A method for preparing a passivator based on tannic acid modified biochar and application thereof in pyrite passivation or ecological restoration

Abstract

This invention relates to the field of metal pollution control technology in mining environments, and particularly to a method for preparing a passivating agent based on tannic acid-modified biochar and its application in pyrite passivation or ecological restoration. The method includes: pyrolyzing bagasse and immersing it in hydrochloric acid for deashing, followed by oxidation in a hydrogen peroxide solution to obtain biochar; mixing the biochar with a tannic acid solution, adjusting the pH of the mixture to 8-8.5, and performing surface modification to obtain the passivating agent. This method solves the problems of biochar materials failing to form a dense and stable protective layer in acidic environments and the insufficient durability of single organic ligand treatments. The passivation layer formed through chemical bonding controls the generation of acidic mine wastewater at its source and improves the oligotrophic environment of tailings ponds, laying the foundation for ecological restoration. Compared to traditional treatments, this invention provides a long-lasting and stable passivation effect, utilizes bagasse as a raw material, has resource advantages, is environmentally friendly, and can improve soil fertility, showing good economic and application prospects.

Description

Technical Field

[0001] This invention relates to the field of metal pollution control technology in mining environments, and in particular to a method for preparing passivating agents based on tannic acid-modified biochar and its application in pyrite passivation or ecological restoration. Background Technology

[0002] Pyrite is a major mineral source of acid mine wastewater (AMD). Large quantities of pyrite are exposed in abandoned tailings ponds, open-pit mines, and wellhead drainage systems. When in prolonged contact with oxygen and water, it continuously releases iron, sulfate, and various heavy metal ions. Direct discharge of untreated AMD into the surrounding environment not only leads to acidification of surface and groundwater and excessive levels of heavy metals, but also causes soil degradation and ecosystem imbalance. This situation seriously threatens the survival of flora and fauna around mining areas, becoming one of the most pressing environmental problems to be addressed in the mining sector.

[0003] Currently, passivating agents and materials used for source control of pyrite still have significant limitations. Materials such as biochar are mainly used as ordinary adsorbents or physical covering media in most studies, and their interfacial interactions with pyrite are mostly limited to physical adsorption and mechanical coating. In strongly acidic environments, these materials struggle to form a dense and long-term stable passivation layer on the mineral surface, thus limiting their inhibitory effect on the continuous oxidation of pyrite. Research on natural organic ligand materials mainly focuses on their complexation and stabilization effects on iron and other metal ions in solution, while systematic research on the orderly adsorption, film formation, and effective protective layer construction of these materials on the pyrite surface remains insufficient. Furthermore, commonly used inorganic passivating agents and neutralizing materials (such as the widely used lime, carbonates, phosphates, and metal oxides) are prone to high salt loads and excessive phosphorus input during long-term use, potentially leading to downstream water eutrophication, soil salinization, or structural deterioration. Some synthetic polymer coatings and organic corrosion inhibitors are also difficult to degrade, which may lead to the accumulation of organic residues in soil and water bodies, and even the formation of secondary solid waste containing heavy metals or toxic organic matter, thus creating new environmental pressures on the ecosystem. It is worth noting that tailings ponds, due to the lack of nutrients such as nitrogen and phosphorus, face enormous challenges in their reclamation and ecological restoration. Therefore, there is an urgent need to develop a highly efficient passivating agent that can function at the mineral source and construct a dense and stable passivation layer on the pyrite surface, while simultaneously improving the oligotrophic environment of tailings ponds, in order to achieve long-term, low-cost, source-based control of acid mine drainage (AMD). Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a passivating agent based on tannic acid-modified biochar and its application in pyrite passivation or ecological restoration. The passivating agent obtained by this method can effectively inhibit pyrite oxidation under room temperature conditions through the passivation effect of modified biochar, providing a promising technical option for large-scale mine restoration and pollution control.

[0005] To achieve the above objectives, the present invention provides a method for preparing a passivating agent based on tannic acid-modified biochar, comprising the following steps: S1. After pyrolyzing the bagasse, it is soaked in hydrochloric acid for deashing, and then placed in hydrogen peroxide solution for oxidation to obtain biochar. S2. Mix biochar with tannic acid solution, adjust the pH of the mixture to 8-8.5, and perform surface modification to obtain a passivating agent.

[0006] In this invention, the bagasse described in S1 preferably undergoes crushing, washing, and drying steps before pyrolysis; the drying temperature is preferably 60°C.

[0007] In this invention, the pyrolysis temperature in S1 is preferably 450-550℃, more preferably 500℃, the holding time is preferably 1.5-2.5h, more preferably 2h, and the heating rate to the pyrolysis temperature is preferably 8-12℃ / min, more preferably 10℃ / min.

[0008] In this invention, the concentration of hydrochloric acid in S1 is preferably 0.05-0.15 mol / L, more preferably 0.1 mol / L, and the deashing treatment time is preferably 1.5-2.5 h, more preferably 2 h; the concentration of hydrogen peroxide solution is preferably 2.5 wt%-3.5 wt%, more preferably 3 wt%, and the oxidation time is preferably 1-2 h.

[0009] In this invention, the deashing treatment in S1 can remove ash and soluble salts from bagasse; oxidation can introduce oxygen-containing functional groups on the surface of bagasse, thereby increasing its hydrophilicity and surface active site density, which facilitates the subsequent adsorption of tannic acid.

[0010] In this invention, bagasse that has undergone deashing is washed with deionized water until the pH of the washing solution reaches 6-7, then dried at 60°C to constant weight, and then oxidized in a hydrogen peroxide solution. The oxidized bagasse is then washed with deionized water until the pH of the washing solution reaches 6-7, and then dried at 60°C to constant weight to obtain biochar.

[0011] In this invention, the concentration of the tannic acid solution in S2 is preferably 2-4 g / L, more preferably 3 g / L, and the mass ratio of biochar to tannic acid solution is preferably 1:45-55, more preferably 1:50.

[0012] In this invention, the preferred method for adjusting the pH of the mixture in step S2 is to add Tris-HCl to the mixture until the pH of the mixture reaches 8-8.5.

[0013] In this invention, the surface modification temperature in S2 is preferably 18-25°C, and the time is preferably 10-14h, more preferably 12h.

[0014] This invention utilizes tannic acid to modify the surface of biochar. The phenolic hydroxyl groups in tannic acid react with the biochar surface, enhancing its adsorption capacity for metal ions. Upon contact with pyrite, the functional groups on the surface of the tannic acid-modified biochar (passivating agent) coordinate with the iron ions on the pyrite surface, forming stable Fe-OC complexes. These complexes adsorb and deposit on the pyrite surface, forming a dense passivation film that effectively prevents contact between the oxidizing medium and pyrite, reduces oxidation reactions, and inhibits the dissolution of pyrite, thus achieving the goal of suppressing the generation of acidic mine wastewater at its source.

[0015] In this invention, after the surface modification is completed, the system is filtered, the filter residue is washed with deionized water, and then dried at 60°C to obtain a passivating agent.

[0016] In this invention, the particle size of the passivating agent in S2 is 48-75 μm.

[0017] The present invention also provides a passivating agent prepared by the above preparation method.

[0018] This invention also provides the application of the above-mentioned passivating agent in pyrite passivation or ecological restoration. The passivating agent can prevent the oxidizing medium from contacting pyrite, thereby inhibiting the dissolution of pyrite and achieving the passivation effect of pyrite. At the same time, the passivating agent can repair the mine soil, improve soil fertility, and play a role in ecological restoration.

[0019] The present invention also provides a method for applying the above-mentioned passivating agent in the passivation of pyrite, comprising the following steps: (1) Mix the above passivating agent with water and sonicate to obtain a suspension; (2) Place the pyrite in a suspension, stir, let stand, and dry.

[0020] In this invention, the mass-to-volume ratio of the passivating agent to water in step (1) is preferably 0.05-0.2 g: 15 mL, the frequency of the ultrasound is preferably 30-50 kHz, more preferably 40 kHz, and the time is preferably 4-6 min, more preferably 5 min; the particle size of the pyrite in step (2) is preferably 75-150 μm, the mass ratio of pyrite to passivating agent is preferably 0.2: 0.05-0.2, the stirring speed is preferably 150-250 r / min, more preferably 200 r / min, the time is preferably 3-5 h, more preferably 4 h, the settling time is preferably 20-28 h, more preferably 24 h, the purpose of settling is to allow the passivating agent to deposit on the surface of the pyrite; the drying method is preferably air drying.

[0021] In this invention, the pyrite has a particle size of 75-150 μm and the passivating agent has a particle size of 48-75 μm. The passivating agent has a finer particle size, which can effectively cover the surface of the larger pyrite particles, thereby increasing the contact area with the pyrite and thus improving the film-forming passivation efficiency.

[0022] The present invention has the following beneficial effects: This invention provides a method for preparing a passivating agent based on tannic acid-modified biochar, comprising the following steps: S1, after pyrolyzing bagasse, immersing it in hydrochloric acid for deashing treatment, and then placing it in a hydrogen peroxide solution for oxidation to obtain biochar; S2, mixing the biochar with a tannic acid solution, adjusting the pH of the mixture to 8-8.5, and performing surface modification to obtain a passivating agent.

[0023] This invention uses bagasse as raw material to prepare a passivating agent and modifies it with tannic acid, successfully replacing the traditional high-cost passivating agent with a natural renewable resource. This significantly improves the passivation performance while greatly reducing the cost of the agent. Moreover, the preparation process does not introduce any potentially toxic or recalcitrant chemicals, and the overall process is both economical and environmentally friendly.

[0024] This invention modifies biochar with tannic acid to introduce active sites on the surface of biochar that can coordinate with iron ions on the surface of pyrite. When the resulting passivating agent comes into contact with pyrite, it can form a denser and more stable chemical passivation film on the pyrite surface, which significantly improves the antioxidant properties of pyrite and achieves a long-term and continuous reduction in its oxidation rate. Compared with the direct passivation method of tannic acid, the passivation layer formed by modified biochar (passivating agent) on the surface of pyrite has a stronger bond and better durability, and can play a more efficient role in inhibiting the leaching of iron ions and other harmful components.

[0025] The method provided by this invention uses widely available and inexpensive raw materials, has mild reaction conditions, is easy to operate, and is suitable for large-scale production.

[0026] This invention also provides the application of passivating agents in pyrite passivation or ecological restoration. When applied to pyrite passivation, the passivating agent can prevent the oxidizing medium from contacting pyrite, thereby inhibiting pyrite dissolution.

[0027] When applied to ecological restoration, passivating agents, when added to the soil, can improve soil aggregates and pore distribution through their porous structure and surface functional groups, thereby enhancing soil aeration and water retention capacity. Simultaneously, their alkaline properties help regulate and buffer soil pH, promoting the replenishment and retention of key nutrients such as carbon, nitrogen, and phosphorus, thus improving soil fertility and nutrient supply capacity. Furthermore, they can provide a suitable habitat for microorganisms, promote nutrient cycling, reduce the risk of pollutant migration, and enhance soil carbon sequestration. These effects collectively improve soil health from structural, chemical, and biological dimensions, ultimately promoting plant growth.

[0028] The passivating agent provided by this invention not only inhibits the dissolution of pyrite, but also repairs mine soil, improves soil fertility, and performs ecological restoration, providing a promising technical option for large-scale mine restoration and pollution control.

[0029] The present invention also provides a method for applying the above-mentioned passivating agent in the passivation of pyrite, comprising the following steps: (1) mixing the above-mentioned passivating agent with water, sonicating, and obtaining a suspension; (2) placing pyrite in the suspension, stirring, letting it stand, and drying.

[0030] The application method provided by this invention has a simple process, mild reaction conditions, and the resulting chemical passivation film has better stability and durability than traditional physical coating methods.

[0031] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0032] Figure 1 This is a graph showing the results of total iron leaching concentration and sulfate leaching concentration in this invention; in, Figure 1 In this context, 'a' represents the total iron leaching concentration. Figure 1 In this context, 'b' represents the sulfate leaching concentration. Figure 2 This is a comparative electrochemical diagram of the present invention; in, Figure 2 In the figure, 'a' represents the potentiodynamic polarization curve. Figure 2 In this context, 'b' represents the AC impedance spectrum. Figure 2 In this context, 'c' represents the Tafel curve. Figure 3 This is a graph showing the infrared spectral analysis results of this invention. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0034] Example 1 S1. After crushing and washing, bagasse is dried at 60°C and then placed in a pyrolysis furnace. The temperature is increased to 500°C at a rate of 10°C / min and held for 2 hours for pyrolysis. Afterward, it is immersed in 0.1 mol / L hydrochloric acid for 2 hours to remove ash. The bagasse after ash removal is washed with deionized water until the pH of the washing solution reaches 7. It is then dried at 60°C to constant weight and placed in a 3 wt% hydrogen peroxide solution for 2 hours to oxidize. The oxidized bagasse is then washed with deionized water until the pH of the washing solution reaches 7 and dried at 60°C to constant weight to obtain biochar.

[0035] S2. The above biochar was mixed with 3 g / L tannic acid solution at a mass ratio of 1:50 to obtain a mixture. Tris-HCl was added to the mixture until the pH value of the mixture reached 8.5. Then, the surface was modified at 25°C for 12 h. After the surface modification was completed, the system was filtered, the filter residue was washed with deionized water, and then dried at 60°C to obtain a passivating agent with a particle size of 48-75 μm.

[0036] Example 2 S1. After crushing and washing, bagasse is dried at 60°C and then placed in a pyrolysis furnace. The temperature is increased to 450°C at a rate of 8°C / min and held for 2.5 hours for pyrolysis. Afterward, it is immersed in 0.05 mol / L hydrochloric acid for 2.5 hours for deashing treatment. The deashed bagasse is washed with deionized water until the pH of the washing solution reaches 7. It is then dried at 60°C to constant weight and placed in a 2.5 wt% hydrogen peroxide solution for oxidization for 2 hours. The oxidized bagasse is then washed with deionized water until the pH of the washing solution reaches 7 and dried at 60°C to constant weight to obtain biochar.

[0037] S2. The above biochar was mixed with 2 g / L tannic acid solution at a mass ratio of 1:55 to obtain a mixture. Tris-HCl was added to the mixture until the pH value of the mixture reached 8. Then, the surface was modified at 18°C ​​for 14 h. After the surface modification was completed, the system was filtered, the filter residue was washed with deionized water, and then dried at 60°C to obtain a passivating agent with a particle size of 48-75 μm.

[0038] Example 3 S1. After crushing and washing, bagasse is dried at 60°C and then placed in a pyrolysis furnace. The temperature is increased to 550°C at a rate of 12°C / min and held for 1.5 hours for pyrolysis. Afterward, it is immersed in 0.15 mol / L hydrochloric acid for 1.5 hours for deashing. The deashed bagasse is washed with deionized water until the pH of the washing solution reaches 7. It is then dried at 60°C to constant weight and placed in a 3.5 wt% hydrogen peroxide solution for 1 hour for oxidation. The oxidized bagasse is then washed with deionized water until the pH of the washing solution reaches 7 and dried at 60°C to constant weight to obtain biochar.

[0039] S2. The above biochar was mixed with 4 g / L tannic acid solution at a mass ratio of 1:45 to obtain a mixture. Tris-HCl was added to the mixture until the pH value of the mixture reached 8.5. Then, the surface was modified at 18°C ​​for 10 h. After the surface modification was completed, the system was filtered, the filter residue was washed with deionized water, and then dried at 60°C to obtain a passivating agent with a particle size of 48-75 μm.

[0040] Comparative Example 1 S1. After crushing and washing, bagasse is dried at 60°C and then placed in a pyrolysis furnace. The temperature is increased to 500°C at a rate of 10°C / min and held for 2 hours for pyrolysis. Afterward, it is immersed in 0.1 mol / L hydrochloric acid for 2 hours to remove ash. The bagasse after ash removal is washed with deionized water until the pH of the washing solution reaches 7. It is then dried at 60°C to constant weight and placed in a 3 wt% hydrogen peroxide solution for 2 hours to oxidize. The oxidized bagasse is then washed with deionized water until the pH of the washing solution reaches 7 and dried at 60°C to constant weight to obtain biochar.

[0041] Application Example 1 (1) Add 0.05 g of the passivating agent prepared in Example 1 to 15 mL of ultrapure water and sonicate at 40 kHz for 5 min to obtain a suspension; (2) Add 0.2g of pyrite with a particle size of 75-150μm to the above suspension, stir at 200r / min for 4h, let stand for 24h, extract the supernatant with a syringe, and air-dry the passivated pyrite for 24h. Record it as modified SBB / py=1 / 4.

[0042] Application Example 2 (1) Add 0.1g of the passivating agent prepared in Example 1 to 15mL of ultrapure water and sonicate at 40kHz for 5min to obtain a suspension; (2) Add 0.2g of pyrite with a particle size of 75-150μm to the above suspension, stir at 200r / min for 4h, let stand for 24h, extract the supernatant with a syringe, and air-dry the passivated pyrite for 24h. Record it as modified SBB / py=1 / 2.

[0043] Application Example 3 (1) Add 0.2g of the passivating agent prepared in Example 1 to 15mL of ultrapure water and sonicate at 40kHz for 5min to obtain a suspension; (2) Add 0.2g of pyrite with a particle size of 75-150μm to the above suspension, stir at 200r / min for 4h, let stand for 24h, extract the supernatant with a syringe, and air-dry the passivated pyrite for 24h. Record it as modified SBB / py=1 / 1.

[0044] Application Example 4 (1) Add 2.5g of pyrite with a particle size of 75-150μm to 50mL of 3g / L tannic acid solution, stir at 200r / min for 2h, let stand for 24h, then extract the supernatant with a syringe, and air-dry the passivated pyrite for 24h. Record as TA+py.

[0045] Application Example 5 (1) Add 0.2 g of the biochar prepared in Comparative Example 1 to 15 mL of ultrapure water and sonicate at 40 kHz for 5 min to obtain a suspension; (2) Add 0.2g of pyrite with a particle size of 75-150μm to the above suspension, stir at 200r / min for 4h, let stand for 24h, then extract the supernatant with a syringe, and air-dry the passivated pyrite for 24h. Record as SBB / py=1 / 1.

[0046] Test example: The air-dried pyrite from Application Examples 1-5 and the unpassivated pyrite raw material (denoted as py) were added to 25 mL of hydrochloric acid with a pH of 2 for leaching experiments. The leaching time was 24 h. After leaching, the supernatant was taken to determine the leaching concentration of iron (Fe) species (including total iron and ferrous ions) and sulfate (SO42-) ions. 2- The leaching concentration was determined to obtain the inhibition rate of total iron (TFe) leaching and the inhibition rate of sulfate leaching. The passivating agent / biochar was separated from pyrite, dried, and then subjected to electrochemical and infrared spectral analysis.

[0047] The results for iron leaching concentration and sulfate leaching concentration are shown in [the table below]. Figure 1 The inhibition rates of total iron leaching and sulfate leaching are shown in Table 1. Electrochemical test results are shown in... Figure 2 The results of the infrared spectroscopy analysis are shown below. Figure 3 .

[0048] Table 1 Results of the inhibition rate of total iron and sulfate leaching

[0049] As shown in Table 1, the passivating agent prepared in this invention has a significant inhibitory effect on the leaching of iron and sulfate ions in pyrite, and the inhibition rate increases significantly with the increase of the amount of passivating agent added. Among them, the 0.20g dosage group (Application Example 3) has the best inhibition effect, with an inhibition rate of 81% for total iron leaching and 88.6% for sulfate leaching. It can also be seen that the passivating effect of the passivating agent prepared in this invention is far superior to that of tannic acid alone (Application Example 4).

[0050] Depend on Figure 1 It can be clearly seen that the leaching concentrations of iron species and sulfate ions in unpassivated pyrite are the highest, with total iron (TFe) reaching 61.12 mg / L and sulfate reaching 89.65 mg / L. The leaching concentrations in Application Examples 1-3 are significantly lower, and the higher the dosage, the lower the leaching concentration. The total iron leaching concentrations are 17.76 mg / L, 15.91 mg / L, and 11.73 mg / L, respectively, while the sulfate ion leaching concentrations are 15.06 mg / L, 12.42 mg / L, and 10.2 mg / L, respectively.

[0051] from Figure 2 It can be seen that the electrochemical activity of Application Examples 1-3 (pyrite treated with tannic acid-modified biochar) is significantly reduced. Figure 2 In example a, the oxidation peak current in the CV curve decreased significantly, with the lowest value observed in example 3. Furthermore... Figure 2 In cases b and c, as the SBB / py ratio increases, the impedance increases, hindering charge transport and leading to a decrease in corrosion current. In application example 3, the current drops to 0.09 μA / cm. 2 This indicates that the passivation layer formed on the surface of pyrite by the modified biochar (the passivating agent prepared in Example 1) effectively reduced its oxidation activity and significantly improved the antioxidant capacity of pyrite. Moreover, the effect was better than that of Application Example 4, which confirms the stabilizing and protective role of the passivation layer.

[0052] from Figure 3 It can be seen that obvious Fe-OC characteristic functional groups can be detected in Application Examples 1-3, especially in Application Example 3, at a wavelength of 570 cm⁻¹. -1 The presence of obvious Fe-O bonds confirms that the functional groups on the surface of the modified biochar undergo a coordination reaction with the iron on the surface of pyrite, forming a complex-type chemical passivation film, revealing the core mechanism of the passivation method of this invention; however, no obvious characteristic peak was found in application example 4, indicating that such a stable chemical passivation layer was not formed.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a passivating agent based on tannic acid-modified biochar, characterized in that, Includes the following steps: S1. After pyrolyzing the bagasse, it is soaked in hydrochloric acid for deashing, and then placed in hydrogen peroxide solution for oxidation to obtain biochar. S2. Mix biochar with tannic acid solution, adjust the pH of the mixture to 8-8.5, and perform surface modification to obtain a passivating agent.

2. The method for preparing a passivating agent based on tannic acid-modified biochar according to claim 1, characterized in that, The pyrolysis temperature described in S1 is 450-550℃, the holding time is 1.5-2.5h, and the heating rate to the pyrolysis temperature is 8-12℃ / min.

3. The method for preparing a passivating agent based on tannic acid-modified biochar according to claim 1, characterized in that, The concentration of hydrochloric acid in S1 is 0.05-0.15 mol / L, and the deashing treatment time is 1.5-2.5 h; The concentration of the hydrogen peroxide solution is 2.5wt%-3.5wt%, and the oxidation time is 1-2 hours.

4. The method for preparing a passivating agent based on tannic acid-modified biochar according to claim 1, characterized in that, The concentration of the tannic acid solution in S2 is 2-4 g / L, and the mass ratio of biochar to tannic acid solution is 1:45-55.

5. The method for preparing a passivating agent based on tannic acid-modified biochar according to claim 1, characterized in that, The surface modification temperature described in S2 is 18-25℃, and the time is 10-14h.

6. The method for preparing a passivating agent based on tannic acid-modified biochar according to claim 1, characterized in that, The passivating agent described in S2 has a particle size of 48-75 μm.

7. The passivating agent prepared by the method for preparing passivating agents based on tannic acid modified biochar as described in any one of claims 1-6.

8. The application of the passivating agent according to claim 7 in pyrite passivation or ecological restoration, characterized in that, The passivating agent can prevent the oxidizing medium from contacting pyrite, thereby inhibiting the dissolution of pyrite and achieving the passivation effect of pyrite. At the same time, the passivating agent can repair the mine soil, improve soil fertility, and play a role in ecological restoration.

9. The method of applying the passivating agent according to claim 8 in the passivation of pyrite, characterized in that, Includes the following steps: (1) The passivating agent of claim 7 is mixed with water and sonicated to obtain a suspension; (2) Place the pyrite in a suspension, stir, let stand, and dry.

10. The method of applying the passivating agent according to claim 9 in the passivation of pyrite, characterized in that, The mass-to-volume ratio of the passivating agent to water in step (1) is 0.05-0.2g:15mL, and the frequency of the ultrasound is 30-50kHz, with a duration of 4-6min. The pyrite in step (2) has a particle size of 75-150 μm, the mass ratio of pyrite to passivating agent is 0.2:0.05-0.2, the stirring speed is 150-250 r / min, the stirring time is 3-5 h, and the settling time is 20-28 h.

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