A method for treating shale gas fracturing flowback fluid

Abstract

This invention relates to the technical field of fracturing flowback fluid treatment, specifically to a method for treating shale gas fracturing flowback fluid. The method includes air flotation, coagulation sedimentation, electrolytic ammonia removal, activated carbon adsorption ultrafiltration, nanofiltration, reverse osmosis (RO), resin adsorption, and an iodine removal system. This invention combines shale gas fracturing flowback fluid treatment with iodine removal technology to remove and recover iodine from the reverse osmosis concentrate, meeting the 1 mg / L iodine content limit for chlor-alkali plants, thus reducing the energy consumption of traditional RO concentrate evaporation and concentration. By utilizing the 3-5% sodium chloride component in the shale gas fracturing flowback fluid, electrolysis is performed to generate chlorine and sodium hydroxide, which then oxidize ammonia nitrogen to nitrogen gas, achieving ammonia nitrogen removal. Combined with the use of an activated carbon submerged ultrafiltration membrane, most of the COD is effectively retained, significantly reducing the COD concentration in subsequent processes, improving the treatment effect of shale gas fracturing flowback fluid, and reducing treatment costs.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fracturing flowback fluid treatment, and particularly relates to a shale gas fracturing flowback fluid treatment method. BACKGROUND

[0002] As a kind of unconventional natural gas resource, shale gas has unique geological characteristics of low porosity and low permeability. In the development process of shale gas, a large amount of fracturing flowback fluid is generated in hydraulic fracturing operation. This kind of wastewater has the characteristics of complex composition, high viscosity, high emulsification and high salinity, which leads to high difficulty in treatment and high risk of environmental pollution. The shale gas fracturing flowback fluid usually contains high-concentration salt, high-concentration ammonia nitrogen and organic pollutants (COD) with poor biodegradability, and has special problems such as slow ammonia nitrogen nitrification rate and abnormal increase of iodine ion concentration.

[0003] At present, the overall shale gas fracturing flowback fluid treatment technology in China is still in the initial stage of research and application, and the technical system is not mature. In the prior art, the concentrated water generated by reverse osmosis is first concentrated to about 15% by electrodialysis technology, or concentrated to about 8% by disc tube reverse osmosis (DTRO) technology, and then subjected to evaporation crystallization process to finally form sodium chloride solid. However, this method generally has technical bottlenecks such as complex and long process flow, high running cost and easy secondary pollution in actual engineering application. In addition, the efficient removal technology of iodine element in shale gas fracturing flowback fluid is still blank. Due to its special chemical properties, iodine element is difficult to be effectively removed in traditional treatment process, which causes serious interference and pollution to the subsequent treatment unit.

[0004] Therefore, it has become a key technical problem to be solved in this field to develop a technical solution that can efficiently remove iodine ions in fracturing flowback fluid while considering the treatment of other pollutants. SUMMARY

[0005] The purpose of the present application is to provide a shale gas fracturing flowback fluid treatment method, which can remove iodine ions in reverse osmosis concentrated water of shale gas fracturing flowback fluid while considering the treatment of other pollutants, so as to facilitate subsequent concentrated water treatment.

[0006] The solution adopted by the present application to achieve the purpose is as follows: a shale gas fracturing flowback fluid treatment method, comprising the following steps: (1) aerating the shale gas fracturing flowback fluid; (2) removing oil from the effluent in step (1) by air flotation; (3) sequentially adding chemicals to the effluent in step (2) for chemical softening and hardness removal, and adding a flocculating agent for coagulation and sedimentation; (4) electrolyzing the effluent in step (3) to remove ammonia; (5) The effluent from step (4) is subjected to powdered activated carbon immersion adsorption ultrafiltration; (6) Separate the effluent from step (5) through a nanofiltration membrane to obtain nanofiltration concentrate and nanofiltration desalinate; (7) The nanofiltration desalinated water obtained in step (6) is subjected to high-salt reverse osmosis separation to obtain reverse osmosis concentrate and reverse osmosis desalinated water; (8) The reverse osmosis freshwater from step (7) is filtered through resin to meet discharge standards; (9) Pass the reverse osmosis concentrate from step (7) into the iodine removal system, and send the iodine-removed solution to the chlor-alkali plant for treatment.

[0007] Preferably, in step (2), the air flotation oil removal includes removing dissolved petroleum, organic matter, and suspended solids by using microbubbles, polyaluminum chloride, and anionic polyacrylamide.

[0008] Preferably, in step (3), the added chemicals include NaOH and NaCO3, and the pH value is adjusted to above 10. The flocculant includes polyaluminum chloride and polyacrylamide.

[0009] Preferably, in step (4), the electrolysis uses a titanium-based ruthenium-plated anode plate and a titanium cathode plate, and the electrolysis voltage is 3~4V / cm and the current is 5~20mA / cm. 2 The DC power supply for electrolysis is related to the water volume and ammonia nitrogen concentration, and the setting range is 0.075~0.1 kWh multiplied by the water volume (m). 3 / h multiplied by the ammonia nitrogen concentration in mg / L, the hydraulic retention time of the reaction after electrolysis is related to the concentration of the incoming water. It requires 1 hour for concentrations below 100 mg / L, and 1 hour for every additional 100 mg / L.

[0010] Preferably, in step (5), the ratio of powdered activated carbon dosage to COD in the powdered activated carbon submerged adsorption ultrafiltration is 5~10:1.

[0011] Preferably, in step (6), the nanofiltration concentrate is subjected to two-stage nanofiltration and high-pressure reverse osmosis for concentration in sequence. The high-pressure reverse osmosis concentrate is evaporated, concentrated and crystallized to obtain NaSO4 and NaCl mixed salts. The high-pressure reverse osmosis desalinated water enters the high-salt reverse osmosis process in step (7).

[0012] Preferably, in step (8), the resin is regenerated with NaCl solution after saturation, and the regenerated waste liquid enters the denitrification reactor for denitrification reaction.

[0013] Preferably, the nitrate removal vessel load in the denitrification reactor is 0.3~1.8 kg / m³. 3 .

[0014] Preferably, by adopting a denitrification process that combines ion exchange with subsequent nitrification and denitrification processes, the effluent can meet the surface water discharge limit of 1 mg / L for total nitrogen. The volume of regenerated waste liquid is less than 5% of the total water volume, reducing the required tank volume and land area for post-biological treatment, thus saving land area.

[0015] Preferably, in step (9), the iodine removal system treatment includes the following steps: (a) Add hydrochloric acid to the reverse osmosis concentrate after high-salt reverse osmosis separation to adjust the pH value to 1.2~2.5; (b) Add an oxidizing agent to the solution from step (a) to oxidize it; (c) Aerate the solution from step (b) in stages; (d) The solution from step (c) is passed through a granular activated carbon adsorption tank to adsorb iodine. (e) Take out the activated carbon from step (d), rinse it with fresh water, drain it, and heat it with protective gas to allow the iodine to volatilize. Then, the iodine element is recovered by condensing it on the cold wall. After that, the temperature is raised again and water vapor is introduced. After cooling, the activated carbon is regenerated and recycled, thus avoiding the generation of hazardous waste.

[0016] Preferred, In step (b), the oxidant includes at least one of hydrogen peroxide, sodium hypochlorite, and ozone, and the dosage is 1.1 to 1.2 times the molar ratio required for oxidizing iodide ions; In step (c), the aeration is a four-stage aeration, lasting 1-6 hours, and after aeration, 0-20 mg / L of iron salt is added to the solution; the iron salt includes at least one of ferric chloride, ferrous chloride, and ferrous sulfate.

[0017] In step (d), the granular activated carbon has a particle size of 1-3 mm, an ash content of less than 5% after acid washing, an iodine adsorption value greater than 1000 mg / g, and a characteristic specific surface area greater than 1000 m². 2 / g, T-Polt method microporous specific surface area greater than 800 m² 2 / g, wherein the adsorption contact time is 2~6h; In step (e), the protective gas includes nitrogen, the heating temperature is 220~230℃, and the secondary heating is raised to 350~450℃.

[0018] Preferred, When the oxidant mentioned in step (b) is sodium hypochlorite, in step (c), 0~20 mg / L of iron salt is added; When the oxidant in step (b) is hydrogen peroxide and ozone, no iron salt is added in step (c).

[0019] Compared with the prior art, the present invention has the following advantages: This invention combines shale gas fracturing flowback fluid treatment with iodine removal technology to remove and recover iodine from reverse osmosis concentrate, ensuring it meets the 1 mg / L iodine content limit for chlor-alkali plants. This reduces the energy consumption of traditional reverse osmosis concentrate evaporation and concentration, and lowers treatment costs.

[0020] This invention utilizes 3-5% sodium chloride in shale gas fracturing flowback fluid, electrolyzes it to produce chlorine and sodium hydroxide, and then reacts the resulting sodium hypochlorite solution with ammonia nitrogen in the water to oxidize the ammonia nitrogen into nitrogen gas, thereby achieving the effect of removing ammonia nitrogen. It has the advantages of small footprint, low reagent dosage, and high stability.

[0021] This invention effectively removes most of the COD by using an activated carbon submerged ultrafiltration membrane, which significantly reduces the COD concentration in the subsequent nanofiltration concentrate and has little impact on the secondary concentration process. The concentrated solution can then be successfully evaporated and crystallized, avoiding coking in the reactor. Attached Figure Description

[0022] Figure 1 This is a flowchart of the shale gas fracturing flowback fluid treatment method in Example 1; Figure 2 This is a process flow diagram of the iodine removal system in Example 1; Figure 3 This is a flowchart of the activated carbon iodine desorption and regeneration process in Example 1. Detailed Implementation

[0023] To better understand the present invention, the following embodiments are further illustrations of the present invention, but the content of the present invention is not limited to the following embodiments.

[0024] Example 1 Taking a shale gas fracturing flowback fluid with a total hardness of 1500-3500 mg / L, total dissolved solids (TDS) of 30000-40000 mg / L, chloride ion concentration of 18000-35000 mg / L, chemical oxygen demand (COD) of 200-800 mg / L, ammonia nitrogen of 40-500 mg / L, oil content of 30-200 mg / L, total phosphorus of 5 mg / L, and iodine concentration of 3-12 mg / L as an example, this study demonstrates its treatment. The flowchart of the shale gas fracturing flowback fluid treatment method used in this embodiment is shown below. Figure 1 As shown.

[0025] (1) The shale gas fracturing flowback fluid is first discharged into the aeration regulating tank, and air is introduced into the bottom of the tank to promote the precipitation and coagulation of oil and modified guar gum in the shale gas fracturing flowback fluid, thereby reducing the chemical oxygen demand.

[0026] (2) Use a raw water lift pump to transport the shale gas fracturing flowback fluid into the flotation tank, add 80~500mg / L of polyaluminum chloride and 1~5mg / L of anionic polyacrylamide for flotation oil removal, which can remove more than 90% of the oil and reduce the suspended solids in the shale gas fracturing flowback fluid. Pass the sludge into the thickening tank and use a plate and frame filter press for dewatering.

[0027] (3) After aeration, the effluent from the flotation tank is introduced into a chemical softening sedimentation tank. NaCO3 and NaOH solutions are added to the tank, and the pH value is adjusted to above 10. Then, polyaluminum chloride and polyacrylamide are added to remove Mg from the wastewater through coagulation and sedimentation. 2+ Ca 2+ Ba 2+ Sr 2+ Equal-hardness ions are used to reduce hardness to below 200 mg / L. The sludge produced by coagulation and hardening is also fed into a thickening tank and dewatered using a plate and frame filter press.

[0028] (4) The effluent from the chemical softening sedimentation tank is fed into the electrolytic ammonia removal unit. The main component of this wastewater is NaCl. Electrolysis is performed using titanium-based ruthenium-plated anode plates and titanium cathode plates. The DC voltage between the electrodes of the electrolysis reaction tank is 3~4V / cm, and the current is 5~20mA / cm. 2 The chlorine gas produced by electrolysis dissolves in the water to form sodium hypochlorite, which reacts with ammonia nitrogen to form nitrogen gas, thus removing most of the ammonia nitrogen. A small amount of nitrate is produced as a side reaction. The change in ammonia nitrogen before and after electrolysis is measured using ammonium ion electrodes to determine the appropriate DC power supply for the electrolytic ammonia removal unit. The DC power supply power is related to the water volume and ammonia nitrogen concentration, and the setting range is 0.1 kWh multiplied by the water volume (m). 3 / h multiplied by the ammonia nitrogen concentration in mg / L; the hydraulic retention time of the reaction after electrolysis is related to the concentration of the incoming water. For concentrations below 100 mg / L, 1 hour is required, and for every additional 100 mg / L, it is preferable to increase by 1 hour.

[0029] (5) The effluent from the electrolytic ammonia removal unit is fed into a submerged ultrafiltration membrane tank with powdered activated carbon. The powdered activated carbon adsorbs COD-type organic pollutants in the water and consumes residual chlorine in the reaction. A centrifugal pump is arranged after the ultrafiltration membrane to form a negative pressure to draw the permeate into the permeate tank. A backwashing pump draws water from the permeate tank for backwashing, which helps to reduce colloidal substances and reduce subsequent nanofiltration membrane clogging. The ratio of powdered activated carbon dosage to COD can be selected as 5~10:1, the hydraulic retention time of the membrane tank is 1~4h, and the membrane flux is selected as 10~25L / m³. 2 h, the preferred design parameter is 15 L / m 2 h.

[0030] (6) The effluent from the powdered activated carbon submerged ultrafiltration membrane tank is fed into a nanofiltration membrane device. Nanofiltration is carried out at a pressure of 1.2~1.5MPa with a nanofiltration concentrate to nanofiltration desalinate ratio of 2:8 to remove most of the COD and SO4.2+ Mg 2+ Ca 2+ Large molecules are enriched in nanofiltration concentrate.

[0031] (7) The nanofiltration concentrate from step (6) is subjected to two-stage nanofiltration to further increase the concentration of ions and COD in the concentrate. Then, it is concentrated by high-pressure reverse osmosis at 10 MPa to obtain high-pressure reverse osmosis fresh water and high-pressure reverse osmosis concentrate. The high-pressure reverse osmosis concentrate is subjected to mechanical vapor recompression (MVR) or multi-effect evaporation for concentration and crystallization to obtain a mixed salt of sodium sulfate and sodium chloride.

[0032] (8) The nanofiltration desalinated water from step (6) and the high-pressure reverse osmosis desalinated water from step (7) are fed into the high-salt reverse osmosis system. The working pressure is 4~6MPa to further separate the salts in the water and obtain reverse osmosis concentrate and reverse osmosis desalinated water.

[0033] (9) The reverse osmosis freshwater is further treated by passing it through a nitrate anion exchange resin tank. The filtration rate of the ion exchange tank is 10~25m / h to adsorb the small amount of residual NO3 in the water. - This ensures that the effluent meets the environmental standard limit of 1 mg / L for nitrogen.

[0034] (10) After the resin in step (9) is saturated, it is regenerated using a 6-10% sodium chloride solution. The regenerated wastewater enters the regeneration conditioning tank to control the salt content at 2%-3.5%, and then enters the denitrification reactor for denitrification. The nitrate removal container load in the denitrification reactor is controlled within the range of 0.3-1.8 kg / m³, preferably a low load of 0.4 kg / m³, to improve the high stability of the biological system. The effluent after the reaction is returned to the regeneration conditioning tank. Among them, the nitrification filter and the denitrification filter need to adopt the salt-tolerant bacteria biofilm technology. During the acclimation process of the salt-tolerant bacteria, the salt concentration should be gradually increased from 1% to 3.5% in a gradient manner. Conventional nutrients such as potassium, calcium, magnesium, sulfur, sodium carbonate, etc. need to be added to the tank, and trace elements including zinc, manganese, iron, copper, and cobalt salts should be supplemented. The nitrification tank converts ammonia nitrogen into nitrite and nitrate through aeration oxidation; the denitrification tank needs to add a carbon source to carry out the denitrification reaction to achieve denitrification. During operation, controlling the magnesium ion concentration at 20-30 mg / L, potassium ion concentration at 40-80 mg / L, and phosphate concentration at 2-5 mg / L helps denitrifying bacteria resist sodium ion osmotic pressure, enhances their proliferation capacity, and thus increases the biochemical rate of nitrate reduction to nitrogen. A BOD:N ratio of 3-4 is recommended for carbon source addition. By subjecting 5% of the total water volume to a 24-hour biochemical reaction in the regenerated wastewater, the total nitrogen content can be reduced to below 10 mg / L.

[0035] (11) The reverse osmosis concentrate from step (8) is fed into the iodine removal system. Hydrochloric acid is added through a pipeline mixer to adjust the pH to 1.5-2, and then the solution enters the oxidation reaction tank. In the oxidation reaction tank, 11% sodium hypochlorite is added in batches, with a total dosage of 1.1 times the molar ratio required for iodine ion oxidation. The solution is then fed into a four-stage aeration tank and aerated for 2-4 hours to form I₂ and I₂Cl. - Ferrous chloride is added at the end of the aeration process, followed by iodine adsorption via a granular activated carbon filter. The preferred coal-based activated carbon has an iodine adsorption value of 1000 mg / g, an ash content of less than 5% after hydrochloric acid washing, and a specific surface area greater than 1000 m² using the BET gas adsorption method for iodine adsorption. 2 / g, T-polt method microporous specific surface area greater than 800 m² 2 / g. The iodine removal system used in this step is such as... Figure 2 As shown.

[0036] (12) When the granular activated carbon in step (11) is saturated with adsorption, it is removed, rinsed with fresh water, and drained with hot air at 60°C. Then, nitrogen is introduced as a protective gas, and the carbon is heated to 220~230°C in an atmosphere furnace and maintained for 2~4 hours to allow the iodine to volatilize and solidify on the cold wall to recover the iodine element. Afterward, the temperature is raised to 350°C and high-temperature steam is introduced to complete the desorption of iodine from the activated carbon. The activated carbon used in this step for iodine desorption and regeneration is as follows: Figure 3 As shown.

[0037] Example 2 The only difference from Example 1 is that 5 mg / L of ferrous oxide is added at the aeration end in step (11) before the activated carbon filter can adsorb iodine.

[0038] Example 3 The only difference from Example 1 is that 20 mg / L of ferrous oxide is added at the aeration end in step (11) and then iodine is adsorbed by activated carbon filter.

[0039] Example 4 The only difference from Example 1 is that the oxidant 11% sodium hypochlorite in step (11) is replaced with the oxidant 28% hydrogen peroxide, and the amount of oxidant added is adjusted accordingly.

[0040] Example 5 The only difference from Example 4 is that 10 mg / L of ferrous oxide is added at the aeration end in step (11) before the activated carbon filter can adsorb iodine.

[0041] Example 6 The only difference from Example 4 is that 20 mg / L of ferrous oxide is added at the aeration end in step (11) and then iodine is adsorbed by activated carbon filter.

[0042] Example 7 The only difference from Example 1 is that the oxidant 11% sodium hypochlorite in step (11) is replaced with ozone, and the dosage of oxidant is adjusted accordingly; and 15 mg / L of ferrous oxide is added at the end of the aeration process, and then iodine is adsorbed in the activated carbon filter.

[0043] Table 1. Iodine removal rate with different oxidants and ferrous chloride dosages

[0044] As shown in Table 1, hydrogen peroxide, sodium hypochlorite, and ozone can all effectively oxidize iodide ions to iodine molecules, which are then removed using the superior adsorption capacity of activated carbon. Considering the complexity of the equipment and the ease of control, the dosage of ozone is difficult to assess, the exhaust gas requires further purification, and the raw material for ozone production is generally liquid oxygen, making the equipment complex. Therefore, hydrogen peroxide or sodium hypochlorite is usually used. Comparing the dosage and effectiveness of hydrogen peroxide and sodium hypochlorite, hydrogen peroxide is superior in terms of cost and efficiency. Furthermore, the addition of an appropriate amount of ferrous chloride can further improve the iodine removal rate. When using sodium hypochlorite as an oxidant, adding 20 mg / L of ferrous chloride is preferred, as it can significantly improve the iodine removal rate.

[0045] Ferric chloride and ferrous sulfate have similar effects to ferrous chloride.

[0046] Table 2 Regeneration recovery rate under different activated carbon regeneration conditions

[0047] As shown in Table 2, considering both iodine recovery efficiency and activated carbon regeneration recovery rate, heating at 230℃ for 3 hours restores approximately 66% of the iodine adsorption capacity. Subsequent regeneration and activation at 350℃~450℃ can restore over 97% of the iodine adsorption efficiency, thus achieving recycling. Considering heating energy consumption and effectiveness, heating at 220℃~230℃ for 3 hours followed by a second activation at 350℃ for activated carbon regeneration is preferred.

[0048] The above description is merely a preferred embodiment of the present invention, and should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for treating shale gas fracturing flowback fluid, characterized in that, Includes the following steps: (1) Aeration treatment of shale gas fracturing flowback fluid; (2) Remove oil from the effluent in step (1) by air flotation; (3) Add chemicals to the effluent from step (2) in sequence to soften and remove hardness, and add flocculant for coagulation and sedimentation; (4) Electrolyze the effluent from step (3) to remove ammonia; (5) The effluent from step (4) is subjected to powdered activated carbon immersion adsorption ultrafiltration; (6) Separate the effluent from step (5) through a nanofiltration membrane to obtain nanofiltration concentrate and nanofiltration desalinate; (7) The nanofiltration desalinated water obtained in step (6) is subjected to high-salt reverse osmosis separation to obtain reverse osmosis concentrate and reverse osmosis desalinated water; (8) The reverse osmosis desalination water from step (7) is filtered through resin to meet discharge standards; (9) Pass the reverse osmosis concentrate from step (7) into the iodine removal system, and send the iodine-removed solution to the chlor-alkali plant for treatment.

2. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (2), the air flotation oil removal includes removing dissolved petroleum, organic matter, and suspended solids by using microbubbles, polyaluminum chloride, and anionic polyacrylamide.

3. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (3), the added chemicals include NaOH and NaCO3, and the pH value is adjusted to above 10. The flocculant includes polyaluminum chloride and polyacrylamide.

4. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (4), the electrolysis uses a titanium-based ruthenium-plated anode plate and a titanium cathode plate, and the electrolysis voltage is 3~4V / cm, and the current is 5~20mA / cm. 2 .

5. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (5), the ratio of powdered activated carbon dosage to COD in the submerged adsorption ultrafiltration of powdered activated carbon is 5~10:

1.

6. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (6), the nanofiltration concentrate is subjected to two-stage nanofiltration and high-pressure reverse osmosis for concentration in sequence. The high-pressure reverse osmosis concentrate is evaporated, concentrated and crystallized to obtain NaSO4 and NaCl mixed salts. The high-pressure reverse osmosis desalinated water enters the high-salt reverse osmosis process in step (7).

7. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (8), the resin is regenerated with NaCl solution after saturation, and the regenerated waste liquid enters the denitrification reactor for denitrification reaction.

8. The method for treating shale gas fracturing flowback fluid according to claim 7, characterized in that, The nitrate removal vessel load in the denitrification reactor is 0.3~1.8 kg / m³.

9. The method for treating shale gas fracturing flowback fluid according to claim 1, characterized in that, In step (9), the iodine removal system treatment includes the following steps: (a) Add hydrochloric acid to the reverse osmosis concentrate after high-salt reverse osmosis separation to adjust the pH value to 1.2~2.5; (b) Add an oxidizing agent to the solution from step (a) to oxidize it; (c) Aerate the solution from step (b) in stages; (d) The solution from step (c) is passed through a granular activated carbon adsorption tank to adsorb iodine. (e) Take out the activated carbon from step (d), rinse it with fresh water, drain it, and heat it with protective gas to allow the iodine to volatilize. Then, the iodine element is recovered by condensing it on the cold wall. After that, the temperature is raised again and water vapor is introduced. After cooling, the activated carbon is regenerated and recycled.

10. The method for treating shale gas fracturing flowback fluid according to claim 9, characterized in that, In step (b), the oxidant includes at least one of hydrogen peroxide, sodium hypochlorite, and ozone, and the dosage is 1.1 to 1.2 times the molar ratio required for oxidizing iodide ions; In step (c), the aeration is a four-stage aeration, lasting 1-6 hours, and after aeration, 0-20 mg / L of iron salt is added to the solution; the iron salt includes at least one of ferrous chloride, ferric chloride, and ferrous sulfate. In step (d), the granular activated carbon has a particle size of 1-3 mm, an ash content of less than 5% after acid washing, an iodine adsorption value greater than 1000 mg / g, and a characteristic specific surface area greater than 1000 m². 2 / g, T-Polt method microporous specific surface area greater than 800 m² 2 / g, wherein the adsorption contact time is 2~6h; In step (e), the protective gas includes nitrogen, the heating temperature is 220~230℃, and the secondary heating is raised to 350~450℃.

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