Method for removing fluorine and silicon in chip production wastewater

Abstract

The application belongs to the technical field of wastewater treatment, and provides a method for removing fluorine and silicon in chip production wastewater, which comprises the following steps: adjusting the pH of the wastewater to obtain alkalized wastewater, adding a calcium-based reagent into part of the alkalized wastewater and stirring to obtain pretreated wastewater, mixing and stirring the pretreated wastewater with the remaining alkalized wastewater to obtain intermediate water, adding a coagulant into the intermediate water, stirring and precipitating to remove fluorine and silicon in the wastewater, and obtaining supernatant. Compared with the prior art, the application has a shorter treatment time, breaks through the technical bottleneck that fluorine and silicon in chip production wastewater are difficult to be removed simultaneously, and greatly relieves the membrane pollution problem in the post-treatment process of chip wastewater.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology and relates to a method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater. Background Technology

[0002] With the ever-increasing demand for chips across various industries, the chip manufacturing industry is experiencing a rapid annual growth rate that is more than doubling. Furthermore, as the chip manufacturing industry is extremely reliant on clean water resources—producing one square meter of wafer generates up to 120 tons of wastewater—thus, thorough purification of wastewater from chip production is essential.

[0003] To achieve zero wastewater discharge, advanced treatment units such as reverse osmosis membrane concentration are needed to ensure the effluent meets reuse standards. This treatment method typically relies on pretreatment processes before reverse osmosis (chemical precipitation + coagulation + ultrafiltration) to prevent fluoride in the chip wastewater from corroding the ultrafiltration membrane in the subsequent reverse osmosis process. Currently, chemical precipitation (calcium salt defluorination) is commonly used to separate fluoride from the wastewater in the form of calcium fluoride particles and coagulants.

[0004] CN105948328A discloses a low-emission purification technology for fluoride-containing wastewater using fluidized bed crystallization. For wastewater with a concentration far exceeding 5000 ppm, the wastewater first flows into a pre-stage chemical coagulation reaction tank (7), where a calcium-containing precipitant is added to cause calcium fluoride sludge to settle and be discharged. The supernatant is diluted and then flows into the fluidized bed (8) to ensure the influent fluoride ion concentration is below 5000 ppm. The pH is adjusted to 5-9, and a support is loaded into the fluidized bed (8). The generated calcium fluoride crystals and settled sludge are discharged, and the treated effluent is either reused or discharged. In this process, the calcium-fluoride ratio (Ca / F) is 0.5-1.2, the water content is as low as 10%, the dosage is low, and the footprint is as small as 20%. The process can remove fluoride from wastewater quickly and efficiently under mild and easily controllable conditions, without secondary pollution. The process is short and easy to operate, with low reagent consumption and low overall treatment cost. The calcium fluoride crystals produced have low water content, are easy to separate, and have high purity, making them suitable for industrial production.

[0005] CN111646591A discloses a method for treating semiconductor fluoride-containing wastewater, belonging to the field of wastewater treatment. The method includes the following steps: adding calcium salts and conditioned sludge to the wastewater to be treated and stirring to obtain a reaction solution; adding a coagulant to the reaction solution and then performing sludge-water separation to obtain calcium fluoride-containing sludge and treated clean water; wherein, the method for obtaining the conditioned sludge includes the following steps: adding calcium salts to the calcium fluoride-containing sludge for conditioning to obtain the conditioned sludge. This invention provides a method for treating semiconductor fluoride-containing wastewater, which can be applied to semiconductor wastewater treatment projects. Its process flow is shortened, and the treated fluoride emissions are stable and meet standards. Furthermore, the amount of precipitating agents such as calcium salts and coagulants used in this method can be reduced by 30%, and the amount of sludge generated can be reduced by 40%.

[0006] While calcium salt defluorination can effectively remove fluorides and prevent reverse osmosis membrane corrosion, existing pretreatment methods neglect dissolved silica, which can still transform into insoluble silica after high-rate membrane concentration, causing scaling and clogging of the reverse osmosis membrane. Therefore, finding a synergistic fluoride and silica removal method is urgently needed to ensure the stable performance and lifespan of the reverse osmosis membrane.

[0007] The removal mechanism of fluoride involves the addition of lime or other calcium salts (such as calcium chloride) to produce calcium fluoride precipitate, with the optimal reaction pH range of 4-7. However, because the resulting calcium fluoride precipitate is very fine, coagulants are needed to accelerate the solid separation process. Silica also requires the addition of coagulants for removal. The optimal silica removal pH for aluminum salt coagulants is 8-9, while that for iron salt coagulants is around 9. The difference in reaction conditions required for the individual removal of fluoride or silica from wastewater makes effective synergistic removal difficult.

[0008] Therefore, finding a method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater is of great significance. Summary of the Invention

[0009] Definitions:

[0010] In this article, PAC refers to polyaluminum chloride.

[0011] In this article, PAM refers to polyacrylamide.

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

[0013] n(PAC):n(Fe 3+In this article, "()" refers to the molar ratio of ferric ions in polyaluminum chloride and ferric chloride.

[0014] In this paper, m(PAC):m(PAM) refers to the mass ratio of polyaluminum chloride to polyacrylamide.

[0015] The purpose of this invention is to provide a method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater, in order to solve the problems existing in the prior art. This method enables the wastewater to meet discharge standards after effective treatment, thereby achieving the synergistic removal of fluorine and silicon from the wastewater, improving the wastewater treatment efficiency, and reducing the damage of dissolved silica to the ultrafiltration membrane during wastewater post-treatment.

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

[0017] A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater includes the following steps:

[0018] (1) Adjust the pH of the wastewater to obtain alkalized wastewater. Add calcium-based reagent to part of the alkalized wastewater and stir to obtain pretreated wastewater.

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

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

[0021] The wastewater in step (1) has the following characteristics: fluoride ion concentration of 50-1000 mg / L, silica concentration of 100-2000 mg / L, and pH range of 1-3.

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

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

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

[0025] More preferably, the dosage of the calcium-based reagent is: n(Ca) based on a molar ratio. 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 fast stirring and then slow stirring; specifically: first stirring at a speed of 200-300 rpm for 2-5 minutes; then stirring at a speed of 50-100 rpm for 5-10 minutes.

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

[0028] Preferably, the mixing and stirring in step (2) is to first stir quickly and then stir slowly; specifically: first stir at a speed of 200-300 rpm for 2-5 minutes; then stir at a speed of 50-100 rpm for 5-10 minutes.

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

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

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

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

[0033] More preferably, the polymeric inorganic salt coagulant is polyaluminum chloride, and the iron-based coagulant is ferric chloride.

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

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

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

[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 first extracts a portion of the alkalized wastewater after pH adjustment and adds calcium-based reagents to generate calcium fluoride with a high supersaturation concentration in the wastewater, generating a large number of calcium fluoride particles, which provide a large number of in-situ crystal nuclei for the subsequent treatment process, saving the process of adding exogenous crystal nuclei.

[0040] (2) The present invention improves the removal efficiency of fluoride by mixing the wastewater treated with calcium-based reagents 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.

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

[0042] (4) This invention provides a large number of in-situ calcium fluoride particles for the growth of calcium fluoride crystals by treating chip manufacturing wastewater in a segmented and distributed manner. This simplifies the defluorination process, promotes fluoride removal, and forms large-volume calcium fluoride crystal nuclei. Under the action of large-volume crystal nuclei, coagulants, and stirring, soluble silica in the wastewater is effectively removed. Compared with the prior art, this invention has a shorter processing time, overcomes the technical bottleneck of the difficulty in the synergistic removal of fluoride and silicon in chip manufacturing wastewater, and greatly alleviates the problem of membrane fouling in the post-treatment of chip wastewater. Detailed Implementation

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

[0044] Whether the fluoride content in the purified water of the embodiments and comparative examples of the present invention is qualified is determined according to the first-level discharge standard of fluoride in the "Integrated Wastewater 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 set at 10 mg / L. This is because after the wastewater to be treated is concentrated by a reverse osmosis membrane by about ten times, the concentration of dissolved silica is very likely to exceed its solubility limit (usually 100-150 mg / L at a near-neutral pH). The resulting insoluble silica will cause scaling and clogging of the reverse osmosis membrane unit.

[0046] Example 1: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater, comprising the following steps:

[0047] The wastewater used in this embodiment has the following characteristics: pH 1, fluoride ion content 1000 mg / L, and silica content 2000 mg / L.

[0048] (1) Pre-adjust pH value:

[0049] The pH of the wastewater was adjusted to 7 to obtain alkalized wastewater.

[0050] (2) Crystal nucleus formation:

[0051] Take 10% by mass of the alkalized wastewater described in step (1), and add calcium chloride in the form of n(Ca 2+ ):n(F - 总 Add the mixture in a ratio of 1:2, then stir at 200 rpm for 2 minutes, and then stir at 50 rpm for 5 minutes to obtain pretreated wastewater.

[0052] (3) Reaction of calcium chloride:

[0053] Add the remaining alkalized wastewater from step (1) to the pretreated wastewater from step (2), then stir at 200 rpm for 2 minutes and then at 50 rpm for 5 minutes to obtain intermediate water.

[0054] (4) Coagulation reaction:

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

[0056] (5) Precipitation:

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

[0058] The fluoride content of the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 1000 mg / L to 9.2 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content in the wastewater decreased from 2000 mg / L to 9.4 mg / L, thus meeting the wastewater purification requirements.

[0059] Example 2: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater, comprising the following steps:

[0060] The wastewater used in this embodiment has the following characteristics: pH 2, fluoride ion content 285 mg / L, and silica content 300 mg / L.

[0061] (1) Pre-adjust pH value:

[0062] The pH of the wastewater was adjusted to 8 to obtain alkalized wastewater.

[0063] (2) Crystal nucleus formation:

[0064] Take 20% by mass of the alkalized wastewater described in step (1), and add calcium chloride in the form of n(Ca 2+ ):n(F - 总 Add a ratio of 1:1.5 to the solution, then stir at 300 rpm for 5 minutes, and then stir at 100 rpm for 10 minutes to obtain pretreated wastewater.

[0065] (3) Reaction of calcium chloride:

[0066] Add the remaining alkalized wastewater from step (1) to the pretreated wastewater from step (2), then stir at 300 rpm for 5 minutes and then at 100 rpm for 10 minutes to obtain intermediate water.

[0067] (4) Coagulation reaction:

[0068] With a molar ratio of n(PAC):n(Fe) 3+ = 6:1, add a total of 24 mmol / L of polyaluminum chloride and ferric chloride to the intermediate water in step (3). The order of addition is: first add polyaluminum chloride, then add ferric chloride; then stir at 300 rpm for 5 min; then stir at 100 rpm for 30 min to obtain the water to be precipitated.

[0069] (5) Precipitation:

[0070] The water to be settled in step (4) is allowed to stand for 30 minutes to settle, and the supernatant is the purified water.

[0071] The fluoride content of the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 285 mg / L to 7.5 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content in the wastewater decreased from 300 mg / L to 5.5 mg / L, thus meeting the wastewater purification requirements.

[0072] Example 3: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater, comprising the following steps:

[0073] The wastewater used in this embodiment has the following characteristics: pH 3, fluoride ion content 50 mg / L, and silica content 100 mg / L.

[0074] (1) Pre-adjust pH value:

[0075] The pH of the wastewater was adjusted to 7.5 to obtain alkalized wastewater.

[0076] (2) Crystal nucleus formation:

[0077] Take 15% by mass of the alkalized wastewater described in step (1), and add calcium chloride in the form of n(Ca 2+ ):n(F - 总 Add a ratio of 1:1.8 to the solution, then stir at 250 rpm for 3 minutes, and then stir at 75 rpm for 8 minutes to obtain pretreated wastewater.

[0078] (3) Reaction of calcium chloride:

[0079] Add the remaining alkalized wastewater from step (1) to the pretreated wastewater from step (2), then stir at 250 rpm for 3 minutes and then at 75 rpm for 8 minutes to obtain intermediate water.

[0080] (4) Coagulation reaction:

[0081] With a molar ratio of n(PAC):n(Fe) 3+ = 5.5:1, add a total of 22 mmol / L of polyaluminum chloride and ferric chloride to the intermediate water in step (3). The order of addition is: first add polyaluminum chloride, then add ferric chloride; then stir at 250 rpm for 3 min; then stir at 75 rpm for 25 min to obtain the water to be precipitated.

[0082] (5) Precipitation:

[0083] The water to be settled in step (4) is allowed to stand for 25 minutes to settle, and the supernatant is the purified water.

[0084] The fluoride content of the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 50 mg / L to 6.4 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content in the wastewater decreased from 100 mg / L to 5.1 mg / L, thus meeting the wastewater purification requirements.

[0085] Comparative Example 1

[0086] This article uses Example 1 disclosed in CN111646591A as a comparative example to explore the removal effect of existing technologies similar to this invention on soluble silica in chip manufacturing wastewater during the treatment of fluoride.

[0087] The experiment was conducted in the same manner as in Example 1 disclosed in CN111646591A, and the treated clean water was obtained. The water quality characteristics of the wastewater used were: pH 1, fluoride ion content 1000 mg / L, and silica content 2000 mg / L.

[0088] The fluoride content in the supernatant was determined using a PXSJ-216F ion meter. The fluoride content in the wastewater decreased from 1000 mg / L to 12.2 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater decreased from 2000 mg / L to 23.7 mg / L, which does not meet the wastewater purification requirements.

[0089] Comparative Example 2: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater

[0090] 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.

[0091] The fluoride content in the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 1000 mg / L to 27.1 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content decreased from 2000 mg / L to 6.6 mg / L, which does not meet the wastewater purification requirements.

[0092] Comparative Example 3: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater

[0093] The difference between this comparative example and Example 1 is that the amount of alkalized wastewater taken in step (2) is different, specifically: 22% of the alkalized wastewater taken in step (1) is used.

[0094] The fluoride content in the supernatant was determined using a PXSJ-216F ion meter. The fluoride content in the wastewater decreased from 1000 mg / L to 15.6 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996). The silicon content in the wastewater decreased from 2000 mg / L to 9.6 mg / L, which does not meet the wastewater purification requirements.

[0095] Comparative Example 4: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater

[0096] 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:

[0097] (2) Crystal nucleus formation:

[0098] Take 10% by mass of the alkalized wastewater described in step (1), and add calcium chloride in the form of n(Ca 2+ ):n(F - 总 Add a ratio of 1:1.4 to the solution, then stir at 180 rpm for 6 minutes, and then stir at 40 rpm for 12 minutes to obtain pretreated wastewater.

[0099] (3) Reaction of calcium chloride:

[0100] Add the remaining alkalized wastewater from step (1) to the pretreated wastewater described in step (2), then stir at 320 rpm for 1 min and then at 110 rpm for 4 min to obtain intermediate water.

[0101] The fluoride content in the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 1000 mg / L to 11.9 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content decreased from 2000 mg / L to 15.5 mg / L, which does not meet the wastewater purification requirements.

[0102] Comparative Example 5: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater

[0103] The difference between this comparative example and Example 1 is that the dosage of polyaluminum chloride and ferric chloride in step (4) is different, specifically: the molar ratio is n(PAC):n(Fe 3+ =4.8:1, and add a total of 22 mmol / L of polyaluminum chloride and ferric chloride to the intermediate water in step (3).

[0104] The fluoride content in the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 1000 mg / L to 13.6 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silicon dioxide content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content decreased from 2000 mg / L to 8.8 mg / L, which does not meet the wastewater purification requirements.

[0105] Comparative Example 6: A method for the synergistic removal of fluorine and silicon from chip manufacturing wastewater

[0106] The difference between this comparative example and Example 1 is that the coagulant mentioned in step (4) is different, specifically: PAM is added in an additional mass ratio of m(PAC):m(PAM) = 25:1.

[0107] The fluoride content of the supernatant was determined using a PXSJ-216F ion meter, and the fluoride content in the wastewater decreased from 1000 mg / L to 9.4 mg / L. The silicon content in the wastewater was determined using the spectrophotometric method for the determination of silica content in industrial circulating cooling water (GB / T 16633-1996), and the silicon content in the wastewater decreased from 2000 mg / L to 13.8 mg / L. Compared with Example 1, although an additional coagulant was added, the final removal efficiency of the remaining silicon decreased by 46%. At the same time, the silicon concentration limit of 10 mg / L in this invention does not meet the wastewater purification requirements.

[0108] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for the simultaneous removal of fluorine and silicon from chip manufacturing wastewater, characterized in that, Includes the following steps: (1) Adjust the pH of the wastewater to obtain alkalized wastewater, add calcium-based reagent to part of the alkalized wastewater and stir to obtain pretreated wastewater; (2) Mix the pretreated wastewater with the remaining alkalized wastewater to obtain intermediate water; (3) Add coagulant to the intermediate water, stir and precipitate to remove fluorine and silicon from the wastewater and obtain supernatant; The pH value of the alkalized wastewater mentioned in step (1) is 7-8; The portion of alkalized wastewater mentioned in step (1) accounts for 10-20% of the total mass of alkalized wastewater; The calcium-based reagent in step (1) is calcium chloride, and the addition amount is: n(Ca 2+ ):n(F - total) = 1:2-1:1.5; The stirring described in step (1) is as follows: first stir at a speed of 200-300 rpm for 2-5 minutes; then stir at a speed of 50-100 rpm for 5-10 minutes. The mixing and stirring described in step (2) is as follows: first stir at a speed of 200-300 rpm for 2-5 minutes; then stir at a speed of 50-100 rpm for 5-10 minutes. The coagulant mentioned in step (3) is a polymeric inorganic salt coagulant and an iron-based coagulant; The molar ratio of the polymeric inorganic salt coagulant to the iron-based coagulant is n(PAC):n(Fe 3+ )=5:1-6:

1.

2. The method of claim 1, wherein, The wastewater in step (1) has the following water quality characteristics: fluoride ion concentration of 50-1000 mg / L, silica concentration of 100-2000 mg / L, and pH range of 1-3.

3. The method of claim 1, wherein, The total dosage of the polymeric inorganic salt coagulant and the iron-based coagulant is 20-24 mmol / L.

4. The method of claim 1, wherein, The stirring in step (3) is as follows: first, stir at a speed of 200-300 rpm for 2-5 minutes; then stir at a speed of 50-100 rpm for 20-30 minutes; the sedimentation time is 20-30 minutes.