Method for synergistically removing fluorine and silicon in chip production wastewater

Abstract

The invention belongs to the technical field of wastewater treatment, and provides a method for synergistically removing fluorine and silicon in chip production wastewater, which comprises the following steps: adjusting the pH value of the wastewater to obtain alkalized wastewater, adding a calcium reagent into part of the alkalized wastewater, and stirring to obtain pretreated wastewater; mixing and stirring the pretreated wastewater and the residual alkalized wastewater to obtain intermediate water; and adding a coagulant into the intermediate water, stirring and precipitating, and removing fluorosilicone in the wastewater to obtain a supernatant. Compared with the prior art, the method has the advantages that the treatment time is shorter, the technical bottleneck that fluorine and silicon in the chip production wastewater are difficult to synergistically remove is broken through, and the problem of membrane pollution in the post-treatment process of the chip wastewater is greatly relieved.

Description

Technical Field

[0001] The invention belongs to the technical field of wastewater treatment and relates to a method for synergistically removing fluorine and silicon from chip production wastewater. Background Art

[0002] With the growing demand for chips in various industries, the average annual growth rate of the chip manufacturing industry has increased rapidly at a rate of two times. At the same time, since the chip manufacturing industry is an industry that is extremely dependent on clean water resources, the production of one square meter of wafers will generate up to 120 tons of wastewater, so it is very necessary to fully purify the wastewater in chip production.

[0003] In order to achieve the requirement of zero wastewater discharge, it is necessary to use deep treatment units such as reverse osmosis membrane concentration to make the effluent water quality meet the standards for reused water. This treatment method usually relies on the pretreatment process (chemical precipitation + coagulation + ultrafiltration) before reverse osmosis to avoid the corrosion of fluoride in the chip wastewater to the ultrafiltration membrane in the subsequent reverse osmosis process. At present, chemical precipitation (calcium salt defluoridation) is usually used to separate fluorine from wastewater in the form of calcium fluoride particles and coagulants.

[0004] CN105948328A discloses a low-emission purification treatment technology for fluorine-containing wastewater by fluidized bed crystallization. When the concentration of fluorine-containing wastewater is much higher than 5000ppm, it first flows into the pre-stage chemical coagulation reaction tank (7), and a calcium-containing precipitant is added to precipitate calcium fluoride sludge and discharge it. The upper clear liquid is diluted and then flows into the fluidized bed (8) to ensure that the fluoride ion concentration of the inlet water is below 5000ppm; the pH is adjusted to 5-9, and a carrier is loaded in the fluidized bed (8), and the generated calcium fluoride crystals are discharged, the precipitated sludge is discharged, and the effluent after the treatment is returned for use or discharged. In the process, the low calcium-fluoride ratio Ca / F=0.5-1.2, the water content is as low as 10%, the dosage is low, and the floor space is as small as 20%. The process can remove fluoride from wastewater quickly and efficiently under mild and easily controllable conditions, without secondary pollution, with a short process and simple operation, low reagent consumption, and low overall treatment cost. The resulting calcium fluoride crystals have low water content, are easy to separate, and have high purity, and can be used in industrial production.

[0005] CN111646591A discloses a method for treating semiconductor fluoride-containing wastewater, which belongs to the field of wastewater treatment. The treatment method comprises the following steps: adding calcium salt and conditioned sludge to the wastewater to be treated for stirring reaction to obtain a reaction solution; adding a coagulant to the reaction solution, and then performing mud-water separation to obtain a sludge containing calcium fluoride and treated clean water; wherein, the method for obtaining the conditioned sludge comprises the following steps: adding calcium salt to the sludge containing calcium fluoride for conditioning to obtain the conditioned sludge. A method for treating semiconductor fluoride-containing wastewater provided in an embodiment of the present invention can be applied to semiconductor wastewater treatment projects, and its process flow is shortened, and the fluoride emissions after treatment are stable and meet the standards. In addition, the amount of precipitation agents such as calcium salts and coagulants used in the method can be saved by 30%, and the amount of sludge generated can be reduced by 40%.

[0006] Although calcium salt defluoridation can effectively remove fluoride to prevent the corrosion of reverse osmosis membranes, the soluble silica ignored by the existing pretreatment method will still be converted into insoluble silica after high-rate membrane concentration, causing scale and blockage of the reverse osmosis membrane. Therefore, in order to ensure the performance and service life of the reverse osmosis membrane, it is urgent to find a method for the coordinated removal of fluorine and silicon.

[0007] The mechanism for removing fluoride is to add lime or other calcium salts (such as calcium chloride) to produce calcium fluoride precipitation. The optimal reaction pH is in the range of 4-7; however, since the calcium fluoride precipitate produced is very fine, a coagulant needs to be added to accelerate the solid separation process. Silica also needs to be removed by adding a coagulant. The optimal silicon removal pH of the aluminum salt coagulant used is 8-9, and the optimal silicon removal pH of the iron salt coagulant is around 9. The difference in reaction conditions required for the removal of fluoride or silica alone in wastewater determines that it is difficult to achieve effective synergistic removal of the two.

[0008] Therefore, it is of great significance to find a method to synergistically remove fluorine and silicon from chip production wastewater. Summary of the invention

[0009] Glossary:

[0010] PAC refers herein to polyaluminium chloride.

[0011] PAM herein refers to polyacrylamide.

[0012] n(Ca 2+ ):n(F - 总 ) in this article refers to the molar ratio of calcium ions added to the calcium-based reagent to all fluoride ions in the wastewater to be treated.

[0013] n(PAC):n(Fe 3+) refers to the molar ratio of polyaluminium chloride to the ferric iron ion in ferric chloride in this article.

[0014] m(PAC):m(PAM) in this article refers to the mass ratio of polyaluminium chloride to polyacrylamide.

[0015] The purpose of the present invention is to provide a method for the synergistic removal of fluorine and silicon in chip production wastewater to solve the problems existing in the prior art, so that the wastewater can be discharged in compliance with the standards after effective treatment, thereby achieving the synergistic removal of fluorine and silicon in the wastewater, improving the treatment efficiency of the wastewater, and reducing the damage of soluble silica to the ultrafiltration membrane in the post-treatment of the wastewater.

[0016] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0017] A method for synergistically removing fluorine and silicon from chip production wastewater comprises the following steps:

[0018] (1) The pH value of the wastewater is adjusted to obtain alkaline wastewater, and a calcium-based reagent is added to a portion of the alkaline wastewater for stirring to obtain pretreated wastewater.

[0019] (2) The pretreated wastewater is mixed with the remaining alkalized wastewater to obtain intermediate water.

[0020] (3) Add a coagulant to the intermediate water, stir and precipitate, remove fluorine and silicon in the wastewater, and obtain a supernatant.

[0021] The water quality characteristics of the wastewater in step (1) are as follows: a fluoride ion concentration of 50-1000 mg / L, a silicon dioxide concentration of 100-2000 mg / L, and a pH range of 1-3.

[0022] Preferably, the pH value of the alkalized wastewater in step (1) is 7-8, more preferably 7.5.

[0023] Preferably, the partially alkalized wastewater in step (1) accounts for 10-20% of the total mass of the alkalized wastewater, more preferably 10-14%, and most preferably 10%.

[0024] Preferably, the calcium-based reagent in step (1) is calcium chloride.

[0025] More preferably, the dosage of the calcium reagent is: n(Ca 2+ ):n(F - 总 )=1:2-1:1.5, optimally, n(Ca 2+ ):n(F - 总 )=1:2.

[0026] Preferably, the stirring in step (1) is first rapid stirring and then slow stirring; specifically: first stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 5-10 min.

[0027] More preferably, the mixture is first stirred at a speed of 250 rpm for 2 min, and then stirred at a speed of 50 rpm for 5 min.

[0028] Preferably, the mixing and stirring in step (2) is firstly rapid stirring and then slow stirring; specifically: firstly stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 5-10 min.

[0029] More preferably, the mixture is first stirred at a speed of 250 rpm for 2 min, and then stirred at a speed of 50 rpm for 5 min.

[0030] Preferably, the coagulant described in step (3) is a polymeric inorganic salt coagulant and an iron-based coagulant.

[0031] More preferably, the order of adding the coagulants is to first add the polymeric inorganic salt coagulant and then add the iron-based coagulant.

[0032] More preferably, the total amount of the polymeric inorganic salt coagulant and the iron coagulant added is 20-24 mmol / L, and the molar ratio is n(PAC):n(Fe 3+ )=5:1-6:1.

[0033] Still more preferably, the polymeric inorganic salt coagulant is polyaluminium chloride, and the iron-based coagulant is ferric chloride.

[0034] Optimally, the molar ratio of polyaluminium chloride to ferric chloride is n(PAC):n(Fe 3+ )=6:1.

[0035] Preferably, the stirring process in step (3) is first rapid stirring and then slow stirring; specifically: first stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 20-30 min.

[0036] More preferably, the mixture is first stirred at a speed of 250 rpm for 2 min, and then stirred at a speed of 50 rpm for 20 min.

[0037] Preferably, the precipitation time in step (3) is 20-30 min; more preferably, the precipitation time is 20 min.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The present invention extracts a portion of the alkaline wastewater after adjusting the pH and adds a calcium reagent to produce a high supersaturated concentration of calcium fluoride in the wastewater, thereby generating a large number of calcium fluoride particles, providing a large number of in-situ crystal nuclei for subsequent treatment processes, thereby saving the process of adding exogenous crystal nuclei.

[0040] (2) The present invention mixes the wastewater treated with the calcium-based reagent with the remaining wastewater, so that the newly generated calcium fluoride and the existing calcium fluoride particles grow and aggregate into larger calcium fluoride crystal nuclei, thereby improving the fluorine removal efficiency.

[0041] (3) The present invention improves the coagulation effect of the coagulant by using the large-volume calcium fluoride crystal nuclei in the generated intermediate water as a promoter of the coagulation process, and achieves the synergistic removal of soluble silica and fluorine.

[0042] (4) The present invention provides a large number of in-situ calcium fluoride particles for the growth of calcium fluoride crystals by treating the chip production wastewater in blocks and distribution, thereby simplifying the defluorination process, promoting the removal of fluoride and forming large-volume calcium fluoride crystal nuclei; under the action of large-volume crystal nuclei, coagulants and stirring, the soluble silicon dioxide in the wastewater is effectively removed. Compared with the prior art, the treatment time of the present invention is shorter, breaking through the technical bottleneck of the difficulty in the coordinated removal of fluorine and silicon in chip production wastewater, and greatly alleviating the problem of membrane pollution in the post-treatment process of chip wastewater. DETAILED DESCRIPTION

[0043] The present invention is described below by way of examples to make the technical solution of the present invention easier to understand and grasp, but the present invention is not limited thereto. The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified; and the performance of products from different sources does not have a significant impact.

[0044] Whether the fluoride content in the water after purification in the embodiment of the present invention and the comparative example is qualified or not is determined according to the first-level fluoride discharge standard of the "Comprehensive Sewage Discharge Standard" GB8978-1996. Specifically, the fluoride concentration should be less than 10 mg / L.

[0045] In the embodiments and comparative examples of the present invention, the concentration limit of silicon in the wastewater is determined to be 10 mg / L. This is because after the wastewater to be deeply treated is concentrated by a reverse osmosis membrane about ten times, the concentration of dissolved silicon dioxide can easily exceed its solubility limit (usually 100-150 mg / L at a pH close to neutral), and the production of insoluble silicon dioxide will cause scaling and clogging of the reverse osmosis membrane unit.

[0046] Example 1 A method for synergistically removing fluorine and silicon from chip production wastewater comprises the following steps:

[0047] The water quality characteristics of the wastewater used in this embodiment are: pH 1, fluoride ion content of 1000 mg / L, and silicon dioxide content of 2000 mg / L.

[0048] (1) Pre-adjustment of pH value:

[0049] The pH value of the wastewater is adjusted to 7 to obtain alkaline wastewater.

[0050] (2) Nucleation:

[0051] Take 10% by mass of the alkalized wastewater described in step (1), and add calcium chloride as n(Ca 2+ ):n(F - 总 )=1:2 was added thereto, followed by stirring at a speed of 200 rpm for 2 min and then at a speed of 50 rpm for 5 min to obtain pretreated wastewater.

[0052] (3) Calcium chloride reaction:

[0053] The remaining alkalized wastewater in step (1) is added to the pretreated wastewater in step (2), and then stirred at a speed of 200 rpm for 2 minutes and then at a speed of 50 rpm for 5 minutes to obtain intermediate water.

[0054] (4) Coagulation reaction:

[0055] The molar ratio is n(PAC):n(Fe 3+ )=5:1, add a total of 20mmol / L of polyaluminium chloride and ferric chloride to the intermediate water described in step (3), and the order of adding is: first add polyaluminium chloride, then add ferric chloride; then stir at a speed of 200rpm for 2min; then stir at a speed of 50rpm for 20min to obtain water to be precipitated.

[0056] (5) Precipitation:

[0057] The water to be precipitated in step (4) is allowed to stand for 20 minutes, and the supernatant is the purified water.

[0058] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 9.2 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 9.4 mg / L, meeting the wastewater purification needs.

[0059] Example 2 A method for synergistically removing fluorine and silicon from chip production wastewater comprises the following steps:

[0060] The water quality characteristics of the wastewater used in this embodiment are: pH 2, fluoride ion content of 285 mg / L, and silicon dioxide content of 300 mg / L.

[0061] (1) Pre-adjustment of pH value:

[0063] The pH value of the wastewater is adjusted to 8 to obtain alkaline wastewater.

[0064] (2) Nucleation:

[0065] Take 20% by mass of the alkalized wastewater described in step (1), and add calcium chloride as n(Ca 2+ ):n(F - 总 )=1:1.5 was added thereto, followed by stirring at 300 rpm for 5 min and then at 100 rpm for 10 min to obtain pretreated wastewater.

[0066] (3) Calcium chloride reaction:

[0067] The remaining alkalized wastewater in step (1) is added to the pretreated wastewater in step (2), and then stirred at a speed of 300 rpm for 5 minutes and then at a speed of 100 rpm for 10 minutes to obtain intermediate water.

[0068] (4) Coagulation reaction:

[0069] The molar ratio is n(PAC):n(Fe 3+ )=6:1, add a total amount of 24mmol / L of polyaluminium chloride and ferric chloride to the intermediate water described in step (3), and the adding order is: first add polyaluminium chloride, then add ferric chloride; then stir at a speed of 300rpm for 5min; then stir at a speed of 100rpm for 30min to obtain water to be precipitated.

[0070] (5) Precipitation:

[0071] The water to be precipitated in step (4) is allowed to stand for 30 minutes, and the supernatant is the purified water.

[0072] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 285 mg / L to 7.5 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 300 mg / L to 5.5 mg / L, meeting the wastewater purification needs.

[0073] Example 3 A method for synergistically removing fluorine and silicon from chip production wastewater, comprising the following steps:

[0074] The water quality characteristics of the wastewater used in this embodiment are: pH 3, fluoride ion content of 50 mg / L, and silicon dioxide content of 100 mg / L.

[0075] (1) Pre-adjustment of pH value:

[0076] The pH value of the wastewater is adjusted to 7.5 to obtain alkaline wastewater.

[0077] (2) Nucleation:

[0078] Take 15% by mass of the alkalized wastewater described in step (1), and add calcium chloride as n(Ca 2+ ):n(F - 总 )=1:1.8 was added thereto, followed by stirring at a speed of 250 rpm for 3 min and then at a speed of 75 rpm for 8 min to obtain pretreated wastewater.

[0079] (3) Calcium chloride reaction:

[0080] The remaining alkalized wastewater in step (1) is added to the pretreated wastewater in step (2), and then stirred at a speed of 250 rpm for 3 minutes and then at a speed of 75 rpm for 8 minutes to obtain intermediate water.

[0081] (4) Coagulation reaction:

[0082] The molar ratio is n(PAC):n(Fe 3+ )=5.5:1, add 22mmol / L of polyaluminium chloride and ferric chloride into the intermediate water in step (3), and the adding order is: first add polyaluminium chloride, then add ferric chloride; then stir at a speed of 250rpm for 3min; then stir at a speed of 75rpm for 25min to obtain water to be precipitated.

[0083] (5) Precipitation:

[0084] The water to be precipitated in step (4) is allowed to stand for 25 minutes, and the supernatant is the purified water.

[0085] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 50 mg / L to 6.4 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 100 mg / L to 5.1 mg / L, meeting the wastewater purification needs.

[0086] Comparative Example 1

[0087] This article uses Example 1 disclosed in CN111646591A as a comparative example to explore the removal effect of soluble silica in wastewater during the treatment of fluorine in chip production wastewater by the existing technology close to the present invention.

[0088] The experiment was carried out in the same manner as Example 1 disclosed in CN111646591A to obtain treated clean water; the water quality characteristics of the wastewater used were: pH 1, fluoride ion content of 1000 mg / L, and silicon dioxide content of 2000 mg / L.

[0089] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 12.2 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 23.7 mg / L, which did not meet the wastewater purification requirements.

[0090] Comparative Example 2 A method for synergistic removal of fluorine and silicon from chip production wastewater

[0091] The difference between this comparative example and Example 1 is that the pH value of the alkalized wastewater in step (1) is different, specifically, the pH value of the alkalized wastewater is 8.2.

[0092] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 27.1 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 6.6 mg / L, which did not meet the wastewater purification requirements.

[0093] Comparative Example 3: A method for synergistic removal of fluorine and silicon from chip production wastewater

[0094] The difference between this comparative example and Example 1 is that the amount of the alkalized wastewater taken in step (2) is different, specifically: 22% of the alkalized wastewater in step (1) is taken.

[0095] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 15.6 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 9.6 mg / L, which did not meet the wastewater purification requirements.

[0096] Comparative Example 4: A method for synergistic removal of fluorine and silicon from chip production wastewater

[0097] The difference between this comparative example and Example 1 is that: the amount of calcium chloride added in step (2) is different; the stirring speed and time in steps (2) and (3) are different, specifically:

[0098] (2) Nucleation:

[0099] Take 10% by mass of the alkalized wastewater described in step (1), and add calcium chloride as n(Ca 2+ ):n(F - 总 )=1:1.4 was added thereto, followed by stirring at a speed of 180 rpm for 6 min and then at a speed of 40 rpm for 12 min to obtain pretreated wastewater.

[0100] (3) Calcium chloride reaction:

[0101] The remaining alkalized wastewater in step (1) is added to the pretreated wastewater in step (2), and then stirred at a speed of 320 rpm for 1 min, and then stirred at a speed of 110 rpm for 4 min; intermediate water is obtained.

[0102] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 11.9 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 15.5 mg / L, which did not meet the wastewater purification requirements.

[0103] Comparative Example 5: A method for synergistic removal of fluorine and silicon from chip production wastewater

[0104] The difference between this comparative example and Example 1 is that the dosage of the polyaluminium chloride and ferric chloride in step (4) is different, specifically: the molar ratio is n(PAC):n(Fe 3+ )=4.8:1, add 22mmol / L of polyaluminium chloride and ferric chloride into the intermediate water in step (3).

[0105] The supernatant was taken and the fluorine content was determined by PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 13.6 mg / L. The silicon content was determined by spectrophotometry according to Determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 8.8 mg / L, which did not meet the wastewater purification requirements.

[0106] Comparative Example 6: A method for synergistic removal of fluorine and silicon from chip production wastewater

[0107] The difference between this comparative example and Example 1 is that the coagulant described in step (4) is different, specifically: PAM is additionally added at a mass ratio of m(PAC):m(PAM)=25:1.

[0108] The supernatant was taken and the fluorine content was determined by a PXSJ-216F ion meter. The fluorine content in the wastewater was reduced from 1000 mg / L to 9.4 mg / L. The silicon content was determined by spectrophotometry according to Determination of Silicon Dioxide Content in Industrial Circulating Cooling Water (GB / T 16633-1996). The silicon content in the wastewater was reduced from 2000 mg / L to 13.8 mg / L. Compared with Example 1, although additional coagulant was added, the removal efficiency of the remaining silicon was reduced by 46%. At the same time, the silicon concentration limit of 10 mg / L in the present invention does not meet the wastewater purification requirements.

[0109] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for the synergistic removal of fluorine and silicon from chip production wastewater, characterized in that: The following steps are involved: (1) adjusting the pH of the wastewater to obtain alkaline wastewater, adding a calcium-based reagent to a portion of the alkaline wastewater and stirring to obtain pretreated wastewater; (2) mixing the pretreated wastewater with the remaining alkalized wastewater to obtain intermediate water; (3) Add a coagulant to the intermediate water, stir and precipitate, remove fluorine and silicon in the wastewater, and obtain a supernatant.

2. The method according to claim 1, characterized in that The water quality characteristics of the wastewater in step (1) are: fluoride ion concentration of 50-1000 mg / L, silicon dioxide concentration of 100-2000 mg / L, and pH range of 1-3.

3. The method according to claim 1, characterized in that: The pH value of the alkalinized wastewater in step (1) is 7-8.

4. The method according to claim 1, characterized in that: The part of the alkalized wastewater in step (1) accounts for 10-20% of the total mass of the alkalized wastewater.

5. The method according to claim 1, characterized in that: The calcium reagent in step (1) is calcium chloride, and the dosage is: n(Ca 2+ ):n(F - 总 )=1:2-1:1.

5.

6. The method according to claim 1, characterized in that: The stirring in step (1) is: first stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 5-10 min.

7. The method according to claim 1, characterized in that: The mixing and stirring in step (2) is: first stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 5-10 min.

8. The method according to claim 1, characterized in that The coagulant described in step (3) is a polymeric inorganic salt coagulant and an iron-based coagulant.

9. The method according to claim 8, characterized in that The total amount of the polymeric inorganic salt coagulant and the iron coagulant added is 20-24 mmol / L, and the molar ratio is n(PAC):n(Fe 3+ )=5:1-6:

1.

10. The method according to claim 1, characterized in that The stirring in step (3) is: first stirring at a speed of 200-300 rpm for 2-5 min; then stirring at a speed of 50-100 rpm for 20-30 min; the precipitation time is 20-30 min.