Method for resourceful treatment of tailings wastewater and simultaneous recovery of valuable metals

Abstract

This invention provides a method for the resource-based treatment of tailings wastewater and the simultaneous recovery of valuable metals, belonging to the field of mine wastewater treatment technology. The method includes: constructing a reaction system, which comprises a transparent reactor with a built-in working electrode, a power-free photovoltaic cell circuit system, a powered photoelectric electrolysis circuit system, and a light source; the photovoltaic cell circuit system and the photoelectric electrolysis circuit system are switched in parallel; the tailings wastewater is introduced into the transparent reactor and sealed; the light source is turned on, and only the photovoltaic cell circuit system is activated, to oxidize ammonia nitrogen into nitrogen gas and nitrate nitrogen, and recover some manganese metal; the light source is turned on, and only the photoelectric electrolysis circuit system is activated, to reduce manganese ions to recover manganese metal and reduce nitrate nitrogen into nitrogen gas. This invention provides a clean and efficient method for simultaneously oxidizing ammonia nitrogen in mine tailings wastewater and recovering valuable metals from the tailings wastewater; it utilizes an interconvertible photovoltaic cell reaction system and a photoelectric electrolysis reaction system to efficiently degrade ammonia-containing pollutants in tailings wastewater and recover manganese metal.

Description

Technical Field

[0001] This invention belongs to the field of mine wastewater treatment technology, and specifically relates to a method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals. Background Technology

[0002] Large amounts of wastewater are generated during the beneficiation, leaching, and washing processes of metal mine resources. Beneficiation wastewater includes process drainage, tailings pond overflow, and mine drainage. Currently, apart from pre-treatment and reuse, some of the beneficiation wastewater enters the tailings pond for natural sedimentation in the form of tailings slurry, forming mine tailings wastewater.

[0003] In practical engineering, clarified water in mine tailings ponds is pumped back for further recycling. However, the overflow water or surface water in the surrounding waters of the mining area contains a certain amount of ammonia nitrogen and metal ions. Therefore, before recycling, the mine tailings wastewater is usually pumped to a tailings treatment plant for denitrification / denitrification or anaerobic ammonia oxidation. Treating mine tailings wastewater requires the addition of large amounts of organic carbon sources, which has drawbacks such as high treatment costs, high energy consumption, and ineffective treatment of metal ions. Currently, the main methods for treating mine tailings wastewater include chemical precipitation and integrated chemical / physical / biological treatment methods.

[0004] Taking chemical precipitation methods such as lime-iron salt-coagulation sedimentation as an example, although they can effectively remove heavy metal ions, they also have problems such as incomplete metal treatment, high cost, and secondary pollution. In addition, regarding the ore washing process, leaching or cascade leaching of mines with clean water or chemical leaching agents (such as potassium chloride, ferrous sulfate, etc.) can remove some ammonia nitrogen, but there are still some ammonium salt residues and metal ions that are slowly released with rainfall and pollute the surrounding environment after ore washing.

[0005] Chemical / physical / biological integrated treatment methods generally combine chemical methods with activated carbon adsorption, electro-oxidation, Fenton oxidation, biological regulation, flocculation sedimentation, and ecological restoration. Although these methods can treat tailings wastewater to a certain extent and reduce the amount of chemical leaching agents used, they still have common problems such as long treatment cycles, low efficiency, high energy consumption, long process flow, cumbersome process, and high cost.

[0006] Therefore, given the coexistence of certain amounts of inorganic nitrogen (ammonia nitrogen and nitrate nitrogen) and metal ions in tailings wastewater after mining and leaching, finding clean, efficient, short-process, and energy-free / low-energy-consumption methods to deeply remove inorganic nitrogen and simultaneously recover metals from mine tailings wastewater is an important requirement for the sustainable, healthy, and green development of mines.

[0007] Photocatalysis, photoelectrolysis, and other methods are widely used in the treatment of domestic and industrial wastewater due to their advantages such as simple installation and minimal chemical additives.

[0008] However, these photovoltaic cells or photoelectrolysis cells still have many shortcomings, such as long process flow, low removal rate and efficiency, incomplete removal of inorganic nitrogen, high or unsustainable cost of electrode materials, and low light efficiency utilization. Summary of the Invention

[0009] Therefore, the present invention aims to provide a method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals, thereby solving at least one of the technical problems in the background art.

[0010] This invention is implemented as follows:

[0011] A method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals, the method comprising the following steps:

[0012] A reaction system is assembled, comprising a transparent reactor with a built-in working electrode, a photovoltaic cell circuit system and a photoelectric decomposition circuit system respectively connected to the working electrode, and a light source externally mounted on the transparent reactor; the photovoltaic cell circuit system and the photoelectric decomposition circuit system are switched in parallel, the photovoltaic cell circuit system has no power supply, and the photoelectric decomposition circuit system has a power supply;

[0013] Tailings wastewater is introduced into the transparent reactor and sealed; the light source is turned on, and only the photovoltaic cell circuit system is turned on to carry out the photovoltaic cell reaction, which is used to oxidize ammonia nitrogen into nitrogen gas and nitrate nitrogen and recover some manganese metal;

[0014] When the light source is turned on and only the photoelectric decomposition circuit system is activated, the photocell reaction is converted into a photoelectric decomposition reaction, which is used to directly or indirectly reduce manganese ions to recover manganese metal and reduce nitrate nitrogen to nitrogen gas.

[0015] Preferably, the working electrode includes a photoanode and a photocathode; the photoanode and photocathode are made of porous carbon felt with a load layer on their surface;

[0016] The loading layer on the surface of the photocathode is a composite of metal oxide and graphite-phase carbon nitride; the loading layer on the surface of the photoanode is a composite of carbon dots and graphite-phase carbon nitride.

[0017] Preferably, the photovoltaic cell circuit system includes a first switch, a first ammeter, and a first external resistor, and the photovoltaic cell circuit system is electrically connected to the photoanode and photocathode; the light source, the transparent reactor, the photovoltaic cell circuit system, and the photoanode and photocathode constitute a photovoltaic cell reaction system;

[0018] The photoelectric desorption circuit system includes a second switch, a second ammeter, a power supply, and a second external resistor. The photoelectric desorption circuit system is electrically connected to the photoanode and photocathode. The light source, the transparent reactor, the photoelectric desorption circuit system, and the photoanode and photocathode together form a photoelectric desorption reaction system.

[0019] Preferably, the metal oxide is selected from at least one of tungsten-molybdenum oxide, iron-manganese oxide, zinc ferrite, and silver phosphate.

[0020] Preferably, the loading amount of the surface loading layer of the photoanode and photocathode is 0.1 mg / cm². 2 ~20mg / cm 2 .

[0021] Preferably, in the loading layer of the photoanode, the mass ratio of the metal oxide to the graphitic carbon nitride is 10:1 to 1:10;

[0022] In the loading layer of the photocathode, the mass ratio of the carbon dots to the graphitic carbon nitride is 10:1 to 1:10.

[0023] Preferably, the first external resistance is 100Ω to 2000Ω;

[0024] When the photovoltaic cell circuit system is running, the potential of the photoanode is -0.4V to -0.3V, and the potential of the photocathode is 0.2V to 0.3V.

[0025] The operating time of the photovoltaic cell reaction is 1.5h to 2.5h.

[0026] Preferably, the second external resistance is 10Ω to 100Ω;

[0027] The power supply applies a voltage of 0.3V to 5.0V;

[0028] When the photoelectrolysis circuit system is running, the potential of the photoanode is 0.1V to 2.5V, and the potential of the photocathode is -2.5V to -0.4V.

[0029] The photoelectrolysis reaction takes 9 to 11 hours to run.

[0030] Preferably, the reaction system further includes a reference electrode and a gas collecting pipe for collecting gas.

[0031] Preferably, the light source intensity is greater than 5.0 mW / cm². 2 .

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. This invention provides a clean and efficient method for removing inorganic nitrogen from mine tailings wastewater and simultaneously recovering valuable metals (such as Mn(II)) from the tailings wastewater; the anode of a photovoltaic cell oxidizes ammonia nitrogen in the mine tailings wastewater into nitrogen gas and nitrate nitrogen, and the photocathode recovers part of the manganese metal with the help of H2O2 mediator; then the photovoltaic cell is switched to a photoelectrolysis cell to reduce the nitrate nitrogen in the mine tailings wastewater into nitrogen gas while further recovering the manganese metal.

[0034] 2. This invention establishes a photovoltaic cell reaction system and a photoelectrolysis reaction system that can be converted into each other. On the basis of efficiently degrading pollutants and recovering valuable metals, it can cleanly use sunlight and ensure the efficiency of light energy utilization, thereby realizing a clean, efficient, and short-process deep treatment of mine tailings wastewater and simultaneous resource recovery of metals.

[0035] 3. This invention uses porous carbon felt with a special loading layer as the working electrode. Carbon-based carbon felt materials are more conducive to the two-electron reaction of O2 to generate H2O2. Graphite-phase carbon nitride (g-C3N4) loaded on porous carbon felt to construct different electrode materials (such as MnFe2O4 / g-C3N4, WO3 / MoO3 / g-C3N4, ZnFe2O4 / g-C3N4, carbon dots / g-C3N4, etc.) can exhibit unique photogenerated electron and hole separation, as well as conduction and valence band properties, after appropriate modification and pretreatment. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the reaction system of the present invention;

[0037] Illustration: 1-Light source; 2-Photoanode; 3-Transparent reaction vessel; 4-Photocathode; 5-Cathode load layer; 6-Reference electrode; 7-Carbon rod; 8-First external resistor; 9-Second external resistor; 10-First ammeter; 11-Second ammeter; 12-Power supply; 13-Second switch; 14-First switch; 15-Gas collecting pipe; 16-Anode load layer.

[0038] Figure 2 This is a graph showing the changes of various pollutants in mine tailings wastewater over operating time in Example 1;

[0039] Figure 3 The graph shows the changes in pH and conductivity of the mine tailings wastewater effluent over operating time in Example 1.

[0040] Figure 4 The graph shows the changes in electrode potential and loop current over time in Example 1.

[0041] Figure 5 This is a cyclic voltammetry curve of the three-electrode system in Example 1;

[0042] Figure 6 E-pH relationships of different inorganic nitrogen and manganese species in mine tailings wastewater in Example 1;

[0043] Figure 7 This is a comparison chart of the removal rates of various pollutants in mine tailings wastewater during the photovoltaic cell stage, between the photovoltaic cell group, the open-circuit control group, and the no-light control group.

[0044] Figure 8Cyclic voltammograms of photoanodes and photocathodes under conditions of illumination and darkness, with and without a load layer;

[0045] Figure 9 Impedance analysis diagrams of photoanodes and photocathodes under conditions of light and darkness, with and without a load layer;

[0046] Figure 10 The images are scanning electron microscope (SEM) images of the photoanode and photocathode after five cycles of operation according to Example 1.

[0047] Figure 11 The X-ray energy spectrum analysis of the photoanode and photocathode after five cycles of operation according to Example 1;

[0048] Figure 12 Impedance analysis and cyclic voltammetry were performed during the five-cycle operation according to Example 1. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0050] The reaction system used in this invention for the resource-based treatment of tailings wastewater using a dual photoelectric synergistic method is as follows: Figure 1 As shown.

[0051] The reaction system includes: a transparent reactor 3 with a built-in working electrode, a photovoltaic cell circuit system connected to the working electrode, a photoelectric desorption circuit system connected to the working electrode, and a light source 1 externally mounted on the transparent reactor 3; the photovoltaic cell circuit system and the photoelectric desorption circuit system are switched in parallel, with the photovoltaic cell circuit system having no power supply and the photoelectric desorption circuit system having a power supply; the photovoltaic cell circuit system and the photoelectric desorption circuit system are externally mounted on the transparent reactor 3. In a specific implementation, the reaction system also includes a gas collecting pipe 15 and a reference electrode 6 mounted on the transparent reactor 3.

[0052] The working electrodes include a photoanode 2 and a photocathode 4. The photoanode 2 is made of porous carbon felt with an anode loading layer 16 on its surface; the photocathode 4 is made of porous carbon felt with a cathode loading layer 5 on its surface. The cathode loading layer 5 is a composite of metal oxide and graphitic carbon nitride, wherein the metal oxide is selected from tungsten molybdenum oxide (WO3 / MoO3), iron manganese oxide (MnFe2O4), and zinc ferrite (ZnFe2O4). 4) At least one of silver phosphate (Ag3PO4); the anode support layer 16 is a composite of carbon dots and graphitic carbon nitride. The loading amount of the support layers on the surfaces of the photoanode and photocathode is 0.1 mg / cm³. 2 ~20mg / cm 2The mass ratio of metal oxide to graphite-phase carbon nitride in the cathode loading layer 5 is 10:1 to 1:10; the mass ratio of carbon dots to graphite-phase carbon nitride in the anode loading layer 16 is 10:1 to 1:10. The substrate material of the working electrode is the key to controlling the in-situ generation of H2O2. In this invention, the use of carbon-based carbon felt material as the substrate for the photoanode 2 and photocathode 4 is more conducive to the two-electron reaction of O2 to generate H2O2. Graphite-phase carbon nitride (g-C3N4) loaded on different electrode materials constructed from porous carbon felt (such as MnFe2O4 / g-C3N4, WO3 / MoO3 / g-C3N4, ZnFe2O4 / g-C3N4, carbon dots / g-C3N4, etc.), after appropriate modification and pretreatment, exhibits unique photogenerated electron and hole separation, as well as conduction and valence band properties.

[0053] The photovoltaic cell circuit system includes a first switch 14, a first ammeter 10, and a first external resistor 8, and is electrically connected to the photoanode 2 and the photocathode 4. The photolysis circuit system includes a second switch 13, a second ammeter 11, a power supply 12, and a second external resistor 9, and is electrically connected to the photoanode 2 and the photocathode 4. In a specific implementation, both the photoanode 2 and the photocathode 4 are connected to the photovoltaic cell circuit system and the photolysis circuit system via carbon rods 7. The first external resistor 8 is 100Ω to 2000Ω, the second external resistor 9 is 10Ω to 100Ω, and the power supply 12 applies a voltage of 0.3V to 5.0V.

[0054] The light source 1, transparent reactor 3, photovoltaic cell circuit system, photoanode 2 and photocathode 4 constitute the photovoltaic cell reaction system. In the photovoltaic cell constructed by the photoanode 2 loaded with carbon point / g-C3N4 and the photocathode 4 loaded with metal oxide / g-C3N4, the different bias voltages generated by the Fermi levels of the two are used to construct the anode and cathode to spontaneously undergo redox reactions. The anode reaction is to convert ammonia nitrogen into nitrate nitrogen and nitrogen gas, and the cathode reaction is to convert dissolved oxygen into H2O2 and indirectly reduce manganese ions.

[0055] The light source 1, transparent reactor 3, photoelectrolysis circuit system, photoanode 2, and photocathode 4 constitute the photoelectrolysis reaction system. Due to the input of a small external voltage (0.3V~5.0V) and the change in internal resistance, the circuit current and electrode potential are improved and increased. This allows the electrode current to work together with the photogenerated electrons of the metal oxide / g-C3N4 loaded photocathode 4 and the photogenerated electrons of the carbon point / g-C3N4 loaded photoanode 2 to directly reduce manganese ions, or to recover manganese metal through an indirect reduction pathway by using dissolved oxygen to generate H2O2 as an intermediate medium. At the same time, the photogenerated electrons in the photoelectrolysis cell stage can also reduce nitrate nitrogen into nitrogen gas, thereby realizing the clean and efficient treatment of mine tailings wastewater and the simultaneous resource recovery of valuable metals.

[0056] This invention establishes a photocell reaction system and a photoelectrolysis reaction system that can be interchanged. Utilizing the oxidizing properties of ammonia nitrogen and the reducing properties of nitrate nitrogen and / or metal ions in mine tailings wastewater, and based on the different bias voltages generated by the Fermi levels of different photoanode and photocathode materials, a photocell that spontaneously oxidizes ammonia nitrogen at the anode while simultaneously reducing dissolved oxygen (H2O2) and metals at the cathode is matched and constructed. A photoelectrolysis cell with a small applied voltage is switched to achieve the process of converting ammonia nitrogen in mine tailings wastewater into nitrogen gas and simultaneously recovering metallic manganese.

[0057] Light source 1 is used to provide visible light towards the working electrode, and the absorbed visible light used is simulated sunlight; in specific implementation, 100W tungsten iodine lamps are placed approximately 15cm away from both the photoanode 2 and the photocathode 4 as light source 1, and the intensity of light source 1 is greater than 5.0mW / cm. 2 At the same time, a small fan is installed to dissipate heat through air convection and regulate temperature changes caused by sunlight.

[0058] Here, taking the hydrothermal method as an example, the preparation method of the load layer of photoanode 2 and photocathode 4 is briefly described. Specifically, the porous carbon felt is placed in the corresponding loading solution, soaked, and then heat-treated at 100℃~150℃. After removal, it is cured (e.g., calcined at 400℃~450℃ for about 1 hour) to form a load layer on the surface of the porous carbon felt. The preparation steps of the loading solution of photocathode 4 are as follows: after mixing metal oxide, graphitic carbon nitride and urea, the pH is adjusted to 12 with alkali and then placed in a stainless steel PTFE-lined reactor at about 200℃ for 12 hours, followed by rinsing with ethanol and deionized water. The preparation steps of the loading solution of photoanode 2 are as follows: after dissolving citric acid and urea in deionized water, the mixture is placed in a stainless steel PTFE-lined reactor at about 180℃ for 5 hours, and the supernatant after centrifugation is the carbon dot solution. This carbon dot solution is mixed with urea and incubated at about 90℃ overnight. In specific implementation, the load layer of photoanode 2 and photocathode 4 can be achieved by any method that can be implemented in the art, such as hydrothermal method, spraying method, electrodeposition method, etc., without specific limitations. The above reaction methods, steps and parameters can be adjusted according to actual needs.

[0059] A method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals includes the following steps:

[0060] S1, according to Figure 1 The assembly reaction system is shown.

[0061] S2. Tailings wastewater is introduced into transparent reactor 3 and sealed. Light source 1 is turned on, and only the photovoltaic cell circuit system is activated to carry out the photovoltaic cell reaction. At this time, the potential of photoanode 2 is -0.4V to -0.3V, and the potential of photocathode 4 is 0.2V to 0.3V. The operation time is 1.5h to 2.5h. In this step, photoanode 2 oxidizes ammonia nitrogen in tailings wastewater into nitrogen gas and nitrate nitrogen, and photocathode 4 recovers part of the manganese metal with the help of H2O2 as a mediator. During the operation of the photovoltaic cell, a two-electron reduction reaction of dissolved oxygen occurs in the cathode reaction. The dissolved oxygen comes from the natural settling of mine tailings wastewater.

[0062] S3. Continue to turn on light source 1, and only start the photoelectrolysis circuit system. The photocell reaction is converted into a photoelectrolysis reaction. The potential of photoanode 2 is 0.1V to 2.5V, and the potential of photocathode 4 is -2.5V to -0.4V. The operation lasts for 9h to 11h. In this step, photocathode 4 and photoanode 2 directly or indirectly reduce manganese ions to recover manganese metal, and reduce nitrate nitrogen to nitrogen gas.

[0063] In practice, actual mine tailings wastewater contains low concentrations of ammonia nitrogen (10 mg / L to 30 mg / L), nitrate nitrogen (5 mg / L to 20 mg / L), manganese ions (10 mg / L to 50 mg / L), and a certain amount of dissolved oxygen (7.0 mg / L to 9.0 mg / L).

[0064] Example 1

[0065] A method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals includes the following steps:

[0066] S1. Place photoanode 2 (a carbon felt electrode with dimensions of 1.0cm × 1.0cm × 0.5cm, covered with carbon dots / g-C3N4) and photocathode 4 (a carbon felt electrode with dimensions of 1.0cm × 1.0cm × 0.5cm, covered with Fe2MnO4 / g-C3N) in a transparent reactor 3 (approximately 25ml) made of plexiglass. Connect them electrically to an external photovoltaic cell circuit system and photoelectrolysis circuit system via carbon rod 7. The circuit system operates in parallel switching mode. The photovoltaic cell circuit system includes a first switch 14, a first ammeter 10, and a first external resistor 8 (set to 1000Ω) connected in parallel. The photolysis circuit system includes a second switch 13, a second ammeter 11, a power supply 12 (voltage set to 3.0V), and a second external resistor 9 (set to 10Ω) connected in parallel. 100W tungsten iodine lamps are placed 15cm away from both the photocathode 4 and photoanode 2 as light sources 1, with the intensity of light source 1 set to 24.3mW / m². 2 Meanwhile, small fans are installed to dissipate heat through air convection and regulate temperature changes caused by sunlight.

[0067] S2. The tailings wastewater is introduced into the transparent reactor 3 and sealed. The light source 1 is turned on, the second switch 13 is opened and the first switch 14 is closed. The photovoltaic cell reaction system is started and runs for 2 hours at room temperature (20℃~25℃).

[0068] S3. Keep light source 1 on and close the second switch 13. Open the first switch 14 to switch the photovoltaic cell reaction system to the photoelectrolysis reaction system and run for about 10 hours.

[0069] 1. Analysis of tailings wastewater treatment efficiency

[0070] During the reaction process in Example 1, liquid samples were periodically taken from the transparent reactor 3 to detect the concentration of target pollutants in the tailings wastewater (e.g., Figure 2 (as shown), pH and conductivity (e.g.) Figure 3 (As shown).

[0071] Specifically, the concentrations of ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, manganese ions, and H2O2 in the liquid phase were analyzed, and the total nitrogen content in the liquid phase, as well as the removal rates of ammonia nitrogen (%), nitrate nitrogen (%), total nitrogen (%), manganese ion (%), and energy utilization rate (%) were calculated.

[0072]

[0073] Energy utilization rate (%) = S2 energy utilization rate + S3 energy utilization rate =

[0074]

[0075] In the formula, ΔG is the Gibbs free energy of the corresponding chemical species, and Δn 氨态氮 , Δn 硝态氮 , Δn 锰离子 , Δn 氢气 These represent the net changes in molar amounts of ammonia nitrogen, nitrate nitrogen, manganese ions, and gaseous hydrogen in the liquid phase during the photovoltaic cell operation phase and the photoelectrolysis cell operation phase, respectively. The energy utilization rate (%) is the ratio of the molar changes in inorganic nitrogen (ammonia nitrogen or nitrate nitrogen) and manganese species in the initial and final states of the tailings wastewater, as well as the molar amount of hydrogen generated, multiplied by the corresponding Gibbs free energy and summed, to the total input energy (the sum of light source and electrical energy).

[0076] 2. Evaluation of the redox performance of the electrode

[0077] Example 1: A reference electrode 6 and a gas collecting pipe 15 are installed on a transparent reactor 3. In the three-electrode system consisting of the working electrode (photoanode 2, photocathode 4) and the reference electrode 6, the changes in electrode potential and current over time are as follows: Figure 4As shown; simultaneously, a carbon felt without any loading layer was used as a control electrode to conduct a control experiment of Example 1. The cyclic voltammograms of the photoanode, photocathode, and control electrode of Example 1 are shown below. Figure 5 As shown, the redox performance of the electrode is then evaluated.

[0078] The E-pH relationships of nitrogen and manganese species are as follows: Figure 6 As shown in Figures A and B.

[0079] 3. The impact of different conditions on the photovoltaic cell stage

[0080] The conditions for adjusting the photovoltaic cell stage are as follows:

[0081] Photovoltaic cell array: Step S2 in Example 1 is a closed-loop reaction condition with illumination;

[0082] Open circuit control group: that is, the first switch is turned off and the light source is turned on, forming an open circuit and a reaction condition with light.

[0083] No light control group: This means closing the first switch to avoid light. Avoiding light means turning off the iodine-tungsten lamp and wrapping the transparent reactor with tin foil to form a closed-loop reaction condition without light.

[0084] After the photovoltaic cell stage, the concentration of target pollutants in the tailings wastewater was measured under different group conditions, and the removal rate was calculated. The results are as follows: Figure 7 As shown.

[0085] 4. The effect of working electrodes under different conditions on the reaction

[0086] Depending on whether illumination is applied to the working electrode and whether a corresponding load layer is formed on the surface of the working electrode, cyclic voltammetry curves and impedance analyses are plotted, as shown below. Figure 8 and Figure 9 As shown.

[0087] 5. Using one photovoltaic cell + one photoelectrolysis cell operation as one cycle, after five intermittent cycles, the concentrations of ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, manganese ions, and total Fe(III) and Fe(II) in the liquid phase of the reaction system were analyzed and measured. The surface morphology and elemental composition of the photoanode and photocathode were observed and analyzed using SEM-EDS. The SEM images of the photoanode and photocathode are shown below. Figure 10 As shown in Figures A and B, the X-ray energy dispersive spectroscopy (EDS) analyses of the photoanode and photocathode are respectively as follows: Figure 11 As shown in Figures A and B; the impedance analysis and cyclic voltammetry for the five cycles are shown in Figures B and C respectively. Figure 12 As shown in Figures A and B, the multi-cycle, long-term operating performance of the carbon dot / g-C3N4 carbon felt electrode (photoanode) and the Fe2MnO4 / g-C3N4 carbon felt electrode (photocathode) is evaluated.

[0088] Combination Figures 2 to 7 It can be seen that, under the S2 photocell reaction mode, after 2 hours of reaction, the ammonia nitrogen content is only 0.12 mg / L ± 0.01 mg / L. Figure 2 This meets the GB / T 14848-2017 Class III Surface Water Discharge Standard (ammonia nitrogen not exceeding 0.50 mg / L); at this point, the ammonia nitrogen removal rate reaches 99.3%. Figure 7 This significantly outperformed the open-circuit control (18.3% ± 2.8%) with only photocatalysis and the dark control (4.3% ± 1.3%) with only current. Figure 7 This indicates that the circuit electronics and photocatalysis of the photovoltaic cell play a significant role in the removal of ammonia nitrogen; furthermore, nitrite nitrogen in the system is almost negligible, while nitrate nitrogen slightly increases from the initial 11.6 mg / L to 12.7 mg / L ± 0.2 mg / L. Figure 2 The total nitrogen concentration decreased from the initial 28.6 mg / L to 12.8 mg / L ± 0.8 mg / L. Figure 2 The total nitrogen removal rate was 55.3% ± 0.7%. Figure 7 The percentage of positive results was significantly higher than that of the open-circuit control (7.1% ± 2.1%) and the dark control (0.7% ± 1.0%). Figure 7 This indicates that while the photovoltaic cell efficiently removes and converts ammonia nitrogen from mine tailings wastewater, the system also adds a small amount of nitrate nitrogen; the nitrogen balance results show that the total nitrogen removed is mainly disposed of as gaseous N2. Combined with... Figure 6 The E-pH relationship curve for the conversion of ammonia nitrogen to nitrogen gas is shown. The photoanode potential of the photovoltaic cell is -0.352V. Figure 4 This is a necessary condition for the system to convert ammonia nitrogen into nitrogen gas. Simultaneously, the manganese ion concentration decreased from the initial 24.7 mg / L to 22.2 mg / L ± 0.6 mg / L. Figure 2 The removal rate of manganese ions was 10.1% ± 2.4%. Figure 7 The concentration was higher than that of the open-circuit control (5.9% ± 1.2%) and the dark control (0.8% ± 1.2%). Figure 7 This indicates that both loop electrons and photocatalysis jointly promote the removal of manganese ions by the photovoltaic cell. The amount of H2O2 in the liquid phase decreased from 650 μg / L after 0.5 hours of operation to 540 μg / L after 2 hours of operation, indicating that the H2O2 and OH- generated from dissolved oxygen in the liquid phase... - This could be due to manganese ion conversion and an increase in liquid phase pH. Figure 3 The main reason is...

[0089] The system was switched to the S3 photoelectrolysis reaction mode (3.0V) and continued to run for 12 hours. Only 0.48 mg / L ± 0.68 mg / L of manganese ions remained in the liquid phase. Figure 2This results in a total manganese ion removal rate of 98.1%, indicating that the photoelectrolysis cell primarily removes manganese ions; combined with the E-pH relationship curve of manganese ions (… Figure 6 (B) The relatively negative photocathode potential of this photoelectrolysis cell is (-1.093V to -0.621V). Figure 4 This is likely the main reason for the (partial) reduction and deposition of manganese ions on the photocathode. Meanwhile, the total nitrogen content in the reaction system decreased to 7.3 mg / L ± 0.3 mg / L. Figure 2 This resulted in a total nitrogen removal rate of 74.5% ± 1.0% in the effluent. Considering that nitrite nitrogen remained almost unchanged, while nitrate nitrogen decreased from 12.7 mg / L ± 0.2 mg / L at the beginning of the photoelectrolysis tank (2 hours) to 7.3 mg / L ± 0.9 mg / L at 12 hours... Figure 2 This indicates that the total nitrogen removal during this stage is mainly attributed to the reduction of nitrate nitrogen to N2. Cyclic voltammetry analysis results show that, compared with the carbon felt control, the carbon dot / g-C3N4 electrode has better oxidation performance, while the Fe2MnO4 / g-C3N4 electrode has better reduction performance. Figure 5 Therefore, the oxidation of ammoniacal nitrogen species occurs at the carbon point / g-C3N4 anode electrode, while the reduction of manganese ions, hydrogen evolution, and the reduction of dissolved oxygen to H2O2 occur at the Fe2MnO4 / g-C3N4 cathode electrode. During this stage, the pH further increases to 6.56 ± 0.23. Figure 3 This indicates that the deposition of metallic manganese is accompanied by a hydrogen evolution reaction, and the system's hydrogen production is 0.0028 m³. 3 / m 2 / d±0.0005m 3 / m 2 / d, the system's energy utilization rate is approximately 15%–25%. The system's solution conductivity decreased from the initial 0.472 mS / cm to 0.435 mS / cm ( Figure 3 In fact, the removal of manganese ions reduces the solution conductivity, while the partial shedding of photocathode material (such as approximately 0.26% iron in the first cycle) increases the conductivity. The net result of both is the main reason for the slight decrease in liquid phase conductivity.

[0090] Figure 8 Cyclic voltammetric analyses were performed under both light and dark conditions, and with and without a loading layer on the photoanode and photocathode. The results corroborated the superior redox performance under light compared to the dark control. Figure 9Impedance analysis of the photoanode in section A shows that, compared with photoanodes without illumination and without a loading layer, the photoanode of this invention (carbon dot / g-C3N4 carbon felt electrode) has a smaller ohmic internal resistance (109Ω vs. 162Ω and 165Ω) and diffusion internal resistance (634Ω vs. 734Ω and 1040Ω), but a larger charge transfer internal resistance (173Ω vs. 116Ω and 145Ω). Similarly, Figure 9 Impedance analysis of the photocathode in section B shows that, compared with photocathodes without light control and without catalyst, the photocathode (Fe2MnO4 / g-C3N4 carbon felt electrode) of the present invention has smaller ohmic internal resistance (122Ω vs. 144Ω and 165Ω), charge transfer internal resistance (112Ω vs. 195Ω and 145Ω), and diffusion internal resistance (400Ω vs. 602Ω and 1040Ω).

[0091] Results from five consecutive cycles showed that the removal of ammonia nitrogen, nitrate nitrogen, total nitrogen, and manganese ions was not significantly different from that of the first cycle (p>0.05). However, the amount of iron removed from the liquid phase was only 0.15%, lower than the 0.26% in the first cycle, and the system pH increased with increasing operating time. Figure 3 This is likely the main reason for the gradual decrease in iron shedding. Scanning electron microscopy observations of the photoanode and photocathode show that the photocathode surface loaded with Fe2MnO4 / g-C3N4 contains a large number of spherical particles (such as...) Figure 10 As shown in Figure B), the photoanode with surface-loaded carbon dots / g-C3N4 is only covered with a thin layer of precipitate (as shown in Figure B). Figure 10 (As shown in A); X-ray energy dispersive spectroscopy analysis further corroborated the presence of a photocathode with Fe2MnO4 / g-C3N4 surface-loaded (as shown in A); Figure 11 (As shown in B) is a carbon point / g-C3N4 photoanode (such as...) Figure 11 As shown in Figure A, a significant amount of manganese was generated on the surface. This result is consistent with the theoretical explanation that a reducing cathode recovers oxidized manganese ions, while the anode adsorbs a small amount of manganese. The continuous accumulation of manganese on the Fe2MnO4 / g-C3N4 photocathode surface over time may have prevented and reduced catalyst shedding, which is also a major reason for the stable operation in the fifth cycle. Figure 12 Impedance analysis shown in Figure A indicates that after five cycles of operation, the internal resistance of all parts of the photoanode decreased (ohmic resistance: 55Ω vs. 109Ω; charge transfer resistance: 53Ω vs. 173Ω; diffusion resistance: 442Ω vs. 634Ω); while the internal resistance of all parts of the photocathode increased to varying degrees. For example... Figure 12Cyclic voltammetry analysis of the photocathode, photoanode, and control electrode shown in Figure B indicates that the capacitance of both the photoanode with carbon dots / g-C3N4 surface loading and the photocathode with Fe2MnO4 / g-C3N4 surface loading increases to varying degrees after prolonged operation. In summary, the photovoltaic cell switching photoelectrolysis unit constructed using the photoanode and photocathode can achieve short-range, efficient, clean, and deep treatment of mine tailings wastewater for simultaneous recovery of manganese metal. The process is clean and pollution-free, offering multiple benefits including environmental, ecological, social, and economic advantages.

[0092] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for resourceful treatment of tailings wastewater and simultaneous recovery of valuable metals, characterized by, The method includes the following steps: A reaction system is constructed, comprising a transparent reactor with a built-in working electrode, a photovoltaic cell circuit system and a photolysis circuit system respectively connected to the working electrode, and a light source externally mounted on the transparent reactor. The photovoltaic cell circuit system and the photolysis circuit system can be switched in parallel; the photovoltaic cell circuit system has no power supply, while the photolysis circuit system has a power supply. The working electrode includes a photoanode and a photocathode. The photoanode and photocathode are made of porous carbon felt with a loading layer on their surfaces. The loading layer on the surface of the photocathode is a composite of metal oxide and graphite-phase carbon nitride; the loading layer on the surface of the photoanode is a composite of carbon dots and graphite-phase carbon nitride. The photovoltaic cell circuit system includes a first switch, a first ammeter, and a first external resistor, and is electrically connected to the photoanode and photocathode. The light source, transparent reactor, photovoltaic cell circuit system, and photoanode and photocathode constitute a photovoltaic cell reaction system. The photolysis circuit system includes a second switch, a second ammeter, a power supply, and a second external resistor, and is electrically connected to the photoanode and photocathode. The light source, transparent reactor, photolysis circuit system, and photoanode and photocathode constitute a photolysis reaction system. Tailings wastewater is introduced into the transparent reactor and sealed; the light source is turned on, and only the photovoltaic cell circuit system is turned on to carry out the photovoltaic cell reaction, which is used to oxidize ammonia nitrogen into nitrogen gas and nitrate nitrogen and recover some manganese metal; When the light source is turned on and only the photoelectric decomposition circuit system is activated, the photocell reaction is converted into a photoelectric decomposition reaction, which is used to directly or indirectly reduce manganese ions to recover manganese metal and reduce nitrate nitrogen to nitrogen gas.

2. A method of beneficiating tailings wastewater and simultaneously recovering valuable metals as claimed in claim 1, wherein, The metal oxide is selected from at least one of tungsten-molybdenum oxide, iron-manganese oxide, and zinc ferrite.

3. The method of beneficiating tailings wastewater and simultaneously recovering value metals as claimed in claim 1, wherein, The loading of the surface supported layer of the photoanode and photocathode is 0.1 mg / cm 2 ~20 mg / cm 2 .

4. The method of beneficiating tailings wastewater and simultaneously recovering value metals of claim 1, wherein, In the loading layer of the photocathode, the mass ratio of the metal oxide to the graphitic carbon nitride is 10:1 to 1:10; In the loading layer of the photoanode, the mass ratio of the carbon dots to the graphitic carbon nitride is 10:1 to 1:

10.

5. The method of beneficiating tailings wastewater and simultaneously recovering value metals as claimed in claim 1, wherein, The first external resistance is 100Ω to 2000Ω; When the photovoltaic cell circuit system is running, the potential of the photoanode is -0.4V to -0.3V, and the potential of the photocathode is 0.2V to 0.3V. The operating time of the photovoltaic cell reaction is 1.5h to 2.5h.

6. The method of beneficiating tailings wastewater and simultaneously recovering value metals of claim 1, wherein, The second external resistance is 10Ω to 100Ω; The power supply applies a voltage of 0.3V to 5.0V; When the photoelectrolysis circuit system is running, the potential of the photoanode is 0.1V to 2.5V, and the potential of the photocathode is -2.5V to -0.4V. The photoelectrolysis reaction takes 9 to 11 hours to run.

7. The method of beneficiating tailings wastewater and simultaneously recovering value metals as claimed in claim 1, wherein, The reaction system also includes a reference electrode and a gas collecting pipe for collecting the gas.

8. The method of beneficiating tailings wastewater and simultaneously recovering value metals of claim 1, wherein, The light source intensity is greater than 5.0 mW / cm². 2 .

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