A combined treatment process for treating refractory acid mine drainage

Abstract

The application discloses a combined treatment process for treating refractory acid mine wastewater, which comprises the following steps: feeding the acid mine wastewater into a pre-sedimentation tank, and introducing hydrogen sulfide gas into the pre-sedimentation tank for pre-sedimentation; feeding supernatant in the pre-sedimentation tank into a biological mineralization tank for biological mineralization treatment; feeding effluent from the biological mineralization tank into a gradient pH control device for treatment, wherein the gradient pH control device comprises a pH primary control device, a primary flocculation and sedimentation device, a pH secondary control device, a secondary flocculation and sedimentation device, and a pH tertiary control device; feeding effluent from the gradient pH tertiary control device into a deep desulfurization tank for deep desulfurization, and recycling hydrogen sulfide gas generated in the deep desulfurization tank into the pre-sedimentation tank. The application overcomes the shortcomings of the prior art, such as difficulty in reaching the standard of heavy metals in effluent, large amount of reagent, large amount of sludge, and high cost of desulfurization, and can recycle by-products such as mineral and sulfidized heavy metal, so that the final effluent can reach the standard stably.

Description

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a combined treatment process for treating difficult-to-treat acidic mine wastewater, suitable for applications with pH values ​​between 2.1 and 3.5 and containing Mn, Cu, Fe, Zn, Al, and SO4. 2- This treatment method is particularly suitable for acidic mine wastewater with excessive levels of certain substances, especially those with pH values ​​between 2.1 and 3.5, Mn content between 250 and 350 mg / L, Fe content between 300 and 400 mg / L, Cu content between 12 and 30 mg / L, Zn content between 90 and 110 mg / L, Al content between 200 and 400 mg / L, and SO4 content between 0.5 and 0.5%. 2- Treatment of acidic mine wastewater with a concentration of 5000~8000 mg / L. Background Technology

[0002] With the continuous development of mineral resources, acidic mine wastewater containing various heavy metals is generated during mining, beneficiation, and tailings disposal. Most of this acidic mine wastewater (AMD) is discharged directly without treatment. AMD has high concentrations of heavy metals and a low pH value, causing serious negative impacts on mining production and the ecological environment, attracting widespread attention from scholars both domestically and internationally. How to effectively treat AMD, protect the ecological environment, and ensure the sustainable development of resources is a crucial issue currently facing us. Physicochemical methods, microbiological methods, and constructed wetland methods are commonly used methods for treating AMD. Among chemical methods, the most commonly used is neutralization precipitation. Neutralization involves adding a neutralizing agent to the AMD to adjust the pH, causing metal ions in the wastewater to precipitate as hydroxides and separate from the water. Common neutralizing agents include lime, limestone, soda ash, and caustic soda. The neutralization precipitation method using lime and limestone is the most widely used in AMD treatment.

[0003] Because these acidic waters are generally rich in substances such as copper, iron, and sulfate, the commonly used treatment method is the stepwise precipitation method. This involves first performing iron removal pretreatment, then recovering valuable copper through sulfidation, and finally using the lime neutralization method (HDS method) to remove other heavy metals from the acidic water. However, this method generates a large amount of neutralization slag. Moreover, because iron ions precipitate together with calcium sulfate in the neutralization stage, the neutralization slag contains not only calcium sulfate but also a large amount of ferric hydroxide and other precipitates. Consequently, the grade of calcium sulfate cannot meet the standards for cement industrial by-product gypsum, making it unsuitable for comprehensive utilization. It can only be stockpiled and disposed of as general industrial solid waste, wasting resources and occupying a large amount of land.

[0004] Furthermore, removing manganese from acidic mine wastewater presents significant technical challenges. Industrially, acidic mine wastewater typically undergoes initial sulfide precipitation to recover or remove copper ions, followed by the addition of lime slurry and aeration to adjust the pH to 7-8 for neutralization. After neutralization, the Cu in the wastewater... 2+ Pb 2+ Zn2+ The concentration of metal ions can generally meet emission standards, while Mn 2+ The concentration is usually higher than 15 mg / L, failing to meet the emission standards (Mn). 2+ The emission standard is <2mg / L).

[0005] To address the technical challenges of manganese removal from acidic mine wastewater and reduce treatment costs, Chinese patent application 202510677508.3 provides a method for manganese removal and recovery from acidic mine wastewater. The method involves treating the acidic mine wastewater to remove iron, obtaining an iron-removing supernatant; adding a manganese-removing agent to the iron-removing supernatant to obtain manganese precipitate and a manganese-removing supernatant; mixing the manganese precipitate with sulfuric acid to obtain a leaching solution and a leaching residue; and electrolyzing the leaching solution to obtain metallic manganese. The manganese-removing agent is sodium silicate and / or sodium metasilicate. However, experimental studies have shown that this method is difficult to apply to environments with pH values ​​between 2.1 and 3.5, Mn concentrations ≥200 mg / L, and the presence of Cu, Fe, Zn, Al, and SO42-. 2- Treatment of acidic mine wastewater with excessive levels of certain substances.

[0006] Therefore, there is an urgent need to develop a low-cost, high-efficiency, and stable effluent treatment method capable of efficiently treating Mn, Cu, Fe, Zn, Al, and SO4. 2- Methods for treating acidic mine wastewater with severely excessive levels of harmful substances. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies, such as the difficulty in treating acidic mine wastewater with high heavy metal content, especially high manganese and sulfate content, as well as the difficulty in achieving heavy metal standards in the treated effluent, large amounts of reagents used, high treatment costs, and large amounts of sludge production. This invention provides a combined treatment process for treating difficult-to-treat acidic mine wastewater. This combined treatment process can not only ensure stable compliance with standards for the effluent from the mine acidic wastewater treatment facility in a highly efficient and energy-saving manner, but also recover by-products such as styrene minerals and heavy metal sulfides.

[0008] To achieve the above-mentioned objectives of this invention, a combined treatment process for treating difficult-to-treat acidic mine wastewater is provided, which is implemented by the following steps:

[0009] S1, Pre-precipitation

[0010] pH range of 2.1-3.5, Mn concentration ≥200 mg / L, Fe concentration ≥300 mg / L, SO4 2- Acidic mining wastewater with a concentration ≥5000 mg / L is transported to a pre-sedimentation tank where hydrogen sulfide gas is introduced for pre-sedimentation. The wastewater retention time in the pre-sedimentation tank is 20-40 minutes, and the amount of hydrogen sulfide gas introduced is determined based on the pH of the acidic mining wastewater and the concentrations of Mn, Cu, Fe, Zn, Al, and SO42-. 2-The concentration is determined by calculation. Pre-precipitation can remove some heavy metal ions from wastewater, such as copper and mercury. Heavy metal sulfides formed at the bottom of the pre-precipitation tank can be sent to flotation for heavy metal recovery.

[0011] S2, biomineralization

[0012] The supernatant from the pre-sedimentation tank is transferred to a biomineralization tank for biomineralization treatment. The biomineralization tank contains uniformly suspended carbon fiber packing material with good electrical conductivity, and an aeration disc is installed at the bottom. The residence time of the supernatant in the biomineralization tank is 30-50 minutes, and the dissolved oxygen level is controlled at 1.5-4 mg / L. The carbon fiber packing material is pre-treated with *Acidithiobacillus ferrooxidans* (At.f.). During this process, under the influence of oxygen and *At.f.* bacteria, the ferrous iron in the wastewater is oxidized to ferric iron or Scheringer's mineral (an iron hydroxyl sulfate mineral) or other iron oxides, which then precipitate. The Scheringer's mineral or other iron oxide precipitates can be recycled as adsorbents or pigments.

[0013] S3, gradient pH control

[0014] (1) pH Level 1 control: The effluent from the biomineralization tank is fed into the pH Level 1 control tank. Alkaline agents are added to raise the pH of the effluent to between 5 and 6. The effluent from the biomineralization tank stays in the pH Level 1 control tank for 40 to 60 minutes.

[0015] (2) Primary flocculation sedimentation: The mixed wastewater, after being regulated by the pH primary control tank, enters the primary flocculation sedimentation tank. A pipeline mixer is installed at the inlet end of the primary flocculation sedimentation tank. Flocculant polyacrylamide is added to the pipeline mixer. After sedimentation in the sedimentation zone of the flocculation sedimentation tank, the mixed wastewater is divided into an upper clear water layer, a middle sludge layer and a bottom sludge layer. A sludge outlet is provided at the bottom of the sedimentation zone of the flocculation sedimentation tank. 28% to 36% of the sludge in the bottom sludge layer is returned to the primary pH control tank, and the remaining sludge is discharged to the sludge tank for subsequent pressure filtration treatment.

[0016] (3) pH secondary control: The supernatant after primary flocculation and sedimentation enters the pH secondary control tank. Alkaline agents are added to raise the pH of the effluent to above 10. The supernatant stays in the pH secondary control tank for 20-40 minutes.

[0017] (4) Secondary flocculation sedimentation: The mixed wastewater, after being regulated by the pH secondary control tank, enters the secondary flocculation sedimentation tank. A pipeline mixer is installed at the inlet end of the secondary flocculation sedimentation tank. Flocculant polyacrylamide is added to the pipeline mixer. After sedimentation in the sedimentation zone of the secondary flocculation sedimentation tank, the mixed wastewater is divided into an upper clear water layer, a middle sludge layer and a bottom sludge layer. A sludge outlet is provided at the bottom of the sedimentation zone of the secondary flocculation sedimentation tank. 28% to 36% of the sludge in the bottom sludge layer is returned to the pH secondary control tank, and the remaining sludge is discharged into the sludge tank for subsequent pressure filtration treatment.

[0018] (5) Three-stage pH adjustment: The supernatant from the secondary flocculation sedimentation tank is fed into the three-stage pH adjustment tank, and oxalic acid is added to adjust the pH value back to between 7 and 8. Most of the heavy metals and some sulfate ions can be removed from the wastewater in this process. Through the gradient pH adjustment process, the addition of alkaline agents can be effectively reduced and the amount of sludge generated can be reduced.

[0019] S4, Deep Desulfurization

[0020] The effluent with a three-stage pH gradient control is fed into a deep desulfurization tank for advanced desulfurization. The retention time of the effluent in the deep desulfurization tank is no less than 20 hours, resulting in a pH of 7-9 and the following concentrations of pollutants: Mn 0.05-1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03-0.5 mg / L, Zn < 0.006 mg / L, Al 0.3-1.5 mg / L, and SO42-. 2- The effluent concentration is ≤1300 mg / L. The advanced desulfurization tank is equipped with uniformly suspended carbon fiber packing material with good electrical conductivity. An exhaust pipe is installed at the top of the advanced desulfurization tank, through which hydrogen sulfide gas generated in the advanced desulfurization tank is circulated back to the pre-precipitation tank described in step S1. The carbon fiber packing material is pre-treated with sulfate-reducing bacteria for microbial biofilm formation. Under anaerobic conditions, the sulfate-reducing bacteria reduce sulfate to divalent sulfur ions through dissimilatory reactions, thereby generating sulfides or hydrogen sulfide. This step can effectively remove sulfate ions and residual heavy metals from the wastewater.

[0021] Furthermore, in step S2, the top of the packing material in the reaction chamber of the biomineralization tank is connected to the anode of the micro-power source by an electrical wire, and there is a connection hole on the side wall of the biomineralization tank reaction chamber; in step S4, the side wall of the deep desulfurization tank reaction chamber is also provided with a connection hole, and the connection hole of the biomineralization tank reaction chamber is connected to the connection hole of the deep desulfurization tank reaction chamber, and a proton exchange membrane is installed between the two connection holes; the micro-power source provides a micro DC current of 5~30mA.

[0022] Furthermore, in step S4, the top of the packing material in the reaction chamber of the deep desulfurization tank is connected to the cathode of a micro power source by an electrical wire, and the micro power source provides a small DC current of 5~30mA.

[0023] Furthermore, in step S3, the alkaline agent is any one or any combination of two or more of quicklime, hydrated lime, and sodium hydroxide.

[0024] Preferably, the acidic mine wastewater fed in step S1 has a pH of 2.1–3.5, a Mn content of 250–350 mg / L, a Cu content of 12–30 mg / L, a Fe content of 300–400 mg / L, a Zn content of 90–110 mg / L, an Al content of 200–400 mg / L, and an SO4 content of [missing information]. 2- The concentration was between 5000 and 8000 mg / L; by adjusting the technical parameters of each step, step S4 achieved a pH of 7 to 9, with the following concentrations of pollutants: Mn 0.08–1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03–0.5 mg / L, Zn < 0.006 mg / L, Al 0.3–1.5 mg / L, and SO42-. 2- 800~1300 mg / L of effluent.

[0025] Compared with existing technologies, this invention provides a combined treatment process for treating difficult-to-treat acidic mine wastewater. Through a physicochemical-biological-micro-electrostatic coupling process, it addresses the shortcomings of acidic mine wastewater effluent failing to meet heavy metal standards and incurring high desulfurization costs. Simultaneously, it can recover byproducts such as styrene minerals and heavy metal sulfides, ensuring efficient and energy-saving maintenance of stable effluent compliance from mine acidic wastewater treatment facilities. Specifically, it offers the following beneficial effects:

[0026] (1) The present invention adopts a physical-chemical-biological micro-electric coupling process, which can effectively increase the pH value of wastewater and remove heavy metals and sulfate from the water. On the other hand, the styrene minerals and heavy metal sulfides generated in the process of treating wastewater can be used as recyclable resources, thereby improving resource utilization and reducing desulfurization costs.

[0027] (2) The present invention adopts a micro-electrochemical catalytic process, which can provide oxygen in the biomineralization stage, reduce the aeration volume, and reduce the energy consumption of the blower. On the other hand, it can catalyze the reduction process of sulfate-reducing bacteria in the deep desulfurization stage, reduce the total amount of organic matter required for the sulfate reduction process, and the power consumption required by the micro-current power supply is quite small and can be ignored. Therefore, the water treatment facility using the process of the present invention can reduce the treatment cost and the amount of reagent used is small, which is especially suitable for the treatment of acidic mine wastewater with high sulfate concentration.

[0028] (3) The present invention adopts a combination of gradient pH control and sludge return process, which can effectively increase pH and improve the utilization rate of alkaline agents. Compared with the traditional neutralization method, it can reduce the consumption of alkaline agents, sludge volume and sludge moisture content.

[0029] (4) Experimental studies have shown that when the pH of the fed acidic mine wastewater is 2.1~3.5, the Mn content is 250~350 mg / L, the Cu content is 12~30 mg / L, the Fe content is 300~400 mg / L, the Zn content is 90~110 mg / L, the Al content is 200~400 mg / L, and the SO4 content is high, the water quality is suitable for the following conditions: 2- With a concentration of 5000-8000 mg / L, the final results showed a pH of 7-9 and concentrations of each pollutant as follows: Mn 0.08-1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03-0.5 mg / L, Zn < 0.006 mg / L, Al 0.3-1.5 mg / L, and SO42-0.5 mg / L. 2- 800~1300 mg / L of effluent. Attached Figure Description

[0030] Figure 1 This is a principle flow diagram of a combined treatment process for treating difficult-to-treat acidic mine wastewater according to the present invention.

[0031] Figure 2 for Figure 1 The principle process flow diagram of the gradient pH control process unit. Detailed Implementation

[0032] To further understand the content of this invention, a detailed description of the invention is provided in conjunction with the embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0033] Example

[0034] The acidic mine wastewater from a certain mine was treated using the method of this invention. The specific treatment process is as follows, and the influent and effluent water quality are shown in Table 1.

[0035] S1, Pre-precipitation

[0036] Acidic mine wastewater is transported to a pre-sedimentation tank for pre-sedimentation, and hydrogen sulfide gas is introduced into the pre-sedimentation tank. The wastewater retention time in the pre-sedimentation tank is 30 minutes.

[0037] S2, biomineralization

[0038] The supernatant from the pre-sedimentation tank is transferred to a biomineralization tank for biomineralization treatment. The biomineralization tank contains uniformly suspended carbon fiber packing material with good electrical conductivity, and an aeration disc is installed at the bottom. The wastewater retention time in the biomineralization tank is approximately 30 minutes, and the dissolved oxygen is controlled at 1.5~4 mg / L. The carbon fiber packing material is pre-treated with *Thiobacillus ferrooxidans* (At.f.) to form a microbial biofilm. The top of the packing material is connected to the anode of a micro-power source via an electrical conductor.

[0039] S3, gradient pH control

[0040] (1) pH primary control: The effluent from the biomineralization tank is fed into the pH primary control tank. A certain amount of 10% quicklime slurry is added to raise the pH of the effluent to between 5 and 6. The retention time in the pH primary control tank is 60 min.

[0041] (2) Primary flocculation and sedimentation: The mixed wastewater, after pH adjustment in the primary pH control tank, enters the primary flocculation and sedimentation tank. A pipeline mixer is installed at the inlet of the primary flocculation and sedimentation tank, and polyacrylamide flocculant is added to the pipeline mixer at a dosage of 1 mg / L. After sedimentation in the sedimentation zone of the flocculation and sedimentation tank, the fed mixed wastewater is divided into an upper clear water layer, a middle sludge layer, and a bottom sediment layer; 1 / 3 of the sludge from the primary flocculation and sedimentation tank is returned to the pH control tank. The sedimentation time is 2 hours.

[0042] (3) pH secondary control: The supernatant after primary flocculation and sedimentation enters the pH secondary control tank. A certain amount of 10% quicklime milk is added to raise the pH of the effluent to above 10. The residence time in the pH secondary control tank is 20 min.

[0043] (4) Secondary flocculation and sedimentation: The mixed wastewater, after being adjusted by the pH secondary control tank, enters the secondary flocculation and sedimentation tank. A pipeline mixer is installed at the inlet of the secondary flocculation and sedimentation tank, and polyacrylamide flocculant is added to the pipeline mixer at a dosage of 1 mg / L. After sedimentation in the sedimentation zone of the secondary flocculation and sedimentation tank, the fed mixed wastewater is divided into an upper clear water layer, a middle sludge layer, and a bottom sediment layer; 1 / 3 of the sludge from the secondary flocculation and sedimentation tank is returned to the pH control tank. The sedimentation time is 2 hours.

[0044] (5) pH three-level adjustment: feed the supernatant of the secondary flocculation sedimentation tank into the pH three-level adjustment tank and add 5% oxalic acid solution to adjust the pH value back to between 7 and 8.

[0045] S4, Deep Desulfurization

[0046] The effluent from the gradient pH control process enters a deep desulfurization tank for further desulfurization. The wastewater retention time is no less than 20 hours. The deep desulfurization tank contains uniformly suspended carbon fiber packing material with good conductivity. An exhaust pipe is installed at the top of the deep desulfurization tank, through which hydrogen sulfide gas generated in the deep desulfurization tank is directed to a pre-precipitation tank. The carbon fiber packing material is pre-treated with sulfate-reducing bacteria to form a microbial biofilm. The top of the packing material is connected to the cathode of a micro-power source via an electrical conductor. A connection hole is located between the sidewall of the reaction chamber of the biomineralization tank and the deep desulfurization tank, and a proton exchange membrane is installed within the connection hole. The micro-power source provides a 20mA micro-DC current.

[0047] Comparative Example

[0048] To better compare treatment effects, the traditional lime neutralization method was used as a control to treat the acidic wastewater from the mine. The treatment steps for the control were as follows:

[0049] Wastewater is pumped by a peristaltic pump to a neutralization reaction bottle for neutralization. The stirring rate is set to 400 r / min, and the dosing rate of the alkaline neutralizing agent is adjusted to stabilize the pH of the effluent between 9.8 and 10.2. The alkaline agent is 10% quicklime slurry, and the neutralization residence time is 0.5 h. The effluent then enters a flocculation bottle for flocculation. The flocculant dosage is 2 mg / L, and the stirring rate is set to 100 r / min. The effluent then enters an inclined plate sedimentation tank for sedimentation, with a residence time of 2 h. The effluent then enters the neutralization bottle for neutralization. A 5% oxalic acid solution is used for acid correction. The dosing rate of the dilute acid peristaltic pump is set to maintain the pH of the effluent from the acid correction bottle between 6 and 9. The effluent then enters an effluent tank.

[0050] Table 1. Influent and Effluent Water Quality of Examples and Comparative Examples

[0051]

[0052] As shown in Table 1, the acidic mine wastewater treated in the examples had a pH of only 2.1-3.3, while the Mn content was as high as 250-350 mg / L, and SO4 content was also high. 2- The concentration of pollutants is as high as 5000-8000 mg / L, with high levels of other elements such as Cu, Fe, Zn, and Al, making it an extremely difficult-to-treat acidic mine wastewater. In the examples, the method of this invention was used to treat a certain acidic mine wastewater, ultimately yielding a solution with a pH of 7-9 and the following concentrations of pollutants: Mn 0.08-1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03-0.5 mg / L, Zn < 0.006 mg / L, Al 0.3-1.5 mg / L, and SO42-120 mg / L. 2-The effluent concentration of 800-1300 mg / L exceeded the requirements of the emission standard for pollutants from iron ore mining and beneficiation industry, GB 28661-2012, achieving unexpected technical results. In contrast, the traditional lime neutralization method resulted in effluent with Mn content frequently exceeding the standard, and the sulfur reduction effect was not significant.

Claims

1. A combined treatment process for treating difficult-to-treat acidic mine wastewater, characterized in that... The following steps are to be taken: S1, Pre-precipitation pH range of 2.1-3.5, Mn concentration ≥200 mg / L, Fe concentration ≥300 mg / L, SO4 2- Acidic mining wastewater with a concentration ≥5000 mg / L is transported to a pre-sedimentation tank where hydrogen sulfide gas is introduced for pre-sedimentation. The wastewater retention time in the pre-sedimentation tank is 20-40 minutes, and the amount of hydrogen sulfide gas introduced is determined based on the pH of the acidic mining wastewater and the concentrations of Mn, Cu, Fe, Zn, Al, and SO42-. 2- The content level is determined by calculation. S2, biomineralization The supernatant in the pre-sedimentation tank is transferred to a biomineralization tank for biomineralization treatment. The biomineralization tank contains uniformly suspended carbon fiber packing material with good electrical conductivity, and an aeration disc is installed at the bottom. The residence time of the supernatant in the biomineralization tank is 30-50 minutes, and the dissolved oxygen level is controlled at 1.5-4 mg / L. The carbon fiber packing material is pre-treated with *Thiobacillus ferrooxidans* for microbial biofilm formation. S3, gradient pH control (1) pH primary control: The effluent from the biomineralization tank is fed into the pH primary control tank, and the pH of the effluent is raised to between 5 and 6 by adding alkaline agents. The residence time of the effluent from the biomineralization tank in the pH primary control tank is 40 to 60 minutes. (2) Primary flocculation sedimentation: The mixed wastewater that has been adjusted by the pH primary control tank enters the primary flocculation sedimentation tank. A pipeline mixer is installed at the inlet end of the primary flocculation sedimentation tank. Flocculant polyacrylamide is added to the pipeline mixer. After the mixed wastewater is settled in the sedimentation zone of the flocculation sedimentation tank, it is divided into an upper clear water layer, a middle sludge layer and a bottom sludge layer. A sludge outlet is provided at the bottom of the sedimentation zone of the flocculation sedimentation tank. 28% to 36% of the sludge in the bottom sludge layer is returned to the primary pH control tank. The remaining sludge is discharged to the sludge tank for subsequent pressure filtration treatment. (3) pH secondary control: The supernatant after primary flocculation and sedimentation enters the pH secondary control tank. Alkaline agents are added to raise the pH of the effluent to above 10. The supernatant stays in the pH secondary control tank for 20-40 minutes. (4) Secondary flocculation sedimentation: The mixed wastewater, after being regulated by the pH secondary control tank, enters the secondary flocculation sedimentation tank. A pipeline mixer is installed at the inlet end of the secondary flocculation sedimentation tank. Flocculant polyacrylamide is added to the pipeline mixer. After sedimentation in the sedimentation zone of the secondary flocculation sedimentation tank, the mixed wastewater is divided into an upper clear water layer, a middle sludge layer and a bottom sludge layer. A sludge outlet is provided at the bottom of the sedimentation zone of the secondary flocculation sedimentation tank. 28% to 36% of the sludge in the bottom sludge layer is returned to the pH secondary control tank, and the remaining sludge is discharged into the sludge tank for subsequent pressure filtration treatment. (5) Three-stage pH adjustment: The supernatant from the secondary flocculation sedimentation tank is fed into the three-stage pH adjustment tank and oxalic acid is added to adjust the pH value back to between 7 and 8; S4, Deep Desulfurization The effluent with a three-stage pH gradient control is fed into a deep desulfurization tank for advanced desulfurization. The retention time of the effluent in the deep desulfurization tank is no less than 20 hours, resulting in a pH of 7-9 and the following concentrations of pollutants: Mn 0.05-1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03-0.5 mg / L, Zn < 0.006 mg / L, Al 0.3-1.5 mg / L, and SO42-. 2- The effluent concentration is ≤1300mg / L; the deep desulfurization tank is uniformly filled with carbon fiber packing material with good electrical conductivity. An exhaust pipe is installed at the top of the deep desulfurization tank. The hydrogen sulfide gas generated in the deep desulfurization tank is circulated to the pre-precipitation tank in step S1 through the exhaust pipe. The carbon fiber filler is pre-treated with sulfate-reducing bacteria to form a microbial film.

2. The combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 1, characterized in that: In step S2, the top of the packing material in the reaction chamber of the biomineralization tank is connected to the anode of the micro-power source by an electrical wire, and there is a connection hole on the side wall of the biomineralization tank reaction chamber; in step S4, the side wall of the deep desulfurization tank reaction chamber is also provided with a connection hole, and the connection hole of the biomineralization tank reaction chamber is connected to the connection hole of the deep desulfurization tank reaction chamber, and a proton exchange membrane is installed between the two connection holes; the micro-power source provides a micro DC current of 5~30mA.

3. The combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 2, characterized in that: In step S4, the top of the packing material in the reaction chamber of the deep desulfurization tank is connected to the cathode of a micro power source by an electrical wire, and the micro power source provides a small DC current of 5~30mA.

4. A combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 1, 2, or 3, characterized in that: In step S3, the alkaline agent is any one or any two or more of quicklime, hydrated lime, and sodium hydroxide.

5. The combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 4, characterized in that: The acidic mine wastewater fed into step S1 has a pH of 2.1–3.3, a Mn content of 250–350 mg / L, a Cu content of 12–30 mg / L, a Fe content of 300–400 mg / L, a Zn content of 90–110 mg / L, an Al content of 200–400 mg / L, and an SO4 content of [missing information]. 2- The concentration was between 5000 and 8000 mg / L; by adjusting the technical parameters of each step, step S4 achieved a pH of 7 to 9, with the following concentrations of pollutants: Mn 0.08–1.8 mg / L, Cu < 0.006 mg / L, Fe 0.03–0.5 mg / L, Zn < 0.006 mg / L, Al 0.3–1.5 mg / L, and SO42-. 2- 800~1300 mg / L of effluent.

6. The combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 4, characterized in that: In step S1, the heavy metal sulfides generated at the bottom of the pre-sedimentation tank are comprehensively recovered using flotation.

7. The combined treatment process for treating difficult-to-treat acidic mine wastewater as described in claim 4, characterized in that: The precipitates of Schist minerals or other iron oxides generated in step S2 can be used as adsorbents or pigments.

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