Method for treating high-concentration sulfate acid mine drainage based on acid-resistant SRB biocathode

Abstract

The application discloses a high-concentration sulfate radical acid mine water treatment method based on acid-resistant SRB biological cathode, which comprises the following steps: after the acid mine water is filtered through a grid, the acid mine water is fed into an adjusting tank; the wastewater in the adjusting tank is fed into a biological membrane electrochemical device to perform an electrochemical reaction; in an anode chamber, Fe 2+ in the wastewater is oxidized into Fe 3+ and releases electrons; in a cathode chamber, the acid-resistant SRB bacterial membrane loaded on the surface of a carbon felt electrode utilizes the transmitted electrons to reduce SO4 2‑ into S 2‑ ; CO2 micro-bubbles are fed into the liquid in the cathode chamber to react with S 2‑ to generate sulfur precipitate, and the mixed solution after the reaction is fed into a cyclone separator to realize gas-solid separation; Fe 3+ in the water in the anode chamber generates Fe(OH)3 precipitate through a hydrolysis reaction, the Fe(OH)3 precipitate is mixed with sodium silicate, and is calcined to be converted into alpha-Fe2O3 with magnetism. The method can effectively remove SO4 2‑ and recover iron mud.

Description

Technical Field

[0001] This invention relates to the field of acid mine wastewater treatment technology, specifically to a method for treating high-concentration sulfate acid mine wastewater based on an acid-resistant SRB biocathode. Background Technology

[0002] Acid mine water (AMD) is naturally formed when sulfide minerals come into contact with oxygen, water, and microorganisms in the environment over a long period of time, resulting in oxidation and catalysis. Mining exposes more sulfide minerals, thus accelerating and causing more AMD to form.

[0003] According to the formation principle of AMD, its characteristics include: (1) low pH value, usually around 2.0~4.0; (2) high concentration of heavy metal ions such as Fe and Mn, which can reach tens to thousands of milligrams per liter; (3) SO4 2- The concentration is high, sometimes reaching several thousand or even tens of thousands of milligrams per liter.

[0004] Because AMD contains extremely high levels of heavy metals and sulfate ions, and is highly acidic, it can cause serious damage to the ecological environment, inhibiting the growth of aquatic organisms, disrupting soil structure, and ultimately endangering the lives and health of plants and animals. Therefore, effective remediation measures must be taken to efficiently control AMD.

[0005] For high concentrations of sulfate (SO4) 2- The following are common treatment methods for acidic mine water with a concentration >2000 mg / L:

[0006] 1. Chemical precipitation methods (such as lime precipitation) are limited by the solubility of calcium sulfate, making it difficult to reduce the sulfate concentration in the effluent to below 1000 mg / L. Furthermore, they generate large amounts of heavy metal-containing sludge, resulting in high disposal costs and the risk of equipment scaling. 2. Membrane separation technologies (reverse osmosis / nanofiltration) face serious membrane fouling problems, requiring complex pretreatment. Additionally, the disposal cost of high-salt concentrates is high, making them uneconomical. 3. Biological methods are limited by stringent environmental requirements, necessitating pre-adjustment of pH and continuous addition of carbon sources. The reactions are slow and easily inhibited by heavy metal toxicity. For example, Chinese patent CN106746228A discloses "A method for treating acidic mine wastewater." Although this method uses SRB (sulfuric acid sludge), it suffers from the following typical problems: ① Additional organic carbon sources must be added: it explicitly requires beer and sucrose as carbon sources (see claim 1 and paragraph

[0016] of the specification), which significantly increases operating costs. Furthermore, the precise addition and control of carbon sources increases operational complexity. ② The raw water needs to be significantly diluted: Because the SRB used is intolerant to high concentrations of pollutants, the influent must be diluted 3-7 times (see example) to reduce SO4 levels. 2-Reducing the concentration from tens of thousands of mg / L to 1000-4000 mg / L before further treatment (claim 2) significantly increases the volume and floor space required for the treatment facility, thus reducing treatment efficiency. ③ Limited removal efficiency: This method removes SO4... 2- The removal rate is only around 50% (see paragraph

[0017] and examples in the specification), which is difficult to meet strict emission or reuse standards. ④ Secondary pollution may occur: sludge containing metal sulfides is generated at the end of the process, and the disposal of sludge after microwave treatment has not been fully considered. If a combined process is used, although the effect can be improved, the system complexity and coordination difficulty will increase significantly, and the investment and operation and maintenance costs will increase by more than 50% compared with a single technology.

[0007] Furthermore, the research progress on treatment methods for high-sulfate mine wastewater (published in Applied Chemical Industry, August 2022) points out that although reverse osmosis and electrodialysis technologies can be used for alkaline high-sulfate mine wastewater, comprehensive consideration is still needed to achieve "low-cost, low-energy consumption and resource utilization of sulfate treatment".

[0008] Therefore, existing technologies generally suffer from common problems such as reliance on exogenous carbon sources, secondary pollution, system complexity, or high costs. Furthermore, most of these technologies focus only on pollutant removal, neglecting the resource recovery and utilization of valuable substances in AMD (such as sulfur and iron). Summary of the Invention

[0009] To address the issues of requiring additional carbon source addition and disposing of heavy metal precipitates in the biological treatment of high-concentration sulfate-containing acidic mine water, and the inefficient recovery of iron sludge, this invention provides a method for treating high-concentration sulfate-containing acidic mine water based on an acid-resistant SRB (Sulfate-Reducing Bacteria) biological cathode, comprising the following steps:

[0010] (1) Cultivate and acclimatize acid-tolerant SRB strains;

[0011] (2) Inoculate the acid-resistant SRB strain obtained in step (1) into the cathode chamber, introduce acidic water with pH 2.0~4.0, control the cathode potential -0.4V vs. Ag / AgCl, and culture continuously for 5~7 days until the thickness of the SRB biofilm on the carbon felt cathode surface reaches 0.5~1.0 mm.

[0012] (3) After the acidic mine water is screened to remove large particulate impurities, it enters the equalization tank to equalize the water quality and quantity.

[0013] (4) The wastewater in the equalization tank is introduced into the anode chamber of the biofilm electrochemical device at a flow rate of 0.3~0.5 m / s, and the electrochemical parameters are set to a current density of 15-18 A / m. 2A cathode potential of -0.4V is used to conduct electrochemical reactions in a biofilm electrochemical apparatus: In the anode chamber, Fe in the wastewater... 2+ It is oxidized to Fe on a titanium-based IrO2 anode plate. 3+ And release electrons; in the cathode chamber, the acid-resistant SRB bacterial film loaded on the surface of the carbon felt electrode uses the transferred electrons to ionize SO42-. 2- Restore to S 2- ;

[0014] (5) Introduce CO2 microbubbles and S into the liquid in the cathode chamber. 2- The reaction produces sulfur precipitate. The mixture after the reaction enters a cyclone separator to achieve gas-solid separation. The separated sulfur is recycled and reused. The tail gas is treated by alkaline absorption and then discharged in compliance with standards.

[0015] (6) The effluent from the anode chamber enters the sedimentation tank, where Fe... 3+ Fe(OH)3 precipitate (also known as iron mud) is generated through hydrolysis. After Fe(OH)3 precipitate is mixed with sodium silicate in a certain proportion, it is calcined to transform into magnetic α-Fe2O3, and then separated by a magnetic separator.

[0016] Preferably, step (1) involves collecting samples from a natural acidic environment, enriching and culturing them under conditions of pH 2.0 to 4.0 and a cathode potential of -0.4V vs. Ag / AgCl, and finally separating and purifying them to obtain an SRB strain with acid resistance and high electrochemical activity.

[0017] Preferably, in the biofilm electrochemical device, the substrate of the anode plate is a porous titanium plate, and the active coating is IrO2-Ta2O5-SnO2; the substrate of the cathode plate is polyacrylonitrile-based carbon felt, and the active coating is polyaniline-graphene-chitosan hydrogel.

[0018] Preferably, the effluent from the cyclone separator in step (5) enters a deep filtration unit; the deep filtration unit includes a security filter and an anion exchange membrane assembly connected in series. The effluent is first pre-filtered by the security filter, and then undergoes deep desalination and fine filtration through the anion exchange membrane. Finally, the SO4 in the product water of the deep filtration unit is removed. 2- The concentration dropped to below 100 mg / L.

[0019] Preferably, in step (6), the calcination is carried out at 450°C for 1 hour.

[0020] Preferably, the magnetic α-Fe2O3 obtained in step (6) is returned to the conditioning tank and used to adsorb heavy metal ions in water at a dosage ratio of 1~3 g / L.

[0021] Preferably, in step (6), the sludge remaining after separating the magnetic α-Fe2O3 by a magnetic separator is dewatered by a plate and frame filter press. The resulting sludge cake is transported off-site, and the resulting filtrate and rinse water are returned to the equalization tank.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1.SO4 2- The removal rate can be increased by 15-28% compared with traditional processes, with a removal rate of ≥95%;

[0024] 2. It utilizes microbial treatment technology, but the carbon source addition cost is zero, thus reducing operating costs;

[0025] 3. The amount of iron sludge generated by the treatment system is zero, achieving zero emissions and reducing secondary emissions and environmental pollution. Attached Figure Description

[0026] Figure 1 This is a flowchart of a method for treating high-concentration sulfate acidic mine water based on an acid-resistant SRB biocathode, according to the present invention. Detailed Implementation

[0027] The present invention will be further described below. It should be noted that the following embodiments are based on the present technical solution and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to these embodiments.

[0028] The main treatment units of the high-concentration sulfate acidic mine wastewater treatment process are as follows:

[0029] 1. Preprocessing unit

[0030] Influent conditioning: Acidic mine water (SO4) 2- After large particulate impurities are removed by the screen (15000-20000 mg / L, pH 2-4), the water enters the equalization tank to homogenize the water quality and quantity.

[0031] 2. Biomembrane electrochemical reaction unit

[0032] Anode chamber (Fe 2+ Oxidation): Wastewater flows through titanium-based IrO2 anode plates (10 mm spacing) at a velocity of 0.3~0.5 m / s; Fe 2+ Oxidized to Fe 3+ And release electrons: 2Fe 2+ → 2Fe 3+ + 2e - Electrons are transferred to the cathode via the external circuit.

[0033] Cathode chamber (SO4) 2-(Reduction): Carbon felt cathode loaded with acid-resistant SRB bacterial membrane, utilizing electron reduction of SO4. 2- SO4 2- + 8H + +8e - → S 2- + 4H2O.

[0034] By employing elemental doping in the anode coating and ternary composite hydrogel modification and pre-film deposition technology in the cathode, efficient and stable operation under extremely acidic environments was achieved. The materials of the anode and cathode plates are shown in Table 1.

[0035] Table 1. Materials for Anode and Cathode Plates

[0036]

[0037] The preparation process of the anodic coating is as follows:

[0038] 1) Pretreatment: The titanium substrate is subjected to sandblasting, alkaline washing, and acid washing activation.

[0039] 2) Coating: A solution of n-butanol containing chloroiridic acid, tantalum chloride and tin chloride is repeatedly brushed or sprayed onto the titanium substrate, wherein the molar ratio of Ir to Ta is 7:3, and the molar amount of Sn accounts for 5% of the total molar amount of the metal elements (Ir + Ta + Sn).

[0040] 3) Sintering: After each coating, sinter at 450-500℃ for 10-15 minutes, repeat 10-15 times until the target thickness is achieved.

[0041] The cathode plate coating preparation process is as follows:

[0042] 1) Raw material preparation:

[0043] Graphene oxide dispersion: concentration 5 mg / mL.

[0044] Polyaniline monomer: needs to be purified by vacuum distillation.

[0045] Chitosan: Deacetylation degree >90%, soluble in 1% acetic acid solution, concentration 2% (w / v).

[0046] Initiator: Ammonium persulfate solution, concentration 0.5 M.

[0047] Crosslinking agent: glutaraldehyde solution (25%).

[0048] Substrate: Polyacrylonitrile-based carbon felt (pre-cleaned ultrasonically with acetone and ethanol, and dried for later use).

[0049] 2) Preparation steps:

[0050] Preparation of the mixed slurry: The polyaniline monomer, graphene oxide dispersion, and chitosan acetate solution were mixed according to a mass ratio of polyaniline monomer: graphene oxide: chitosan = 3:1:1. The mixture was magnetically stirred in an ice-water bath for 2 hours to form a uniform, viscous black slurry.

[0051] Vacuum-assisted impregnation: The cleaned carbon felt is completely immersed in the above slurry. The entire system is placed in a vacuum dryer, evacuated to -0.1 MPa, and maintained for 15-20 minutes.

[0052] Objective: To use vacuum pressure to expel air from the pores of carbon felt fibers, ensuring that the slurry completely penetrates the interior of the carbon felt and achieves three-dimensional uniform loading.

[0053] 3) In-situ polymerization and crosslinking:

[0054] Remove the impregnated carbon felt and drain off excess slurry. Expose it to ammonium persulfate vapor and glutaraldehyde vapor (the carbon felt can be suspended above the container containing these two solutions in a sealed container). React at 60°C for 6-8 hours.

[0055] During the process:

[0056] In-situ polymerization: Ammonium persulfate initiates the polymerization of polyaniline monomers on the backbone of graphene oxide and chitosan to form a conductive polyaniline network.

[0057] Chemical cross-linking: Glutaraldehyde reacts with the amino groups on chitosan molecules to form a stable three-dimensional network structure.

[0058] Simultaneous reduction: The polymerization process of aniline itself releases reducing substances, which partially reduce graphene oxide and restore its conductivity.

[0059] 4) Post-processing:

[0060] Gently rinse the coated carbon felt with deionized water to remove unreacted monomers and reagents. Store in a humid environment at 4°C until used for the next step of strain fixation.

[0061] The steps for loading acid-resistant SRB strains onto the cathode plate are as follows:

[0062] 1) Preparation of bacterial culture:

[0063] The bacterial strain was cultured in a specific culture medium until the late logarithmic growth phase.

[0064] 2) Immobilization steps:

[0065] Bacterial solution impregnation: Immerse the hydrogel-coated carbon felt in a high-concentration bacterial solution. Perform a short-term vacuum assisted process (-0.05 MPa, 5 minutes) to ensure that the bacterial solution penetrates into the hydrogel.

[0066] Adsorption and activation: In an anaerobic environment at room temperature for 12-24 hours, SRB cells are allowed to spontaneously adsorb into the hydrophilic network structure of the hydrogel.

[0067] Nutrient activation: The medium was replaced with an acidic diluted medium (pH=4.0) containing micronutrients (such as yeast extract, 0.01%) and activated by microcurrent for 48 h at a cathode potential of -0.4 V (vs. Ag / AgCl).

[0068] Objective: To allow SRB bacteria to initially adapt and begin metabolism under the actual working potential, forming an active biofilm.

[0069] 3. Iron sludge recycling unit

[0070] Precipitation collection: Fe in the effluent from the anode chamber 3+ Hydrolysis produces Fe(OH)3:

[0071] Fe 3+ +3H₂O→Fe(OH)₃↓+3H +

[0072] Low-temperature regeneration: Fe(OH)3 sludge is mixed with sodium silicate (Si:Fe=1:5) and calcined at 450℃ for 1 h to convert it into magnetic α-Fe2O3. The α-Fe2O3 after low-temperature regeneration (which has strong magnetic properties and high adsorption activity) will be returned to the pretreatment unit of the system for reuse in heavy metal adsorption.

[0073] The specific process flow for treating acidic mine water with high concentrations of sulfate is as follows:

[0074] Acidic mine wastewater containing high concentrations of sulfate first flows into a collection well, where suspended or floating solids are intercepted by a basket screen. It is then pumped to a regulating tank, where a mixer homogenizes the water quality and quantity. The regulated wastewater enters a biofilm electrochemical reactor, which operates continuously and consists of two treatment units: an anode chamber and a cathode chamber. In the anode chamber, Fe... 2+ It is oxidized to Fe on a titanium-based IrO2 anode plate. 3+ And release electrons; in the cathode chamber, the acid-resistant SRB bacterial film loaded on the surface of the carbon felt electrode uses the transferred electrons to ionize SO42-. 2- Restore to S 2- Simultaneously, the introduced CO2 microbubbles react with S²⁻ to generate high-purity sulfur precipitate. The mixture containing sulfur particles generated in the cathode chamber first enters a cyclone separator, where most of the sulfur is separated and recovered under the action of CO2. Subsequently, the effluent from the cyclone separator (with most of the sulfur removed but containing trace amounts of fine powder) enters a deep filtration unit. This unit first undergoes pre-filtration through a security filter, and then deep desalination and fine filtration through an anion exchange membrane to completely remove residual SO4. 2-and suspended solids, ensuring SO4 levels in the effluent. 2- Concentration <100mg / L. Finally, the water produced by the deep filtration unit enters the clear water tank, where the pH is finely adjusted to 6-8 before being discharged or reused.

[0075] The specific reactions within the cathode chamber are as follows:

[0076] CO2 dissolves in water to form carbonic acid, which lowers the pH: CO2 + H2O H2CO3 H + + HCO3 -

[0077] Under acidic conditions and the action of CO2, S generated at the cathode 2- It is converted into H2S; subsequently, the H2S is reacted with trace amounts of dissolved oxygen or Fe in the system. 3+ Oxidation produces elemental sulfur (S) 0 ).

[0078] H2S is oxidized by oxygen: 2H2S + O2 → 2S 0 + 2H2O (This reaction is the basis of the Claus process for desulfurization in industry)

[0079] H2S was affected by Fe 3+ Oxidation: H₂S + 2Fe 3+ → S 0 + 2Fe 2+ + 2H +

[0080] The specific composition of a depth filtering unit:

[0081] In practical engineering, this unit is usually designed as a multi-stage filter to ensure effectiveness and operational stability.

[0082] First stage: Mechanical security filtration

[0083] Type: Bag filter or spiral wound filter element. Accuracy: 10-25 microns.

[0084] Function: To trap trace amounts of slightly larger particles remaining in the effluent from the cyclone separator, serving as a pre-protection for downstream precision filtration.

[0085] Second stage: Precision / membrane filtration

[0086] Form: Anion exchange membrane module, ultrafiltration (UF) membrane or precision filter cartridge (e.g., 1-5 microns).

[0087] Function: Anion exchange membrane: selectively removes residual SO4. 2-Ionization; Ultrafiltration / Precision Filtration: Further removes all suspended sulfur particles and colloidal matter, ensuring that the suspended solids (SS) in the effluent are close to zero.

[0088] Fe in the effluent from the anode chamber 3+ Fe(OH)3 precipitate is generated through hydrolysis. The precipitated sludge is then mixed with sodium silicate in a certain proportion and calcined at 450℃ for 1 hour to transform it into magnetic α-Fe2O3. The regenerated product can be reused in the pretreatment unit to adsorb heavy metals.

[0089] The iron sludge system includes units such as a magnetic separator, an iron sludge collection tank, and a plate and frame filter press. The magnetic separator is responsible for separating the Fe2O3 complex containing heavy metals; the remaining sludge is temporarily stored in the sludge tank and then dewatered by the plate and frame filter press; the resulting filtrate and wash water are returned to the equalization tank for reprocessing, realizing a closed-loop operation of the entire process.

[0090] Example: 200m 3 / d-scale acidic mine water treatment system application

[0091] The water quality conditions of the treated acidic mine water are shown in Table 2.

[0092] Table 2 Water Quality Conditions

[0093]

[0094] The equipment configuration is as follows:

[0095] Biomembrane electrochemical reactor module:

[0096] Quantity: 5 sets in parallel (each set has a processing capacity of 40 m³ / day)

[0097] Single unit dimensions: 2m (L) × 1m (W) × 2.5m (H)

[0098] Electrode plate arrangement: 12 pairs of alternating anodes and cathodes (10mm spacing).

[0099] Power system operating parameters: Voltage 48V, Current 360A, Power 17.28kW

[0100] The startup and running steps are as follows:

[0101] Step 1: Microbial film culture

[0102] ① Inoculate the acid-resistant SRB strain into the cathode chamber at an inoculation rate of 15 g / L (wet weight).

[0103] ② Introduce simulated wastewater (SO4) 2- =10000 mg / L, pH=3.0), controlling the cathode potential at -0.4 V;

[0104] ③ After culturing for 7 consecutive days, the frozen section samples were observed using a laser scanning confocal microscope (LSCM) until the thickness of the SRB biofilm was confirmed to be 0.5~1.0 mm.

[0105] Step 2: System Integration and Operation

[0106] ① Water inlet adjustment:

[0107] The water enters the anode chamber at a flow rate of 0.3 m / s through a perforated water distribution pipe (10 mm diameter).

[0108] ② Electrochemical parameter settings:

[0109] Current density 15-18 A / m², cathode potential -0.4 V

[0110] ③ Iron sludge regeneration:

[0111] Collect Fe(OH)3 sludge from the anode chamber every 8 hours;

[0112] Sodium silicate was added at a Si:Fe ratio of 1:5, and calcined at 450℃ for 1 h to obtain α-Fe₂O₃.

[0113] Table 3 Treatment effect of acidic mine water

[0114]

[0115] Table 3 shows the treatment effect of acidic mine water, indicating that this solution effectively treats SO4. 2- The removal rate is greater than 99.2%, for Fe 2+ The recovery rate is greater than 99.9%, achieving the goal of "zero carbon source and zero iron sludge".

[0116] For those skilled in the art, various corresponding changes and modifications can be made based on the above technical solutions and concepts, and all such changes and modifications should be included within the protection scope of the claims of this invention.

Claims

1. A method for treating high-concentration sulfate acidic mine water based on an acid-resistant SRB biocathode, characterized in that, Includes the following steps: (1) Cultivate and acclimatize acid-tolerant SRB strains; (2) Inoculate the acid-resistant SRB strain obtained in step (1) into the cathode chamber, introduce acidic water with pH 2.0~4.0, control the cathode potential -0.4V vs. Ag / AgCl, and culture continuously for 5~7 days until the thickness of the SRB biofilm on the carbon felt cathode surface reaches 0.5~1.0 mm. (3) After the acidic mine water is screened to remove large particulate impurities, it enters the equalization tank to equalize the water quality and quantity. (4) The wastewater in the equalization tank is introduced into the anode chamber of the biofilm electrochemical device at a flow rate of 0.3~0.5 m / s, and the electrochemical parameters are set to a current density of 15-18 A / m. 2 A cathode potential of -0.4V is used to conduct electrochemical reactions in a biofilm electrochemical apparatus: In the anode chamber, Fe in the wastewater... 2+ It is oxidized to Fe on a titanium-based IrO2 anode plate. 3+ And release electrons; in the cathode chamber, the acid-resistant SRB bacterial film loaded on the surface of the carbon felt electrode uses the transferred electrons to ionize SO42-. 2 Restore to S 2- ; (5) CO2 microbubbles are introduced into the liquid in the cathode chamber. CO2 dissolves in water to form carbonic acid, which lowers the pH. Under acidic conditions and the action of CO2, the sulfur produced at the cathode increases. 2- It is converted into H2S; subsequently, the H2S is reacted with trace amounts of dissolved oxygen or Fe in the system. 3+ Oxidation produces sulfur precipitate. The mixture after the reaction enters a cyclone separator to achieve gas-solid separation. The separated sulfur is recycled and reused. The tail gas is treated by alkaline absorption and then discharged in compliance with standards. (6) The effluent from the anode chamber enters the sedimentation tank, where Fe... 3+ Fe(OH)3 precipitate is generated through hydrolysis. After the Fe(OH)3 precipitate is mixed with sodium silicate in a certain proportion, it is calcined to transform into magnetic α-Fe2O3, and then separated by a magnetic separator. In the biomembrane electrochemical device, the substrate of the anode plate is a porous titanium plate, and the active coating is IrO2-Ta2O5-SnO2; the substrate of the cathode plate is polyacrylonitrile-based carbon felt, and the active coating is polyaniline-graphene-chitosan hydrogel.

2. The method according to claim 1, characterized in that, Step (1) involves collecting samples from a natural acidic environment and enriching them under conditions of pH 2.0 to 4.0 and a cathode potential of -0.4V vs. Ag / AgCl. Finally, the SRB strain with acid resistance and high electrochemical activity is isolated and purified.

3. The method according to claim 1, characterized in that, In step (5), the effluent from the cyclone separator enters the deep filtration unit; the deep filtration unit includes a security filter and an anion exchange membrane assembly connected in series. The effluent is first pre-filtered by the security filter, and then undergoes deep desalination and fine filtration through the anion exchange membrane. Finally, the SO4 in the product water of the deep filtration unit is removed. 2 The concentration dropped to below 100 mg / L.

4. The method according to claim 1, characterized in that, In step (6), the sample is calcined at 450°C for 1 hour.

5. The method according to claim 1, characterized in that, The magnetic α-Fe2O3 obtained in step (6) is returned to the conditioning tank and used to adsorb heavy metal ions in water at a dosage ratio of 1~3 g / L.

6. The method according to claim 1, characterized in that, In step (6), after separating the magnetic α-Fe2O3 by a magnetic separator, the remaining sludge is dewatered by a plate and frame filter press. The resulting sludge cake is transported off-site, and the resulting filtrate and rinse water are returned to the equalization tank.

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