Lead-acid battery wastewater treatment method

Abstract

The invention discloses a lead-acid battery wastewater treatment method. The method comprises the steps of separate collection and homogeneous adjustment, introducing the homogenized wastewater into a crystallization reactor, adding lead sulfate seed crystals or heterogeneous nucleation carrier seed crystals, and selectively separating out lead in the form of lead sulfate coarse particle crystals by adopting segmented pH regulation and low supersaturation closed-loop control; carrying out solid-liquid separation on crystallization effluent, and recovering a coarse particle solid phase as a seed crystal cycle; the supernate is subjected to secondary weak base precipitation and then enters an electric flocculation unit to deeply remove residual colloidal / complex lead; then carrying out flocculent precipitation and filtration, and arranging an ion exchange / adsorption or membrane separation polishing unit according to standard reaching or reuse requirements; and dehydrating the generated solid phase or sludge and recovering the seed crystal. Through seed crystal induced crystallization and seed crystal circulation, generation of fine grains is reduced, solid-liquid separation and sludge dewatering properties are improved, the lead removal stability is improved, and the operation cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and specifically relates to a method for treating lead-acid battery wastewater. Background Technology

[0002] The production, repair, and regeneration of lead-acid batteries generate a certain amount of lead-containing acidic wastewater. The main sources include plate formation and rinsing, acid overflow and equipment acid washing, workshop floor cleaning, and lead dust rinsing. This type of wastewater typically contains high concentrations of sulfate ions and free acid (calculated as sulfuric acid), and also carries soluble lead ions (Pb²⁺), fine lead salt particles, suspended lead dust, and small amounts of residual organic additives. It is characterized by large acidity fluctuations, complex lead forms (ionic, colloidal, and particulate forms coexisting), and rapid changes in treatment load. In engineering applications, relying solely on conventional precipitation or filtration methods often fails to consistently meet emission or reuse standards under fluctuating operating conditions.

[0003] Current lead-acid battery wastewater treatment commonly employs an alkaline neutralization-flocculation sedimentation-filtration route. For example, lime slurry, sodium hydroxide, or sodium carbonate is used to rapidly raise the wastewater pH to the alkaline range, causing lead to precipitate as hydroxides or carbonates. Solid-liquid separation is then achieved through flocculation sedimentation and pressure filtration. While this route is simple in equipment and has low investment, it is prone to the following problems in actual operation: First, rapid pH increases can easily cause localized supersaturation, generating a large number of fine precipitates or colloidal particles. Slow settling speed and large fluctuations in supernatant turbidity lead to high subsequent filtration loads and insufficient effluent stability. Second, lead-containing sludge is fine, has high water content, and is difficult to dewater, resulting in large sludge volumes and high disposal costs. Third, in systems with high sulfate content, the form of lead is significantly affected by sulfate, pH, and ionic strength; relying solely on a single neutralization and sedimentation process often fails to simultaneously achieve the goals of "deep lead removal" and "low sludge production / easy dewatering." Some processes also introduce sulfide precipitation or chemical complexation to improve lead removal efficiency, but this may bring risks of reagent safety, odor and secondary pollution, or lead to complex operation control and insufficient stability. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for treating lead-acid battery wastewater, which is suitable for sulfate-containing systems, can suppress fine crystal formation and improve solid-liquid separation and dehydration performance, while also ensuring stability in deep lead removal.

[0005] The technical solution provided by this invention is as follows: A method for treating lead-acid battery wastewater includes the following steps: S1. Separate collection and homogenization: The acidic and lead-containing wastewater generated during the production or regeneration of lead-acid batteries is collected separately according to the acid strength and suspended solids content and then enters the homogenization tank for stirring and homogenization to remove large particulate suspended solids. S2, Primary lead removal: The effluent from S1 is introduced into the crystallization reactor. Lead sulfate seed crystals or heterogeneous nucleation carrier seed crystals are added to the crystallization reactor, and the pH is adjusted in stages under stirring conditions so that lead sulfate crystals are formed in the crystallization window and grow into sedimentable coarse granular crystals. The segmented pH control includes at least: adjusting the pH to 3.5-4.5 and maintaining it to induce nucleation and epitaxial growth, and then slowly adjusting the pH to 5.5-6.5 to promote crystal growth; the pH control is achieved by controlling the dosing rate of alkaline agents through an online pH signal closed loop, so that the system is in a low supersaturation crystallization state to inhibit the formation of fine crystals and colloids. S3. Solid-liquid separation and seed recovery: The effluent from S2 is subjected to sedimentation or cyclone separation to obtain crystalline solid phase and supernatant, and at least part of the crystalline solid phase is recovered as seed crystals for recycling in S2. S4, Secondary lead removal: The supernatant obtained from S3 is introduced into the secondary reaction tank, and the pH is adjusted to 8.5-9.5 to generate metal hydroxide flocs and achieve co-precipitation lead removal. Then it enters the electrocoagulation unit, where flocculants are generated in situ through sacrificial anodes to remove residual colloidal lead and complexed lead. S5. Flocculation, sedimentation and filtration: Add flocculant to the S4 effluent for sedimentation and separation and then filter to obtain the effluent. S6. Deep Polishing: The S5 filtered water is then fed into an adsorption / ion exchange or membrane separation polishing unit to further reduce the lead ion concentration in the effluent. S7. Sludge treatment: The solid phase or sludge generated by S3 and S5 is dewatered, and the coarse particles in the dewatering product are recycled for S2 seed crystal recycling.

[0006] In some embodiments, the lead sulfate seed crystals are derived from the coarse-particle component obtained by fractionation of the dehydrated solid phase obtained in S3 or S7, wherein the volume fractionation particle size D50 of the coarse-particle component is 20–200 μm.

[0007] In some embodiments, the heterogeneous nucleation support seed crystal is one or more of calcium sulfate particles, inert ceramic particles, silica sand or alumina particles, and its surface is used to provide heterogeneous nucleation sites for lead sulfate to promote the epitaxial growth of lead sulfate.

[0008] In some embodiments, the alkaline agent in S2 is one or more of lime milk, sodium carbonate solution, sodium hydroxide solution, or magnesium hydroxide slurry; and closed-loop control is performed using online pH and influent flow signals to slowly raise the pH from 3.5 to 4.5 to 5.5 to 6.5 over a period of 10 to 90 minutes.

[0009] In some embodiments, the crystallization reactor is one or more of a continuously stirred tank reactor with stirring, a circulating fluidized bed crystallizer, or a crystallizer with internal circulation, and the hydraulic residence time of the crystallization reactor is 20 to 180 minutes.

[0010] In some embodiments, the solid-liquid separation of S3 employs one or more of an inclined plate sedimentation tank, a high-efficiency sedimentation tank, or a hydrocyclone separator, and a flocculation promotion step is set before solid-liquid separation to increase the settling rate of coarse lead sulfate crystals.

[0011] In some embodiments, the electrocoagulation unit in S4 uses an electrode combination of iron anode / stainless steel cathode, aluminum anode / stainless steel cathode, or iron anode / graphite cathode; the current density of electrocoagulation is 5 to 50 A / m², and the energizing time is 5 to 60 minutes.

[0012] In some embodiments, the flocculant in S5 is polyaluminum chloride, polyferric sulfate, or a mixture thereof, the coagulant aid is anionic or nonionic polyacrylamide, and the filtration after flocculation and sedimentation is one or more of sand filtration, bag filtration, or microfiltration.

[0013] In some embodiments, the S6 deep polishing unit is one or more of a chelating ion exchange resin adsorption unit, a selective adsorption material adsorption unit containing thiol / amino functional groups, an ultrafiltration unit, or a nanofiltration unit; and the effluent from the polishing unit is reused for in-plant rinsing or circulating water systems.

[0014] In summary, the beneficial effects of this invention are: (1) This invention uses seed-induced selective crystallization of lead sulfate and low supersaturation control with segmented slow pH increase to make lead preferentially precipitate in the form of settleable coarse lead sulfate particles, reducing the formation of fine precipitates and colloidal lead salts and reducing solid-liquid separation fluctuations. On this basis, it is combined with secondary weak alkaline precipitation and electrocoagulation to enhance the removal of residual colloidal / complexed lead. Resin or membrane separation and polishing units can be configured as needed, so that the effluent indicators can still meet the long-term stable standards or reuse requirements under the conditions of fluctuating influent acidity and lead load.

[0015] (2) The present invention makes the lead-containing solid phase mainly composed of granular crystals, which has good settling performance and is easier to form high solid content filter cake during filter pressing, reducing flocculation sedimentation of fine mud and entrainment, increasing the solid content of filter cake and reducing sludge volume; at the same time, the crystallization efficiency and system anti-fluctuation ability are improved through crystal seed recovery and recycling, reducing the ineffective consumption of alkaline agents and flocculants, thereby reducing sludge disposal costs and overall operating costs. Attached Figure Description

[0016] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0017] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. The following embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0018] like Figure 1 As shown, the present invention provides a method for treating lead-acid battery wastewater, which is composed of the following process units in sequence: a fractional collection system, a homogenization and conditioning tank, a crystallization reactor, a solid-liquid separation unit, a secondary reaction tank, an electrocoagulation unit, a flocculation sedimentation tank, a filtration unit, a deep polishing unit, and a sludge dewatering and seed crystal recovery unit.

[0019] The system includes a separation collection system to separate wastewater from different sources and with different acidity / suspended solids content into corresponding branches; a homogenization equalization tank for water quantity and quality buffering; a crystallization reactor for seed-induced crystallization reaction; a solid-liquid separation unit for separating coarse lead sulfate crystals; a secondary reaction tank and electrocoagulation unit for deep removal of residual colloidal or complexed lead; a flocculation sedimentation tank and filtration unit for further solid-liquid separation; a deep polishing unit for further reducing the lead concentration in the effluent to a stable level that meets standards or can be reused; and a sludge dewatering and seed recovery unit for dewatering lead-containing solids and recovering coarse seed crystals for recycling.

[0020] S1, Separate Collection and Homogenization Adjustment The wastewater from the factory area is classified into at least two categories based on its acidity and suspended solids content: 1) High-acid wastewater (e.g., equipment pickling, overflow acid collection liquid, etc.); 2) General rinsing wastewater (such as workshop floor rinsing water, equipment rinsing water, etc.); After various types of wastewater enter the homogenization and equalization tank, they are stirred and homogenized. The homogenization and equalization tank can be equipped with a screen or settling zone to remove large suspended solids and reduce the ineffective solid load in the downstream crystallizer. The residence time in the homogenization and equalization tank can be 2 to 12 hours, preferably 4 to 8 hours. An online pH probe and flow meter are installed in the tank to enable closed-loop control of chemical dosing in the downstream stage.

[0021] S2, Seed-induced selective crystallization of lead sulfate for primary lead removal The homogenized wastewater is fed into the crystallization reactor continuously or intermittently. The crystallization reactor can be a continuous stirred tank reactor (CSTR), an internal circulation crystallizer, or a circulating fluidized bed crystallizer. An agitator and an online pH probe are installed inside the reactor; optionally, a conductivity probe is installed to reflect changes in the ionic strength of the system, to help determine the dosing rate and supersaturation state.

[0022] Specifically, the first step is seed crystal addition and circulation: lead sulfate seed crystals or heterogeneous nucleation carrier seed crystals are added to the crystallization reactor. The lead sulfate seed crystals can be derived from the lead-containing solid phase obtained from subsequent solid-liquid separation or sludge dewatering, and the coarse particles obtained through classification; their particle size can be controlled to D50: 20-200μm to improve settling properties and reduce the formation of fine crystals.

[0023] When using heterogeneous nucleation support seed crystals, one or more of the following can be selected: calcium sulfate particles, inert ceramic particles, silica sand, or alumina particles, to provide heterogeneous nucleation sites, promote the epitaxial growth of lead sulfate on the support surface, and form settleable particles.

[0024] Next, a crystallization window is formed through segmented pH control: an alkaline agent is added to the crystallization reactor to achieve segmented pH control. The alkaline agent can be one or more of lime slurry, sodium carbonate solution, sodium hydroxide solution, or magnesium hydroxide slurry.

[0025] A two-stage control method is preferred: First stage: Adjust the pH of the reaction system to 3.5-4.5 and maintain it for a certain period of time to promote the preferential heterogeneous nucleation and epitaxial growth of lead ions on the seed crystal surface, form initial crystal nuclei and avoid instantaneous supersaturation that would lead to the formation of a large number of fine crystals; Second stage: While maintaining stirring, slowly continue to add alkaline reagents to further raise the pH to 5.5-6.5 to promote the growth of lead sulfate crystals and the formation of coarse granular crystals.

[0026] Finally, low supersaturation closed-loop control (inhibiting fine crystals and facilitating solid-liquid separation) is employed: To achieve a crystallization state of "low supersaturation, promoting crystal growth, and inhibiting fine crystals," the addition of alkaline agents is preferably controlled by online pH signal closed-loop control, combined with feedforward correction based on influent flow rate. The pH increase process is controlled as a slow ramp-up, with the pH rising from 3.5–4.5 to 5.5–6.5 in 10–90 minutes, preferably 20–60 minutes.

[0027] Under this control mode, lead sulfate mainly forms coarse-grained crystals through seed epitaxy, reducing the generation of fine colloidal lead salts and difficult-to-settle sludge, thereby reducing the load on downstream flocculation and filter press and improving effluent stability.

[0028] The hydraulic residence time in the crystallization reactor can be 20 to 180 minutes, preferably 40 to 120 minutes; the stirring intensity should be such that the seed crystals are suspended uniformly and do not break excessively.

[0029] S3, Solid-Liquid Separation and Seed Crystal Recovery The effluent from the crystallization reaction enters a solid-liquid separation unit, which can be one or more of an inclined plate sedimentation tank, a high-efficiency sedimentation tank, or a hydrocyclone separator. The crystalline solid phase and supernatant are obtained through sedimentation or hydrocyclone separation.

[0030] The crystalline solid phase further undergoes a classification and recovery process: the coarse particles with good settling properties are recovered and returned to the crystallization reactor as seed crystals for recycling; the remaining fine particles and inclusions are sent to the sludge treatment unit. To improve the purity of the recovered seed crystals, they can be subjected to simple water washing or classification screening.

[0031] S4, secondary weak alkaline precipitation and electrocoagulation for deep lead removal The supernatant obtained from S3 enters the secondary reaction tank, where the pH is adjusted to 8.5–9.5 to cause residual lead and associated metal ions to precipitate as hydroxides or undergo co-precipitation reactions, further reducing the concentration of dissolved lead. An electrocoagulation unit is connected in series after the secondary reaction tank to remove any remaining colloidal lead, complexed lead, and fine particles that are difficult to remove by conventional precipitation.

[0032] The electrocoagulation unit can employ a combination of iron anode / stainless steel cathode, aluminum anode / stainless steel cathode, or iron anode / graphite cathode; the current density can be 5–50 A / m², and the energizing time can be 5–60 minutes. During the electrocoagulation process, the iron / aluminum hydroxyl complex generated in situ by the sacrificial anode has adsorption bridging and scavenging flocculation effects, which can enhance the removal effect on fine lead particles and complexed lead, and improve the stability of the effluent under fluctuating conditions.

[0033] S5, Flocculation and Sedimentation and Filtration The effluent from electrocoagulation enters a flocculation sedimentation tank, where inorganic flocculants and coagulants are added to enhance solid-liquid separation. Inorganic flocculants can be polyaluminum chloride (PAC), polyferric sulfate (PFS), or their mixtures; coagulants can be anionic or nonionic polyacrylamide (PAM). After flocculation and sedimentation, the supernatant enters a filtration unit to further remove fine suspended solids. The filtration unit can employ one or more of sand filtration, bag filtration, or microfiltration to obtain clarified effluent.

[0034] S6, Deep Polishing To further reduce the lead ion concentration in the effluent and improve long-term stability, the S5 filtered water enters the deep polishing unit. The deep polishing unit can employ one or more of the following: chelating ion exchange resin adsorption unit, selective adsorption material adsorption unit containing thiol / amino functional groups, ultrafiltration unit, or nanofiltration unit.

[0035] When there is a need for reuse within the plant, polishing effluent can be reused for rinsing water or circulating water systems; when the goal is to meet emission standards, the polishing unit serves as an end-point safeguard, which can reduce the risk of occasional exceedances due to fluctuations in influent or chemicals.

[0036] S7, Sludge Treatment and Seed Crystal Closed-Loop Recycling Solids or sludge from S3 and S5 enter a dewatering system (e.g., plate and frame filter press or diaphragm filter press) to form a filter cake. The filter cake can be particle-separated: the larger, more settling lead sulfate particles are returned to S2 as seed crystals, forming a closed-loop seed crystal cycle; the remaining fine particles and impurities are disposed of through resource recovery or compliant transfer in accordance with lead-containing solid waste regulations. This closed-loop seed crystal cycle reduces the amount of external seed crystals used and stabilizes the crystallization process, further lowering operating costs.

[0037] Compared to the traditional single neutralization and precipitation route, this invention incorporates a seed-induced selective crystallization step for lead sulfate and uses low supersaturation closed-loop control to suppress fine crystal formation, allowing the lead-containing solid phase to precipitate in a coarse particle form that is more conducive to sedimentation and pressure filtration. Simultaneously, secondary weak alkaline precipitation and electrocoagulation work together to deeply remove residual colloidal / complexed lead, and the final polishing unit ensures stable compliance with or reuse requirements for lead levels in the effluent. Furthermore, the seed crystal recovery and recycling process reduces chemical consumption and solid waste production, improving the economic efficiency and stability of the system operation.

[0038] The effects of the present invention will be further illustrated by the following examples.

[0039] Example 1 A total of 10 m³ / d of wastewater from a lead-acid battery workshop was collected. The influent water quality was as follows: pH=2.10, Pb (as total lead)=86.0 mg / L, SO₄²⁻=8.2 g / L, SS=220 mg / L, COD=95 mg / L.

[0040] Wastewater, after passing through a screen, enters a homogenization and equalization tank for 6 hours of homogenization. After settling for 30 minutes, the supernatant enters a crystallization reactor (continuous stirred tank, 25℃, stirring speed 120 rpm, hydraulic retention time 90 minutes). Lead sulfate seed crystals (1.50 g / L, sourced from coarse particles obtained from the previous filter cake classification) are added to the crystallization reactor. A 10% (mass fraction) magnesium hydroxide slurry is used for segmented pH control: the pH is first adjusted to 4.00 and maintained for 20 minutes, then slowly increased to pH 6.00 using a closed-loop control method (climb time 40 minutes), and the reaction continues at pH 6.00 for 30 minutes. The crystallization effluent enters an inclined plate sedimentation tank for solid-liquid separation. The underflow crystallized solid phase is washed with water and then enters a hydrocyclone classifier. The amount of coarse seed crystals obtained from the classification is 1.00 g / L (converted to the crystallizer dosage), and the remaining fine solid phase enters the sludge system. The supernatant enters a secondary reaction tank, is adjusted to pH=9.20 with 20% sodium hydroxide solution and stirred for 10 min, and then enters the electrocoagulation unit (iron anode / stainless steel cathode, plate spacing 12 mm, current density 20 A / m², energized for 20 min). After electrocoagulation, 80 mg / L of polyferric sulfate (PFS) and 2.0 mg / L of anionic PAM are added to the effluent for flocculation and sedimentation. The effluent is then filtered through a quartz sand filter, and the filtered water enters a chelating resin polishing column (operating flow rate 1 BV / h, single column resin packing volume 20 L) to obtain the final effluent.

[0041] In this embodiment, the final effluent total lead content was 0.08 mg / L; the total lead content before polishing (sand filtration effluent) was 0.24 mg / L; the system produced a dewatered filter cake with a solid content of 48%, which translates to a dry sludge volume of 0.42 kg / m³ wastewater.

[0042] Example 2 Take 30 m³ / d of flushing wastewater from another production line. The influent water quality is: pH=1.85, Pb=52.0 mg / L, SO4²⁻=6.5g / L, SS=140 mg / L, COD=60 mg / L.

[0043] After homogenization in the equalization tank for 4 hours, the solution enters the internal circulation crystallizer (25℃, internal circulation flow ratio 5:1, equivalent residence time 70 min). Lead sulfate seed crystals at 1.20 g / L are added, and pH is adjusted in stages using 10% lime slurry: pH is adjusted to 4.20 and held for 15 min, then increased to pH=5.80 under closed-loop control (climb time 35 min), and held for 20 min. Solid-liquid separation is performed in a high-efficiency sedimentation tank; after bottom flow solid-phase pressure filtration, 0.80 g / L of seed crystals (converted to crystallizer dosage) are reused in stages. The supernatant is adjusted to pH=9.00 and then enters the electrocoagulation unit (aluminum anode / stainless steel cathode, current density 12 A / m², energized for 25 min). Subsequently, flocculation and sedimentation were carried out using PAC 60 mg / L and PAM 1.5 mg / L. The effluent was then filtered through a bag filter (5 μm) and then passed through an ultrafiltration membrane (molecular weight cutoff 100 kDa, operating flux 35 L·m⁻²·h⁻¹) to obtain the final effluent.

[0044] In this embodiment, the total lead content in the final effluent was 0.12 mg / L; the total lead content in the effluent before ultrafiltration (bag filter effluent) was 0.31 mg / L; the solid content of the dewatered filter cake was 44%, which translates to a dry sludge volume of 0.38 kg / m³ wastewater.

[0045] Example 3 Take 20 m³ / d of mixed wastewater containing dust and general flushing water. The influent water quality is: pH=2.35, Pb=110.0 mg / L, SO4²⁻=9.0 g / L, SS=480 mg / L, COD=120 mg / L.

[0046] After homogenization and conditioning for 6 hours and initial grit removal, the wastewater enters a circulating fluidized bed crystallizer (25℃, upflow velocity 0.85 cm / s, residence time 80 min). Seed crystals are added: 0.60 g / L calcium sulfate particles and 0.20 g / L inert ceramic microparticles (mixed as a heterogeneous nucleation carrier), and 0.70 g / L of coarse lead sulfate particles recovered from the previous cycle are reused. The pH is controlled in stages using a 10% sodium carbonate solution: pH=3.90 is maintained for 25 min, then increased to pH=6.10 (climb time 45 min). The crystallization effluent first passes through a hydrocyclone separator to remove coarse crystal particles, then enters an inclined plate sedimentation tank. After solid phase recovery and classification, a total of 0.90 g / L of seed crystals (calculated up to the crystallizer) is reused. The supernatant was adjusted to pH 9.30 in the secondary reaction tank and then entered ferroelectric flocculation (current density 25 A / m², energized for 15 min). Subsequently, it was flocculated and precipitated using PFS 70 mg / L and PAM 2.5 mg / L, followed by sand filtration. To achieve reuse, the sand filtration effluent entered a nanofiltration system (operating pressure 0.8 MPa, recovery rate 70%) to obtain recycled water.

[0047] In this embodiment, the total lead content in the nanofiltration permeate is 0.02 mg / L; the total lead content in the nanofiltration influent (sand filtration effluent) is 0.20 mg / L; the solid content of the dewatered filter cake is 50%, which translates to a dry sludge volume of 0.46 kg / m³ wastewater.

[0048] Comparative Example 1 Wastewater from the same source as in Example 1 (pH=2.10, Pb=86.0 mg / L, SO4²⁻=8.2 g / L, SS=220 mg / L) was homogenized and adjusted for 6 h. 10% lime slurry was added directly to raise the pH to 9.20 in one step. After stirring for 15 min, 110 mg / L PFS and 3.0 mg / L PAM were added for flocculation and sedimentation. The effluent was obtained after sand filtration. No seed crystallization, no electrocoagulation, and no seed recovery and recycling were set up.

[0049] The total lead content in the sand filtration effluent of the comparative example was 0.35 mg / L; the solid content of the dewatered filter cake was 32%, which translates to a dry sludge volume of 0.85 kg / m³ of wastewater, and the turbidity of the sedimentation tank fluctuated significantly.

[0050] Comparative Example 2 Take wastewater from the same source as in Example 1, set up a crystallizer but do not recover the crystal seeds and do not perform slow pH control: quickly adjust the pH from 2.10 to 6.00 in one go (using 20% ​​NaOH solution, completion time 5 min), do not add external crystal seeds and do not reuse filter cake coarse particles; the remaining steps are the same as in Example 1 (including electrocoagulation, flocculation filtration, resin polishing).

[0051] The total lead in the final effluent of this comparative example was 0.18 mg / L; the total lead before polishing was 0.42 mg / L; the solid content of the dewatered filter cake was 40%, which translates to a dry sludge volume of 0.62 kg / m³ wastewater; and the solid-liquid separation stage showed a high proportion of fine crystals and a slow settling rate.

[0052] The following table 1 shows a comparison of the processing effects of the examples / comparative examples.

[0053] Table 1 As shown in Table 1, compared with Comparative Example 1, which uses traditional lime neutralization and precipitation combined with conventional flocculation filtration, Examples 1-3 of this invention, by setting up seed-induced selective crystallization of lead sulfate and using segmented, slow pH increases with low supersaturation control, allow lead to preferentially precipitate as settleable coarse-grained lead sulfate particles, significantly reducing the ratio of fine crystals to colloidal lead salts. This simultaneously improves the upstream removal load and downstream solid-liquid separation conditions without increasing system complexity. Specifically, the total lead before polishing / membrane preparation further decreases from 0.35 mg / L to 0.20–0.31 mg / L, the solid content of the dewatered filter cake increases from 32% to 44%–50%, and the dry sludge volume decreases from 0.85 kg / m³ to 0.38–0.46 kg / m³. Meanwhile, although Comparative Example 2 incorporates crystallization and end-stage polishing, the lack of seed addition / recovery and rapid pH increases lead to finer crystals, reducing solid-liquid separation efficiency and increasing the downstream load, with the total lead before polishing rising to 0.42 mg / L. The total lead content in the effluent was only 0.18 mg / L, while Examples 1-3 of this invention, through the synergistic effect of crystallization stabilization and electrocoagulation for deep lead removal, achieved better and more stable effluent indicators. Specifically, Example 1, which used chelating resin polishing, had a total lead content of 0.08 mg / L in the effluent; Example 2, which used ultrafiltration polishing, had a total lead content of 0.12 mg / L in the effluent; and Example 3, which used a nanofiltration reuse path, further reduced the total lead content in the effluent to 0.02 mg / L. This demonstrates that the present invention has significant technical effects and engineering application value in terms of stable effluent compliance, reduced sludge production, and improved sludge dewatering.

[0054] The foregoing description illustrates and describes preferred embodiments of the present invention. As previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A method for treating lead-acid battery wastewater, characterized in that, Includes the following steps: S1. Separate collection and homogenization: The acidic and lead-containing wastewater generated during the production or regeneration of lead-acid batteries is collected separately according to the acid strength and suspended solids content and then enters the homogenization tank for stirring and homogenization to remove large particulate suspended solids. S2, Primary lead removal: The effluent from S1 is introduced into the crystallization reactor. Lead sulfate seed crystals or heterogeneous nucleation carrier seed crystals are added to the crystallization reactor, and the pH is adjusted in stages under stirring conditions so that lead sulfate crystals are formed in the crystallization window and grow into sedimentable coarse granular crystals. The segmented pH control includes at least: adjusting the pH to 3.5-4.5 and maintaining it to induce nucleation and epitaxial growth, and then slowly adjusting the pH to 5.5-6.5 to promote crystal growth; the pH control is achieved by controlling the dosing rate of alkaline agents through an online pH signal closed loop, so that the system is in a low supersaturation crystallization state to inhibit the formation of fine crystals and colloids. S3. Solid-liquid separation and seed recovery: The effluent from S2 is subjected to sedimentation or cyclone separation to obtain crystalline solid phase and supernatant, and at least part of the crystalline solid phase is recovered as seed crystals for recycling in S2. S4, Secondary lead removal: The supernatant obtained from S3 is introduced into the secondary reaction tank, and the pH is adjusted to 8.5-9.5 to generate metal hydroxide flocs and achieve co-precipitation lead removal. Then it enters the electrocoagulation unit, where flocculants are generated in situ through sacrificial anodes to remove residual colloidal lead and complexed lead. S5. Flocculation, sedimentation and filtration: Add flocculant to the S4 effluent for sedimentation and separation and then filter to obtain the effluent. S6. Deep Polishing: The S5 filtered water is then fed into an adsorption / ion exchange or membrane separation polishing unit to further reduce the lead ion concentration in the effluent. S7. Sludge treatment: The solid phase or sludge generated by S3 and S5 is dewatered, and the coarse particles in the dewatering product are recycled for S2 seed crystal recycling.

2. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The lead sulfate seed crystals are derived from the coarse particle component obtained by fractionation of the dehydrated solid phase obtained in S3 or S7, and the volume fractionation particle size D50 of the coarse particle component is 20-200 μm.

3. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The heterogeneous nucleation support seed crystal is one or more of calcium sulfate particles, inert ceramic particles, silica sand or alumina particles, and its surface is used to provide heterogeneous nucleation sites for lead sulfate to promote the epitaxial growth of lead sulfate.

4. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The alkaline agent in S2 is one or more of lime milk, sodium carbonate solution, sodium hydroxide solution, or magnesium hydroxide slurry; and closed-loop control is performed through online pH and influent flow signals to slowly raise the pH from 3.5 to 4.5 to 5.5 to 6.5 in a time of 10 to 90 minutes.

5. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The crystallization reactor is one or more of the following: a continuous stirred tank reactor with stirring, a circulating fluidized bed crystallizer, or a crystallizer with internal circulation, and the hydraulic residence time of the crystallization reactor is 20 to 180 minutes.

6. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The solid-liquid separation in S3 employs one or more of an inclined plate sedimentation tank, a high-efficiency sedimentation tank, or a hydrocyclone separator, and a flocculation promotion step is set before solid-liquid separation to increase the settling speed of coarse lead sulfate crystals.

7. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The electrocoagulation unit in S4 uses an electrode combination of iron anode / stainless steel cathode, aluminum anode / stainless steel cathode, or iron anode / graphite cathode; the current density of electrocoagulation is 5-50 A / m², and the energizing time is 5-60 minutes.

8. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The flocculant in S5 is polyaluminum chloride, polyferric sulfate or their compound, the coagulant aid is anionic or nonionic polyacrylamide, and the filtration after flocculation and sedimentation is one or more of sand filtration, bag filtration or microfiltration.

9. The method for treating lead-acid battery wastewater according to claim 1, characterized in that, The S6 deep polishing unit is one or more of the following: a chelating ion exchange resin adsorption unit, a selective adsorption material adsorption unit containing thiol / amino functional groups, an ultrafiltration unit, or a nanofiltration unit; and the effluent from the polishing unit is reused for in-plant rinsing or circulating water systems.

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