, with an addition amount of 2.5 to 3.5 mg / L. The stirring speed during coagulation and sedimentation is 30 to 100 r / min, the stirring time is 5 to 10 min, and the sedimentation is allowed to stand for 15 to 20 min.

8. The method according to claim 1, characterized in that, The membrane separation described in step (5) is a combination of ultrafiltration and reverse osmosis. The ultrafiltration membrane has a molecular weight cutoff of 5,000 to 100,000 Da, the reverse osmosis operating pressure is 1.0 to 4.0 MPa, and the pH is adjusted to 6.5 to 6.8 before ultrafiltration.

9. The method according to claim 1, characterized in that, The evaporation and concentration in step (6) uses an MVR evaporator or a multi-effect evaporator.

10. The method according to any one of claims 1 to 9, characterized in that, The treated wastewater is reused in the NdFeB waste recycling process.