A process for treating heavy metal wastewater

Abstract

The application provides a heavy metal wastewater treatment process, and belongs to the technical field of industrial wastewater treatment. The process first performs homogenization adjustment and first-stage sedimentation treatment on the heavy metal wastewater to obtain first-stage sedimentation effluent; then performs residual heavy metal form regulation on the first-stage sedimentation effluent, and then performs second-stage deep capture, coagulation, flocculation and sedimentation treatment, and the second-stage sedimentation effluent enters a subsequent comprehensive treatment system. The application can improve the removal stability of residual copper, nickel, zinc, chromium and other heavy metals, improve the sedimentation performance of the captured sediment, and reduce effluent fluctuation and subsequent treatment load.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, and more specifically, relates to a treatment process for heavy metal wastewater. Background Technology

[0002] Industrial processes such as electroplating, electroless plating, metal surface treatment, stripping, pickling, cleaning, and rinsing typically generate heavy metal wastewater containing ions of copper, nickel, zinc, chromium, iron, and aluminum. The concentration of heavy metals in this type of wastewater fluctuates significantly and may also contain fluoride ions, complexing agents, organic additives, surfactants, and suspended particles. Because heavy metal pollutants are highly toxic, difficult to degrade, and easily accumulate in the environment, inadequate treatment can adversely affect subsequent biological treatment systems, reuse systems, and the quality of discharged wastewater. Therefore, heavy metal wastewater usually needs to be collected, regulated, and settled separately before entering a comprehensive treatment system.

[0003] Existing heavy metal wastewater treatment processes typically employ a combination of methods, including neutralization precipitation, sulfide precipitation, heavy metal capture, coagulation and flocculation, and sedimentation separation. Neutralization precipitation primarily works by adjusting the wastewater pH to cause metal ions such as copper, nickel, zinc, and chromium to form hydroxide precipitates. Sulfide precipitation involves adding agents such as sodium sulfide and sodium hydrosulfide to induce some heavy metals to form metal sulfide precipitates with lower solubility. Coagulation and flocculation promote the aggregation and growth of fine precipitate particles, achieving solid-liquid separation. These processes are effective for treating heavy metal wastewater with high concentrations and relatively simple morphologies.

[0004] However, in actual engineering operations, the pollutant forms of heavy metal wastewater are usually quite complex. Some heavy metals such as copper, nickel, zinc, and chromium may form weakly complexed pollutants with organic acid complexing agents, surfactants, or cleaning additives, or they may be stably dispersed in water as colloidal metal hydroxides, colloidal metal sulfides, or fine precipitate particles. For such complex systems, it is often difficult to stably transform different forms of heavy metals into settleable forms simply by relying on a single neutralization precipitation or sulfide precipitation.

[0005] To improve effluent stability, existing processes often include secondary sedimentation, secondary coagulation, or advanced trapping treatment after primary sedimentation. For example, in a factory heavy metal wastewater treatment device disclosed in Chinese Patent Publication No. CN209602278U, after the first-stage sedimentation treatment removes most of the heavy metal ions, the effluent from the first-stage sedimentation is further treated with heavy metal trapping agents, coagulants, and flocculants to complete the second-stage sedimentation treatment. In a multi-stage treatment process for electroplating wastewater in a centralized industrial park disclosed in Chinese Patent Publication No. CN1803659A, the effluent from the first-stage reaction enters the secondary treatment reaction tank, where heavy metal ion trapping and precipitating agents, PAC, and PAM are added for solid-liquid separation. Chinese Patent Publication No. CN102603097A also discloses a process for treating wastewater containing heavy metal ions through primary sedimentation, secondary sedimentation, and subsequent advanced treatment.

[0006] However, the above-mentioned treatment approaches typically focus on reducing the concentration of residual heavy metals by adding precipitants, trapping agents, coagulants, or flocculants again, without usually setting specific control steps for the existing forms of residual heavy metals in the primary sedimentation effluent. Low concentrations of dissolved heavy metals, weakly complexed heavy metals, colloidal metal hydroxides, colloidal metal sulfides, and fine precipitate particles may still be present in the primary sedimentation effluent. These pollutants are characterized by low concentration, dispersed forms, strong colloidal stability, and poor settling performance. If they are directly introduced into the secondary coagulation sedimentation process or if heavy metal trapping agents are directly added, problems such as low trapping agent utilization, fine particles of the trapped products, unstable solid-liquid separation, and fluctuations in the concentration of heavy metals in the effluent may still occur.

[0007] Furthermore, in processes employing sulfides in primary sedimentation treatment, a small amount of sulfides may remain in the primary sedimentation effluent. Excessive residual sulfide concentration can adversely affect subsequent treatment units; conversely, focusing solely on removing residual sulfides may increase reagent consumption and sludge load. Therefore, balancing the residual heavy metal speciation, the stability of fine colloidal particles, and the impact of residual sulfides before the primary sedimentation effluent enters the secondary advanced treatment stage is a key factor affecting the stability of advanced heavy metal wastewater treatment.

[0008] Therefore, existing heavy metal wastewater treatment processes still have the following shortcomings: the form of residual heavy metals after primary sedimentation is not effectively controlled, and secondary treatment is prone to repeated chemical precipitation; residual weakly complexed, colloidal, and low-concentration dissolved heavy metals are difficult to remove stably; heavy metal scavenging agents are ineffectively consumed, and the settling performance of scavenged products is unstable; the concentrations of heavy metals such as copper, nickel, zinc, and chromium in the effluent are prone to fluctuation, and subsequent integrated treatment systems may still be affected. Summary of the Invention

[0009] To address the existing problems in heavy metal wastewater treatment processes, such as insufficient control of residual heavy metal speciation in primary sedimentation effluent, low utilization rate of secondary trapping agents, insufficient stability of solid-liquid separation in trapping and sedimentation, and easy fluctuations in effluent heavy metal concentration, this invention provides a heavy metal wastewater treatment process that can improve the stability of deep removal of residual heavy metals in primary sedimentation effluent, improve the settling performance of secondary trapping and sedimentation, and reduce the heavy metal shock load on subsequent integrated treatment systems.

[0010] To solve the above problems, the present invention adopts the following technical solution.

[0011] A process for treating heavy metal wastewater includes the following steps: S1. Homogenize and adjust the heavy metal wastewater to obtain homogenized heavy metal wastewater; S2. Perform primary precipitation treatment on the homogeneous heavy metal wastewater to precipitate at least some of the heavy metals in the wastewater. S3. Solid-liquid separation is performed on the wastewater after primary sedimentation treatment to obtain primary sedimentation effluent; S4. The residual heavy metal speciation of the primary sedimentation effluent is regulated to convert the residual dissolved heavy metals, weakly complexed heavy metals or colloidal heavy metals in the primary sedimentation effluent into an easily captured state, thereby obtaining speciation-regulated effluent. S5. Perform secondary deep capture treatment on the morphology-controlled effluent to form insoluble precipitates of residual heavy metals. S6. The wastewater after secondary deep capture treatment is subjected to coagulation, flocculation and sedimentation treatment to obtain secondary sedimentation effluent; S7. The secondary sedimentation effluent is sent to an intermediate water tank or a subsequent integrated treatment system.

[0012] Furthermore, the heavy metal wastewater originates from electroplating, electroless plating, metal surface treatment, stripping, pickling, cleaning, rinsing, or heavy metal-containing wastewater discharge processes. The heavy metal wastewater contains one or more of the following: copper, nickel, zinc, chromium, iron, aluminum, fluoride ions, complexing agents, organic additives, or surfactants.

[0013] Further, in step S2, the homogeneous heavy metal wastewater is sent to a primary reaction tank, the pH of the wastewater is adjusted to 9.0-11.0, and an alkaline regulator and sulfide are added for primary precipitation treatment. The alkaline regulator is one or more of sodium hydroxide, lime milk, and sodium carbonate; the sulfide is one or more of sodium sulfide, sodium hydrosulfide, and polysulfides; and the reaction time for the primary precipitation treatment is 15–40 min.

[0014] Furthermore, in step S3, coagulant and flocculant are added to the wastewater after primary sedimentation treatment to carry out primary coagulation, flocculation and sedimentation treatment; The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 50–200 mg / L; the flocculant is PAM, with a dosage of 1–5 mg / L; and the primary sedimentation time is 1–3 h.

[0015] Furthermore, in step S4, the pH of the primary precipitate effluent is adjusted to 8.6–9.6, and an iron salt-type speciation regulator is added to regulate the speciation of residual heavy metals. During the residual heavy metal speciation process, the redox potential is controlled to be -50 to +180 mV, and the reaction time is 10 to 30 min; the dosage of the iron salt speciation modifier is 10 to 100 mg / L based on Fe.

[0016] Furthermore, the iron salt-type speciation modifier includes ferrous salts, ferric salts, or a mixture of ferrous salts and ferric salts; The ferrous salt is one or both of ferrous sulfate and ferrous chloride; the ferric salt is one or more of ferric sulfate, ferric chloride, polyferric sulfate, and polyferric chloride; when the ferric salt morphology regulator is a compound of ferrous salt and ferric salt, the molar ratio of ferrous salt to ferric salt is 1:0.3 to 1:2.0.

[0017] Furthermore, in step S4, the concentration of residual sulfide in the primary precipitate is adjusted to 0.1–1.0 mg / L; The iron salt-type speciation modifier forms iron-sulfur micro-flocs with residual sulfides, and forms iron hydroxyl flocs through iron salt hydrolysis, thereby destabilizing, adsorbing or co-precipitating colloidal metal precipitates, weakly complexed heavy metals or low-concentration dissolved heavy metals in the primary precipitate water.

[0018] Further, in step S5, the morphology-controlled effluent is sent to a secondary deep-collection reaction tank, the pH is controlled at 8.8-9.8, and an organic sulfur heavy metal precipitant is added for secondary deep-collection treatment. The dosage of the organic sulfur heavy metal capture agent is calculated as 1.05 to 1.80 times the total molar amount of residual copper, nickel, zinc, and chromium in the effluent, based on the modulus of the form control; the reaction time for the secondary deep capture treatment is 15 to 40 minutes.

[0019] Furthermore, the organosulfur heavy metal scavenger is one or more of the following: dithiocarbamate scavengers, trithiotriazine scavengers, xanthate scavengers, and polythiol scavengers; The organic sulfur heavy metal scavenger is added in two stages. The first stage accounts for 60% to 80% of the total scavenger dosage, and the second stage accounts for 20% to 40% of the total scavenger dosage. The interval between the two stages is 5 to 15 minutes.

[0020] Furthermore, in step S6, coagulants and flocculants are added to the wastewater after the secondary deep capture treatment for secondary coagulation, flocculation and sedimentation treatment; The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 30–150 mg / L; the flocculant is PAM, with a dosage of 0.5–3 mg / L; the secondary sedimentation time is 1–2.5 h; After the secondary sedimentation effluent enters the intermediate water tank, it enters one or more of the following treatment units: biochemical treatment, filtration treatment, ion exchange treatment, or comprehensive reuse treatment.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention introduces a residual heavy metal speciation control step before the primary sedimentation effluent enters the secondary deep trapping stage. This transforms the low-concentration dissolved, weakly complexed, and colloidal heavy metals remaining after primary sedimentation into more easily trapped, flocculated, and settled forms. Then, a secondary deep trapping stage using an organosulfur-based heavy metal trapping agent causes residual heavy metals such as copper, nickel, zinc, and chromium to form stable, insoluble precipitates. This process improves the stability of residual heavy metal removal at depth, reduces ineffective trapping agent consumption, improves the settling performance of secondary sediment flocs, reduces fluctuations in effluent heavy metal concentration, and alleviates the impact load on subsequent integrated treatment systems. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of a heavy metal wastewater treatment process according to the present invention. Detailed Implementation

[0023] The specific embodiments of the present invention are described below. It should be understood that the following content is used to explain the technical solution of the present invention and is not intended to limit the scope of protection of the present invention. Without departing from the technical concept of the present invention, those skilled in the art can make adaptive adjustments to the type of reagent, dosage, reaction time, and treatment unit form according to the actual wastewater quality, quantity, heavy metal concentration, complexing agent content, and discharge requirements.

[0024] This invention provides a process for treating heavy metal wastewater. This process is mainly applicable to heavy metal wastewater generated from electroplating, electroless plating, metal surface treatment, stripping, pickling, cleaning, rinsing, or discharge of heavy metal-containing wastewater. This type of wastewater may contain one or more of the following: copper, nickel, zinc, chromium, iron, aluminum, fluoride ions, complexing agents, organic additives, or surfactants.

[0025] Compared to conventional two-stage sedimentation processes for heavy metal wastewater, the focus of this invention is not simply adding a primary or secondary sedimentation unit, but rather on: before the primary sedimentation effluent enters the secondary deep collection stage, the residual heavy metals in the primary sedimentation effluent are first subjected to speciation control, transforming residual dissolved heavy metals, weakly complexed heavy metals, colloidal heavy metals, and fine precipitate particles from a difficult-to-capture and difficult-to-settle state into a easily-captured, easily-flocculated, and easily-settle state; subsequently, secondary deep collection and secondary coagulation sedimentation are carried out. Thus, the secondary treatment stage has a clear speciation control and depth fixation function, rather than being a simple repetition of the primary sedimentation treatment.

[0026] Specifically, the heavy metal wastewater treatment process of the present invention may include the following steps.

[0027] I. Homogenization and Conditioning of Heavy Metal Wastewater First, the heavy metal wastewater is collected separately in an equalization tank to balance its volume, quality, and pH, resulting in homogeneous heavy metal wastewater.

[0028] Heavy metal wastewater is typically characterized by intermittent discharge, concentration fluctuations, and significant pH fluctuations. If it enters the subsequent precipitation reaction unit directly without homogenization, it can easily lead to problems such as fluctuations in reagent dosage, excessively high or low local pH levels, and incomplete heavy metal precipitation. Therefore, the homogenization step is mainly used to reduce influent shock and provide a relatively stable reaction basis for subsequent primary precipitation treatment.

[0029] This step can be implemented using a conventional equalization tank, stirring device, booster pump, and online pH monitoring device. This step itself is a standard pretreatment step in wastewater treatment, and in this invention, it primarily serves as the foundation unit for subsequent staged sedimentation and speciation control.

[0030] II. Primary Sedimentation Treatment The homogeneous heavy metal wastewater is sent to a primary reaction tank to adjust the pH of the wastewater. Alkaline regulators and sulfides are added for primary precipitation treatment, causing the main heavy metals in the wastewater to precipitate.

[0031] In one specific embodiment, the pH of the homogenized heavy metal wastewater is adjusted to 9.0–11.0. The alkaline adjuster is one or more of sodium hydroxide, lime milk, and sodium carbonate. The sulfide is one or more of sodium sulfide, sodium hydrosulfide, and polysulfides. The reaction time for the primary precipitation treatment is 15–40 minutes.

[0032] During the primary precipitation process, alkaline regulators cause some heavy metal ions such as copper, nickel, zinc, and chromium in the wastewater to form metal hydroxide precipitates; sulfides, on the other hand, can react with copper, nickel, zinc, and other metal ions to form metal sulfide precipitates with lower solubility. For heavy metal ions with higher concentrations and simpler forms, this step can reduce the main pollution load.

[0033] However, primary sedimentation treatment is usually insufficient to completely remove all forms of heavy metals. This is because actual wastewater often contains cleaning additives, surfactants, organic acid complexing agents, or other complexing components, and some heavy metals may exist in a weakly complexed state. Simultaneously, under conditions of high pH and sulfide-related reactions, small-sized metal hydroxides, metal sulfides, and their complex colloidal particles may also form in the wastewater. These residual pollutants are easily and stably dispersed in the water and enter subsequent treatment stages along with the primary sedimentation effluent.

[0034] Therefore, the primary precipitation treatment in this invention mainly serves to remove the main heavy metals, and its purpose is to reduce the heavy metal load entering the subsequent core treatment steps, rather than as a final deep treatment method.

[0035] III. Primary Solid-Liquid Separation Process Solid-liquid separation is performed on the wastewater after primary sedimentation treatment to obtain primary sedimentation effluent.

[0036] In one specific embodiment, a coagulant and a flocculant are added to the wastewater after primary sedimentation treatment for primary coagulation, flocculation, and sedimentation. The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 50–200 mg / L; the flocculant is PAM, with a dosage of 1–5 mg / L; and the primary sedimentation time is 1–3 hours.

[0037] After primary solid-liquid separation, most of the metal hydroxide precipitates, metal sulfide precipitates, and suspended particles are separated. However, the following residual pollutants may still exist in the primary precipitate effluent: Firstly, low concentrations of dissolved heavy metals such as copper, nickel, zinc, and chromium; Secondly, weakly complexed heavy metals stabilized by organic acid complexing agents, surfactants, or cleaning additives; Third, colloidal metal hydroxides or colloidal metal sulfides with smaller particle size, stronger surface charge, and less tendency to settle naturally; Fourth, the trace amounts of sulfides remaining during the primary sulfide precipitation process and the finely dispersed metal precipitates induced by them.

[0038] The aforementioned residual components are characterized by low concentration, dispersed form, strong colloidal stability, and poor settling performance. If the effluent from the primary sedimentation directly enters the secondary coagulation sedimentation, it is usually difficult to stably remove these pollutants. If a large amount of heavy metal precipitating agent is added directly, the precipitating agent may be consumed by colloidal particles, residual sulfides, or competing components in the water, resulting in a decrease in the effective utilization rate of the precipitating agent. Furthermore, the precipitated particles formed are small, and the subsequent settling stability is insufficient.

[0039] To address the aforementioned issues, this invention incorporates a residual heavy metal speciation control step between the primary sedimentation effluent and the secondary deep capture.

[0040] IV. Control of residual heavy metal speciation in primary sedimentation effluent The primary sedimentation effluent is sent to a residual heavy metal speciation control reaction tank to regulate the speciation of residual heavy metals in the primary sedimentation effluent, thus obtaining speciation-controlled effluent.

[0041] In one specific embodiment, the pH of the primary sedimentation effluent is adjusted to 8.6–9.6, and an iron salt-type speciation modifier is added to regulate the speciation of residual heavy metals. During speciation regulation, the redox potential is controlled at -50–+180 mV, and the reaction time is 10–30 min. The dosage of the iron salt-type speciation modifier, calculated as Fe, is 10–100 mg / L.

[0042] Ferrous salt-type speciation modifiers include ferrous salts, ferric salts, or mixtures of ferrous and ferric salts. The ferrous salt is one or both of ferrous sulfate and ferrous chloride; the ferric salt is one or more of ferric sulfate, ferric chloride, polyferric sulfate, and polyferric chloride. When the ferrous salt-type speciation modifier is a mixture of ferrous and ferric salts, the molar ratio of ferrous to ferric iron is 1:0.3 to 1:2.0.

[0043] In a further specific embodiment, the concentration of residual sulfide in the primary precipitate is adjusted to 0.1–1.0 mg / L.

[0044] This step is one of the core steps of this invention. Its function is not limited to ordinary coagulation, but rather to regulate the existing form of residual pollutants in the primary precipitate.

[0045] First, a small amount of sulfides may remain in the wastewater after primary sedimentation treatment. Conventionally, residual sulfides are often considered interfering components that need to be removed because excessively high concentrations can produce odors, affect subsequent biochemical treatments, or interfere with subsequent chemical reactions. However, this invention does not aim to completely remove residual sulfides, but rather to control their concentration within a low range of 0.1–1.0 mg / L, transforming them from purely interfering components into auxiliary components involved in speciation regulation.

[0046] Within this concentration range, residual sulfides can react with added iron salt-type speciation modifiers to form iron-sulfur micro-flocs. These iron-sulfur micro-flocs have certain adsorption and nucleation effects, providing a microscopic adhesion basis for subsequent capture and precipitation of residual heavy metals. If the residual sulfide concentration is too low, insufficient iron-sulfur micro-floc formation will hinder its nucleation and coagulation-aiding effects; if the residual sulfide concentration is too high, it may increase the burden on subsequent treatments and adversely affect the capture agent or biochemical system. Therefore, maintaining the residual sulfide concentration within a controlled low range can balance sulfide risk control with the subsequent capture, precipitation, and nucleation effects.

[0047] Secondly, the iron salt-type speciation regulator undergoes hydrolysis under weakly alkaline conditions to form iron hydroxyl flocs. These iron hydroxyl flocs can adsorb colloidal metal hydroxides, colloidal metal sulfides, and fine precipitate particles. Through charge neutralization, adsorption bridging, and sweeping, they weaken the stability of colloidal particles, transforming the fine particles that were originally stably dispersed in the primary precipitate into a state that is prone to flocculation.

[0048] Furthermore, for weakly complexed heavy metals, the hydrolysis products of ferric salts and iron-based flocculants can provide an adsorption and co-precipitation interface, allowing some weakly complexed heavy metals such as copper, nickel, zinc, and chromium to transform from a stable dissolved or semi-dissolved state to an adsorbed or co-precipitated state. This transformation process reduces the competitive interference of residual complexing components on subsequent trapping agents, enabling subsequent organosulfur trapping agents to act more concentratedly on the residual heavy metals.

[0049] Furthermore, this invention controls the pH of the speciation regulation step to 8.6–9.6. This pH range is lower than the highly alkaline environment of some primary sedimentation stages, which reduces the risk of some metals forming soluble hydroxyl complexes or finely dispersed colloids under high pH conditions; at the same time, it maintains a weakly alkaline environment, which is conducive to the formation of iron salt hydrolysis flocs and subsequent organic sulfur capture reactions. Thus, the primary sedimentation effluent is transformed from a low-concentration, dispersed, and difficult-to-settle residual heavy metal system into a speciation regulation system containing iron-sulfur micro-flocs, iron-hydroxyl flocs, and destabilized metal pollutants.

[0050] Compared to conventional two-stage precipitation processes, this step alters the target of secondary deep trapping, thereby improving the effectiveness and stability of subsequent trapping reactions. Traditional secondary treatments typically involve the direct and repeated addition of precipitants, trapping agents, or coagulants, targeting uncontrolled, low-concentration dissolved, weakly complexed, or colloidal metal pollutants. This invention, by first modifying the speciation, alters the target of secondary deep trapping, ensuring that subsequent trapping agents deal with destabilized, adsorbed, or nucleated residual heavy metals. This pre-constitutional speciation modification enhances the effectiveness and stability of subsequent secondary deep trapping.

[0051] V. Secondary Depth Collection Processing The effluent with morphology-controlled conditions is sent to a secondary deep-collection reaction tank, where organic sulfur-based heavy metal precipitants are added for secondary deep-collection treatment, causing the residual heavy metals after morphology control to form insoluble precipitates.

[0052] In one specific embodiment, the pH of the effluent with form-controlled precipitation is controlled to 8.8–9.8, and an organosulfur heavy metal scavenger is added. The reaction time for the secondary deep scavenging treatment is 15–40 min. The dosage of the organosulfur heavy metal scavenger is calculated as 1.05–1.80 times the total molar amount of residual copper, nickel, zinc, and chromium in the form-controlled effluent.

[0053] Organic sulfur heavy metal scavengers are one or more of the following: dithiocarbamate scavengers, trithiotriazine scavengers, xanthate scavengers, and polythiol scavengers.

[0054] In a further specific embodiment, the organosulfur heavy metal scavenger is added in two stages. The first addition accounts for 60% to 80% of the total scavenger dosage, and the second addition accounts for 20% to 40% of the total scavenger dosage, with an interval of 5 to 15 minutes between the two additions.

[0055] The secondary deep capture step has a different function from the primary precipitation treatment. The primary precipitation treatment is mainly used to reduce the main heavy metal load, relying on alkaline adjustment and sulfide precipitation to form metal hydroxides or metal sulfides; the secondary deep capture treatment is mainly used to treat the residual low concentration, weakly complexed or colloidal heavy metals in the primary precipitation effluent, relying on the sulfur coordination groups in organosulfur-based capture agents to form stable and insoluble precipitates with the residual heavy metals.

[0056] Since secondary deep capture is carried out after morphology regulation, the residual heavy metals in the primary precipitate have already been partially adsorbed onto the surface of iron-based flocs, or are in a state of colloidal destabilization and weakened complexation. At this point, adding organosulfur heavy metal precipitants allows the precipitants to react more effectively with the residual metal ions, forming organosulfur metal precipitates. Simultaneously, these precipitates can further aggregate and grow based on the iron-hydroxyl flocs or iron-sulfur micro-flocs formed in the previous step, forming a composite precipitate structure composed of organosulfur metal precipitates, iron-hydroxyl flocs, and iron-sulfur micro-flocs.

[0057] Compared to the fine precipitates formed by directly adding precipitants, this composite precipitation structure exhibits better flocculation growth capacity and sedimentation stability. This is because the iron-hydroxyl flocs provide strong adsorption bridging and sweeping effects, the iron-sulfur micro-flocs provide nucleation and coagulation aid interfaces, and the organic sulfur precipitant provides strong coordination precipitation of residual heavy metals such as copper, nickel, zinc, and chromium. These three elements work synergistically in a continuous process, transforming the residual heavy metals from a difficult-to-capture and difficult-to-settle state into an easily-capture and easily-settle state.

[0058] The two-stage addition of the trapping agent further improves the stability of the secondary deep trapping process. The first addition is used to rapidly trap the main residual heavy metals in the effluent, causing them to form initial trapping precipitates. The second addition is used to supplement the trapping of low-concentration residual metals and metal pollutants that are still in a weakly complexed or slightly dissolved state after the first reaction. Compared with a single large-scale addition, the two-stage addition reduces the risk of reagent waste and the formation of fine dispersed precipitates due to excessively high local trapping agent concentrations, making the trapping reaction more stable and the trapped products more likely to grow and settle in the subsequent coagulation and flocculation stages.

[0059] Therefore, this invention, by linking residual heavy metal speciation control with secondary deep trapping, transforms secondary deep trapping from a simple repetition of primary precipitation into a fine treatment step specifically designed for the speciation of residual pollutants after primary precipitation. This step can improve the stability of residual heavy metal removal with relatively low trapping agent dosage and enhance the settling performance of the trapped and precipitated particles.

[0060] VI. Secondary coagulation, flocculation and sedimentation treatment The wastewater after secondary deep capture treatment is subjected to coagulation, flocculation and sedimentation treatment to obtain secondary sedimentation effluent.

[0061] In one specific embodiment, a coagulant and a flocculant are added to the wastewater after secondary deep sedimentation treatment. The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 30–150 mg / L; the flocculant is PAM, with a dosage of 0.5–3 mg / L; and the secondary sedimentation time is 1–2.5 h.

[0062] After secondary deep capture, residual heavy metals, iron-based flocculants, iron-sulfur micro-flocculants, and some fine colloidal particles have formed in the wastewater. Through secondary coagulation, flocculation, and sedimentation treatment, these particles further aggregate to form larger flocs and achieve solid-liquid separation.

[0063] Since the aforementioned morphology regulation and secondary deep capture have improved the particle morphology and sedimentation basis of residual pollutants, the load required for the secondary coagulation and sedimentation stage is reduced, the concentration of heavy metals such as copper, nickel, zinc and chromium in the precipitated water is more stable, and the turbidity and suspended solids in the effluent are easier to control.

[0064] VII. Intermediate Water Tank and Subsequent Comprehensive Treatment The effluent from the secondary sedimentation is sent to an intermediate water tank for buffering of water volume and quality before entering the subsequent comprehensive treatment system. This system includes one or more of the following: biological treatment, filtration, ion exchange, or integrated reuse treatment units.

[0065] This step is mainly used to connect the entire plant's integrated wastewater treatment process. Since the upstream has already achieved stable reduction of heavy metal pollutants through primary main sedimentation, residual speciation control, secondary deep capture, and secondary sedimentation, the heavy metal shock load entering the subsequent integrated treatment system is reduced, which is conducive to improving the operational stability of the subsequent biological system, filtration system, or reuse system.

[0066] Through the above-described process, this invention incorporates a residual heavy metal speciation control step between the primary sedimentation treatment and the secondary deep capture treatment, ensuring that the pollutant speciation of the primary sedimentation effluent is transformed before entering the secondary capture stage. This design transforms the secondary treatment stage from a simple repetition of the primary sedimentation into a deep treatment targeting the speciation characteristics of residual heavy metals after primary sedimentation.

[0067] Specifically, primary sedimentation treatment is mainly used to remove major heavy metals from wastewater, but low concentrations of dissolved, weakly complexed, colloidal, or fine particulate heavy metals may still remain in the effluent. If this part of the wastewater directly enters the secondary coagulation sedimentation or if a precipitant is directly added, problems such as high precipitant consumption, fine precipitate particles, unstable solid-liquid separation, and fluctuations in the concentration of heavy metals in the effluent are likely to occur.

[0068] This invention transforms residual heavy metals in the primary precipitate from a dispersed, stable, and difficult-to-settle state into a state that is easier to capture and flocculate through a residual heavy metal speciation control step. Subsequently, a secondary deep capture step further fixes the speciation-controlled residual heavy metals into insoluble precipitates. Finally, solid-liquid separation is achieved through secondary coagulation, flocculation, and precipitation. This forms a continuous treatment path encompassing primary precipitation, residual speciation control, deep capture, and solid-liquid separation.

[0069] This continuous treatment pathway establishes a clear division of labor among the steps: primary sedimentation reduces the overall heavy metal load; residual heavy metal speciation improves the capture and settling conditions of residual pollutants in the primary sedimentation effluent; secondary deep capture fixes the speciation-modified residual heavy metals; and secondary coagulation sedimentation separates the captured products from fine particles. Each step is interconnected in terms of the target substances and treatment objectives, thereby improving the stability of deep removal of residual heavy metals such as copper, nickel, zinc, and chromium.

[0070] Therefore, the technical effect of this invention does not stem from simply adding a secondary sedimentation unit, but rather from the coordination between the regulation of the effluent morphology from the primary sedimentation stage and the secondary deep trapping stage. This coordination can reduce the ineffective consumption of trapping agents, improve the flocculation and sedimentation performance of the secondary trapping sedimentation, reduce the fluctuation of heavy metal concentration in the secondary sedimentation effluent, and alleviate the heavy metal shock load on the subsequent integrated treatment system.

[0071] The present invention will be further described below with reference to the embodiments. All embodiments and comparative examples use the same batch of heavy metal wastewater as the treatment object. After homogenization, primary composite sedimentation, and primary solid-liquid separation, the heavy metal wastewater yields primary precipitate effluent. To highlight the technical effects of the core steps of the present invention, the embodiments and comparative examples mainly focus on comparing the control of residual heavy metal speciation in the primary precipitate effluent and the secondary deep capture step.

[0072]

[0073] The concentrations of copper, nickel, zinc, and chromium refer to the mass concentration of the corresponding metal elements in the wastewater in all forms, including dissolved, weakly complexed, colloidal, and other detectable forms.

[0074] Example 1: This example uses the heavy metal wastewater treatment process of the present invention. The heavy metal wastewater is fed into an equalization tank for homogenization and then enters a primary reaction tank. The pH of the wastewater is adjusted to 9.0, and sodium hydroxide and sodium sulfide are added for primary precipitation treatment, with a reaction time of 15 minutes. Subsequently, PAC and PAM are added for primary coagulation, flocculation, and precipitation treatment to obtain primary precipitated effluent.

[0075] The primary sedimentation effluent was sent to a residual heavy metal speciation control reaction tank. The pH was adjusted to 8.6, and ferrous sulfate was added as an iron salt speciation control agent at a dosage of 10 mg / L (calculated as Fe). The redox potential was controlled at -50 mV, the reaction time was 10 min, and the residual sulfide concentration was adjusted to 0.10 mg / L.

[0076] The effluent with modified form was then sent to a secondary deep capture reaction tank, where the pH was controlled at 8.8. A dithiocarbamate-based heavy metal capture agent was added at a dosage of 1.05 times the total molar amount of residual copper, nickel, zinc, and chromium in the effluent with modified form. The capture reaction time was 15 minutes. After capture, PAC and PAM were added for secondary coagulation, flocculation, and sedimentation treatment to obtain secondary precipitated effluent.

[0077] Example 2: This example uses the same overall process as Example 1, except that: In the residual heavy metal speciation control step, the pH of the primary precipitate effluent was adjusted to 9.1, and a compound of ferrous sulfate and polyferric sulfate was added as an iron salt speciation control agent, wherein the molar ratio of ferrous iron to ferric iron was 1:1.0, the dosage was 50 mg / L based on Fe, the redox potential was controlled at +60 mV, the reaction time was 20 min, and the residual sulfide concentration was controlled to 0.50 mg / L.

[0078] In the secondary deep capture step, the pH is controlled at 9.3, and a trithiotriazine heavy metal capture agent is added. The dosage is calculated as 1.40 times the total molar amount of residual copper, nickel, zinc, and chromium in the effluent, and the capture reaction time is 25 minutes. The capture agent is added in two stages: the first addition accounts for 70% of the total capture agent dosage, and the second addition accounts for 30% of the total capture agent dosage, with an interval of 10 minutes between the two additions.

[0079] The remaining steps are the same as in Example 1.

[0080] Example 3: This example uses the same overall process as Example 1, except that: In the residual heavy metal speciation control step, the pH of the primary precipitate effluent was adjusted to 9.6, and a compound of ferrous chloride and polyferric chloride was added as an iron salt speciation control agent. The molar ratio of ferrous iron to ferric iron was 1:2.0, the dosage was 100 mg / L (calculated as Fe), the redox potential was controlled at +180 mV, the reaction time was 30 min, and the residual sulfide concentration was controlled to 1.00 mg / L.

[0081] In the secondary deep capture step, the pH is controlled at 9.8, and a dithiocarbamate heavy metal capture agent is added. The dosage is calculated as 1.80 times the total molar amount of residual copper, nickel, zinc, and chromium in the effluent, and the capture reaction time is 40 minutes. The capture agent is added in two stages: the first addition accounts for 80% of the total capture agent dosage, and the second addition accounts for 20% of the total capture agent dosage, with an interval of 15 minutes between the two additions.

[0082] The remaining steps are the same as in Example 1.

[0083] Comparative Example 1: This comparative example uses a conventional single-stage composite sedimentation treatment process. After homogenization and conditioning, the heavy metal wastewater undergoes only single-stage composite sedimentation, single-stage coagulation, flocculation, and sedimentation treatment, without any residual heavy metal speciation control steps or secondary deep capture steps.

[0084] Comparative Example 2: This comparative example is basically the same as Example 2, except that: the residual heavy metal speciation of the primary sedimentation effluent is not regulated, and it directly enters the secondary deep capture reaction tank, where a trithiotriazine heavy metal capture agent is added for capture treatment. The amount of capture agent added is 1.40 times the total molar amount of residual copper, nickel, zinc, and chromium in the primary sedimentation effluent. The reaction time and secondary coagulation, flocculation, and sedimentation conditions are the same as in Example 2.

[0085] Comparative Example 3: This comparative example is basically the same as Example 2, except that only pH and redox potential are adjusted in the residual heavy metal speciation control step, and no iron salt speciation control agent is added.

[0086] Comparative Example 4: This comparative example is basically the same as Example 2, except that: iron salt-type speciation modifier is added in the residual heavy metal speciation control step, but the residual sulfide concentration is not controlled. After speciation control, the residual sulfide concentration in the wastewater is 1.65 mg / L.

[0087] Comparative Example 5: This comparative example is basically the same as Example 2, except that: after adjusting the residual heavy metal speciation, no organic sulfur heavy metal capture agent is added, and secondary sedimentation effluent is obtained only through secondary coagulation, flocculation and sedimentation treatment.

[0088] Comparative Example 6: This comparative example is basically the same as Example 2, except that in the secondary deep capture step, the organic sulfur heavy metal capture agent is added in a single dose, without being added in multiple doses. The total amount of capture agent added is the same as in Example 2.

[0089]

[0090]

[0091]

[0092] As shown in Table 4, Comparative Example 1, which only used a single-stage composite precipitation treatment, still had high concentrations of copper, nickel, zinc, and chromium in the effluent. Although Comparative Example 2 underwent a two-stage deep capture, the heavy metal removal effect was significantly lower than that of Example 2 due to the lack of residual heavy metal speciation control. Comparative Example 3, without the addition of an iron salt-type speciation modifier, failed to fully destabilize colloidal metal precipitates and weakly complexed heavy metals, resulting in a decrease in the effectiveness of the two-stage deep capture. Examples 1 to 3 all significantly reduced the heavy metal concentration in the effluent from the two-stage precipitation process, with Examples 2 and 3 showing more stable treatment effects.

[0093]

[0094] Note: Comparative Example 1 did not undergo secondary treatment, and its final effluent was primary sedimentation effluent.

[0095] As shown in Table 5, the residual sulfide, turbidity, and SS in the primary sedimentation effluent of Comparative Example 1 were all relatively high. Although an iron salt-type speciation modifier was added to Comparative Example 4, the residual sulfide concentration was not controlled, and a high residual sulfide concentration still existed, indicating that the control of residual sulfide concentration has a significant impact on the stability of subsequent treatment. Examples 1 to 3, through continuous treatment of speciation and secondary deep capture, were able to simultaneously reduce residual sulfide, turbidity, and SS, indicating that residual heavy metals and fine colloidal particles were effectively converted and separated.

[0096]

[0097] Note: The fluctuation range of heavy metals in the effluent refers to the fluctuation range of the combined concentration of copper, nickel, zinc, and chromium in the secondary precipitate effluent relative to the average value during continuous operation. The sludge interface height after 30 minutes of settling is the ratio of the sludge layer height to the height of the original liquid column after 30 minutes of settling; the lower the value, the more complete the settling.

[0098] As shown in Table 6, Comparative Examples 2 and 3, with the same amount of trapping agent as Example 2, exhibited significantly poorer settling performance and effluent stability. Although Comparative Example 6 used the same total amount of trapping agent, its single-dose addition resulted in weaker settling performance and effluent stability compared to Example 2. Example 2, through residual heavy metal speciation control and multi-stage trapping agent addition, facilitated the formation of stable flocs from the collected sediment, significantly improving both settling performance and effluent stability. Example 1 achieved good treatment results even with a lower trapping agent dosage, indicating that the speciation control step improved the effective utilization rate of the trapping agent.

[0099] The above embodiments and comparative examples demonstrate that the treatment effect of the present invention mainly stems from the continuous coordination between the regulation of residual heavy metal speciation in the primary sedimentation effluent and the secondary deep capture. After primary sedimentation, the residual heavy metals in the effluent are not all present in easily precipitated forms, but may exist in low-concentration dissolved, weakly complexed, colloidal, and fine particulate forms. If secondary capture or secondary coagulation sedimentation is directly carried out, the utilization rate of the capture agent and the sedimentation stability are both limited.

[0100] This invention utilizes pH adjustment, iron salt-based speciation modifiers, and residual sulfide concentration control to transform residual heavy metals in the primary precipitate into a more easily captured and settled state. Subsequently, a secondary deep capture process is performed using an organosulfur heavy metal capture agent, followed by solid-liquid separation through secondary coagulation and flocculation sedimentation. This continuous treatment method reduces the residual concentrations of copper, nickel, zinc, and chromium, decreases residual sulfides and suspended particles, improves the settling performance of the precipitated flocs, and reduces fluctuations in effluent heavy metal concentration, thereby enhancing the stability of deep treatment of heavy metal wastewater.

Claims

1. A treatment process for heavy metal wastewater, characterized in that, Includes the following steps: S1. Homogenize and adjust the heavy metal wastewater to obtain homogenized heavy metal wastewater; S2. Perform primary precipitation treatment on the homogeneous heavy metal wastewater to precipitate at least some of the heavy metals in the wastewater. S3. Solid-liquid separation is performed on the wastewater after primary sedimentation treatment to obtain primary sedimentation effluent; S4. The residual heavy metal speciation of the primary sedimentation effluent is regulated to convert the residual dissolved heavy metals, weakly complexed heavy metals or colloidal heavy metals in the primary sedimentation effluent into an easily captured state, thereby obtaining speciation-regulated effluent. S5. Perform secondary deep capture treatment on the morphology-controlled effluent to form insoluble precipitates of residual heavy metals. S6. The wastewater after secondary deep capture treatment is subjected to coagulation, flocculation and sedimentation treatment to obtain secondary sedimentation effluent; S7. The secondary sedimentation effluent is sent to an intermediate water tank or a subsequent integrated treatment system.

2. The heavy metal wastewater treatment process according to claim 1, characterized in that: The heavy metal wastewater originates from electroplating, chemical plating, metal surface treatment, stripping, pickling, cleaning, rinsing or discharge of waste liquid containing heavy metals. The heavy metal wastewater contains one or more of the following: copper, nickel, zinc, chromium, iron, aluminum, fluoride ions, complexing agents, organic additives, or surfactants.

3. The treatment process for heavy metal wastewater according to claim 1, characterized in that: In step S2, the homogeneous heavy metal wastewater is sent to a primary reaction tank, the pH of the wastewater is adjusted to 9.0-11.0, and an alkaline regulator and sulfide are added for primary precipitation treatment. The alkaline regulator is one or more of sodium hydroxide, lime milk, and sodium carbonate; the sulfide is one or more of sodium sulfide, sodium hydrosulfide, and polysulfides; and the reaction time for the primary precipitation treatment is 15–40 min.

4. The heavy metal wastewater treatment process according to claim 1, characterized in that: In step S3, coagulant and flocculant are added to the wastewater after primary sedimentation treatment to carry out primary coagulation, flocculation and sedimentation treatment; The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 50–200 mg / L; the flocculant is PAM, with a dosage of 1–5 mg / L; and the primary sedimentation time is 1–3 h.

5. The treatment process for heavy metal wastewater according to claim 1, characterized in that: In step S4, the pH of the primary precipitate effluent is adjusted to 8.6-9.6, and an iron salt-type speciation regulator is added to regulate the speciation of residual heavy metals. During the residual heavy metal speciation process, the redox potential is controlled to be -50 to +180 mV, and the reaction time is 10 to 30 min; the dosage of the iron salt speciation modifier is 10 to 100 mg / L based on Fe.

6. The heavy metal wastewater treatment process according to claim 5, characterized in that: The iron salt-type morphology modifiers include ferrous salts, ferric salts, or mixtures of ferrous salts and ferric salts; The ferrous salt is one or both of ferrous sulfate and ferrous chloride; the ferric salt is one or more of ferric sulfate, ferric chloride, polyferric sulfate, and polyferric chloride; when the ferric salt morphology regulator is a compound of ferrous salt and ferric salt, the molar ratio of ferrous salt to ferric salt is 1:0.3 to 1:2.

0.

7. A treatment process for heavy metal wastewater according to claim 5 or 6, characterized in that: In step S4, the concentration of residual sulfide in the primary precipitate is adjusted to 0.1–1.0 mg / L; The iron salt-type speciation modifier forms iron-sulfur micro-flocs with residual sulfides, and forms iron hydroxyl flocs through iron salt hydrolysis, thereby destabilizing, adsorbing or co-precipitating colloidal metal precipitates, weakly complexed heavy metals or low-concentration dissolved heavy metals in the primary precipitate water.

8. The treatment process for heavy metal wastewater according to claim 1, characterized in that: In step S5, the morphology-controlled effluent is sent to a secondary deep-collection reaction tank, the pH is controlled at 8.8-9.8, and an organic sulfur heavy metal precipitant is added for secondary deep-collection treatment. The dosage of the organic sulfur heavy metal capture agent is calculated as 1.05 to 1.80 times the total molar amount of residual copper, nickel, zinc, and chromium in the effluent, based on the morphological regulation; the reaction time for the secondary deep capture treatment is 15 to 40 minutes.

9. The heavy metal wastewater treatment process according to claim 8, characterized in that: The organosulfur heavy metal scavenger is one or more of the following: dithiocarbamate scavengers, trithiotriazine scavengers, xanthate scavengers, and polythiol scavengers; The organic sulfur heavy metal scavenger is added in two stages. The first stage accounts for 60% to 80% of the total scavenger dosage, and the second stage accounts for 20% to 40% of the total scavenger dosage. The interval between the two stages is 5 to 15 minutes.

10. The heavy metal wastewater treatment process according to claim 1, characterized in that: In step S6, coagulants and flocculants are added to the wastewater after the secondary deep capture treatment for secondary coagulation, flocculation and sedimentation treatment; The coagulant is one or more of PAC, polyferric sulfate, and polyferric chloride, with a dosage of 30–150 mg / L; the flocculant is PAM, with a dosage of 0.5–3 mg / L; the secondary sedimentation time is 1–2.5 h; After the secondary sedimentation effluent enters the intermediate water tank, it enters one or more of the following treatment units: biochemical treatment, filtration treatment, ion exchange treatment, or comprehensive reuse treatment.

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