Alkali fractionation-coagulation treatment agent for alkali-reduction wastewater and alkali-reduction wastewater treatment method

Abstract

The application discloses an alkali-deduction wastewater alkali precipitation-coagulation treatment agent and a treatment method thereof. The alkali precipitation-coagulation treatment agent comprises alkali precipitation treatment agent I, 14wt.%-16wt.% of Al2(SO4)3 aqueous solution, alkali precipitation treatment agent II, 19wt.%-21wt.% of CaCl2 aqueous solution, coagulation treatment agent I, 4wt.%-6wt.% of polyaluminum chloride aqueous solution, and coagulation treatment agent II, 0.08wt.%-0.12wt.% of composite polymer aqueous solution, which is composed of cationic starch and cationic polyacrylamide with a weight ratio of 4:6-6:4. The alkali precipitation-coagulation treatment agent is used to treat the alkali-deduction wastewater by sequentially performing alkali precipitation, coagulation and pressure filtration dehydration, so that the pollutants in the alkali-deduction wastewater are treated without adding acid and adjusting pH, TA in the wastewater is recovered, and the caustic alkali in the wastewater is also maximally reserved.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and in particular to an alkali precipitation-coagulation treatment agent and a method for treating alkali-reduced wastewater. Background Technology

[0002] Alkali weight reduction is a performance improvement technology for polyester fiber fabrics. Specifically, it involves immersing polyester in a high-concentration NaOH solution at a certain temperature to induce a hydrolysis reaction, causing some substances on the fiber surface to peel off and dissolve, thus achieving weight reduction. During this process, the polyester PEG macromolecules undergo hydrolysis, ester bonds break, and soluble condensation polymers of varying degrees of polymerization are continuously formed, ultimately producing sodium terephthalic acid (TA) and ethylene glycol (EG). After weight reduction through alkali erosion, the polyester fibers become finer, softer, and lighter in total weight, giving the polyester fabric silk-like softness and wrinkle resistance. The specific chemical reaction formula is as follows:

[0003]

[0004] With the development of alkali reduction technology, the characteristics of alkali reduction wastewater, such as high TA salt content, high pH value, difficulty in biological treatment, and large fluctuations in water quality and quantity, have become increasingly prominent, making its treatment increasingly difficult.

[0005] Currently, common treatment methods for alkali reduction wastewater include acid precipitation, activated carbon adsorption, coagulation, and alkali precipitation-coagulation. Acid precipitation involves adding acid to the alkali reduction wastewater to neutralize the insoluble TA salt produced after polyester hydrolysis, adjusting the pH to a certain value to precipitate the TA. While this method is simple, has a high TA recovery rate, low cost, and convenient operation, it suffers from drawbacks such as high acid consumption, large salt production, high requirements for corrosion resistance of treatment equipment, certain safety hazards, and high impurity content in the recovered products, poor settling performance, poor dewatering, and poor filtration. Activated carbon adsorption involves mixing powdered adsorbents such as activated carbon with wastewater, or passing the wastewater through a filter bed, allowing pollutants in the wastewater to be adsorbed onto the activated carbon surface and removed by filtration. While this method is simple, has high COD and turbidity removal rates, and can be operated continuously, it has drawbacks such as high equipment requirements, the need for regular replacement or regeneration of the activated carbon adsorbent, high operating costs, and suitability only for treating low-concentration alkali reduction wastewater. The specific operation of the coagulation method involves using coagulants such as ferrous chloride and magnesium sulfate. Their hydrolysis or polymerization products cause large flocs in the wastewater to aggregate through adsorption and bridging, and then the wastewater is purified by sedimentation. Although this method has low investment cost, small equipment footprint, large processing capacity, and the coagulant has a flocculation effect, making the precipitate easy to dewater, the flocculation treatment effect is prone to fluctuation with changes in the properties of the alkali reduction wastewater, and the process needs to be changed frequently, making actual production management difficult.

[0006] Therefore, in view of the shortcomings of the existing treatment methods, it is necessary to develop a treatment method suitable for alkali reduction wastewater with high alkalinity characteristics, so as to achieve a highly efficient operation method that can directly achieve flocculation and particle precipitation without adjusting pH. Summary of the Invention

[0007] The purpose of this invention is to provide an alkali precipitation-coagulation treatment agent for alkali reduction wastewater that solves the technical problem.

[0008] Another object of the present invention is to provide a method for treating alkali-reducing wastewater using the above-mentioned alkali-reducing wastewater treatment agent.

[0009] Therefore, the technical solution of the present invention is as follows:

[0010] An alkali-precipitation-coagulation treatment agent for alkali reduction wastewater comprises alkali precipitation agent I, alkali precipitation agent II, coagulation agent I, and coagulation agent II; wherein, alkali precipitation agent I is a 14wt.% to 16wt.% aqueous solution of Al2(SO4)3, alkali precipitation agent II is a 19wt.% to 21wt.% aqueous solution of CaCl2, coagulation agent I is a 4wt.% to 6wt.% aqueous solution of polyaluminum chloride, and coagulation agent II is a 0.08wt.% to 0.12wt.% aqueous solution of a composite polymer, wherein the composite polymer is composed of cationic starch and cationic polyacrylamide in a weight ratio of 4:6 to 6:4.

[0011] Preferably, polyaluminum chloride is PAC28.

[0012] Preferably, the cationic polyacrylamide used is a cationic polyacrylamide with an ionicity of 30 and a molecular weight of 13 million Da.

[0013] Preferably, the cationic starch is a cationic starch with a degree of substitution of 0.035 and a molecular weight of 2 million Da.

[0014] A method for treating alkali reduction wastewater using the above-mentioned alkali precipitation-coagulation treatment agent comprises the following steps:

[0015] S1. Add alkali precipitation agent I and alkali precipitation agent II to the alkali reduction wastewater simultaneously, and mix and stir evenly to obtain alkali precipitation treatment solution; wherein, the addition amount of alkali precipitation agent I is: 1.3 kg to 1.7 kg of Al2(SO4)3 per ton of wastewater, and the addition amount of alkali precipitation agent II is: 2.3 kg to 2.7 kg of CaCl2 per ton of wastewater;

[0016] S2. First, add coagulant I to the alkaline precipitation treatment liquid. After mixing and stirring evenly, add coagulant II and mix and stir evenly. The dosage of coagulant I is: 0.4 kg to 0.6 kg of polyaluminum chloride per ton of wastewater, and the dosage of coagulant II is: 15 g to 25 g of composite polymer per ton of wastewater.

[0017] S3. Utilize gravity sedimentation to allow the flocs formed in the wastewater obtained from step S2 to settle; use a plate and frame filter press to dewater and recover the white mud-like solids, namely terephthalate and its oligomers.

[0018] In the above-mentioned alkaline reduction wastewater treatment steps, step S1 is the alkaline precipitation step, which is crucial for recovering terephthalic acid (TA) from the alkaline reduction wastewater. Compared with the acid precipitation method described in the background art, although the acid precipitation method has a higher TA recovery rate, the TA obtained during the acid precipitation process is particulate, with a slow settling speed, requiring a large amount of reagents (such as FeCl2, MgSO4, HPAM) for coagulation aid. At the same time, due to the strong water retention of particulates, the efficiency of pressure filtration is significantly reduced. In contrast, the alkaline precipitation step of this invention, without adding any acid (i.e., without adjusting the pH), directly utilizes the precipitant aluminum sulfate and the Al in calcium chloride. 3+ Ca 2+ These two high-valence ions react with terephthalate ions through ion exchange to form insoluble aluminum and calcium salts, thus forming primary flocs. However, the problem that still exists after the alkaline precipitation step is that the precipitate particles are too small to settle easily. Therefore, further treatment is required to allow the precipitate to settle sufficiently.

[0019] In this alkaline precipitation step, Al is used 3+ With Ca 2+ Two ions react with TA in the wastewater, converting it into water-insoluble aluminum salt TAAl and calcium salt TACa, which precipitate out of the water. The chemical reaction formula is as follows:

[0020]

[0021] At the same time, the low molecular weight homopolymer of TA will also react with the above-mentioned ions to form insoluble high-valence metal salts, which will precipitate out together. That is, the final precipitate is the salt of TA and its oligomers.

[0022] In the above-mentioned alkaline reduction wastewater treatment steps, step S2 is the coagulation treatment step, which is a follow-up step to alkaline precipitation. Although the insoluble calcium and aluminum salts of terephthalic acid and its oligomers obtained after alkaline precipitation are larger than those obtained by acid precipitation, the particle size is still small, the settling velocity is slow, and the water holding capacity is large. In order to further improve the floc structure, increase the floc particle size, and reduce the water holding capacity, it is necessary to add polyaluminum chloride (PAC), cationic acrylamide (CPAM), and cationic starch (CS) to form an inorganic-organic coagulant, which increases the floc density, enhances the settling performance, and improves the separation efficiency. In addition, in the coagulation step, TA has been converted into insoluble high-valence metal salts during the alkaline precipitation stage. Therefore, the amount of coagulant used in the coagulation stage should be minimized to ensure that it can maintain stable charge neutralization and bridging capabilities by carrying a strong positive charge in a high pH environment, thereby rapidly forming stable flocs, improving the floc structure, and enhancing the settling performance.

[0023] Compared with existing technologies, when using this alkali precipitation-coagulation treatment agent to treat alkali reduction wastewater, the purpose of treating and recovering TA in the alkali reduction wastewater can be achieved by directly adding alkali precipitation reagent and coagulation reagent to the wastewater in sequence without adding acid or adjusting pH. At the same time, the treatment of alkali reduction wastewater using this alkali precipitation-coagulation treatment agent can retain caustic alkali to the greatest extent, so that the water sample after TA salt and its oligomers are recovered can be further recovered for caustic alkali or reused. Attached Figure Description

[0024] Figure 1 The infrared analysis comparison spectrum of the white mud sample recovered through Example 1 of the present invention and the TA standard is shown. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0026] Example 1

[0027] The following steps describe the treatment of alkali reduction wastewater A using an alkali precipitation-coagulation treatment agent:

[0028] S1. Alkali precipitation: Based on the weight of alkali reduction wastewater A, determine the dosage of Al2(SO4)3 and CaCl2; the dosage of Al2(SO4)3 is 1.3 kg / T, 1.5 kg / T, and 1.7 kg / T, and a 15 wt.% Al2(SO4)3 aqueous solution is prepared; the dosage of CaCl2 is 2.5 kg / T, and a 20 wt.% CaCl2 aqueous solution is prepared; the mixture of Al2(SO4)3 aqueous solution and CaCl2 aqueous solution prepared by the above method is added to alkali reduction wastewater A, and then stirred for 3 minutes to allow for full reaction, generating aluminum and calcium salt precipitates of TA and its oligomers, forming primary flocs;

[0029] S2. Coagulation: Based on the weight of alkali reduction wastewater A, determine the dosage of coagulant I to be 0.5 kg / T, and prepare a 5 wt.% PAC aqueous solution; add the aqueous solution of coagulant I to the wastewater treated in step S1, and then continue stirring for 3 minutes; then, based on the weight of alkali reduction wastewater A, determine the dosage of coagulant II to be 20 g / T, and prepare a 0.1 wt.% mixed aqueous solution; continue adding the aqueous solution of coagulant II to the wastewater, and then continue stirring for 5 minutes; wherein, coagulant I is PAC28, and coagulant II is a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the degree of ionization of cationic polyacrylamide is 30 and the molecular weight is 13 million Da, and the degree of substitution of cationic starch is 0.035 and the molecular weight is 2 million Da;

[0030] S3. Utilize gravity settling to allow the flocs formed in the wastewater treated in step S2 to settle; use a plate and frame filter press for dewatering to recover the white mud-like solids.

[0031] As a comparative example 1, the amount of Al2(SO4)3 added in the above treatment process was replaced with 1.2 kg / T, 1.8 kg / T and 5.0 kg / T respectively, while the other steps were the same.

[0032] The treatment results of alkali reduction wastewater A using the methods of Example 1 and Comparative Example 1 are shown in Table 1.

[0033] Table 1:

[0034]

[0035] The processing results in Table 1 show that:

[0036] When the dosage of Al2(SO4)3 gradually increased from 1.2 kg / T to 5.0 kg / T, while other conditions remained the same, the COD removal rate, light transmittance, and white mud yield all gradually increased. This indicates that increasing the dosage of Al2(SO4)3 helps reduce pollution indicators in fiber-opening wastewater, improves water quality, and increases TA recovery rate. Specifically, when the dosage of Al2(SO4)3 was <1.3 kg / T, the wastewater treatment effect did not meet the qualified standard (COD removal rate <25%). When the dosage of Al2(SO4)3 exceeded 1.7 kg / T, the COD in the wastewater increased significantly. - The significant decrease in sodium hydroxide (NaOH) in the wastewater is due to the reaction of excess aluminum sulfate with hydroxide ions, resulting in a substantial reduction in NaOH retention and alkali loss. This hinders the subsequent recovery of TA salts and their oligomers, making it difficult to recover or recycle the remaining alkali in the wastewater sample. Furthermore, it increases the cost of chemical treatment. Therefore, considering factors such as COD removal rate, light transmittance, white mud yield, chemical cost, and alkali recovery efficiency, the appropriate dosage of Al2(SO4)3 is [missing information].

[0037] 1.3kg / T~1.7kg / T.

[0038] Compared with the water quality test results of alkali reduction wastewater A, when the Al2(SO4)3 dosage is 1.3kg / T to 1.7kg / T, the COD removal rate is 43.5wt.% to 44.8wt.%, the transmittance increases by 56.4 times to 95 times, the white mud yield is 10.04g / L to 12.08g / L, and the caustic alkali retention rate is 85.2% to 92.6%.

[0039] Example 2

[0040] The following steps describe the treatment of alkali reduction wastewater B using an alkali precipitation-coagulation treatment agent:

[0041] S1. Alkali precipitation: Based on the weight of alkali reduction wastewater B, determine the dosage of Al2(SO4)3 and CaCl2; the dosage of Al2(SO4)3 is 1.5 kg / T, and a 15 wt.% Al2(SO4)3 aqueous solution is prepared; the dosage of CaCl2 is 2.3 kg / T, 2.5 kg / T, and 2.7 kg / T, and a 20 wt.% CaCl2 aqueous solution is prepared; the mixture of Al2(SO4)3 aqueous solution and CaCl2 aqueous solution prepared by the above method is added to alkali reduction wastewater B, and then stirred for 3 minutes to allow for full reaction, generating aluminum and calcium salt precipitates of TA and its oligomers, forming primary flocs;

[0042] S2. Coagulation: Based on the weight of the alkali reduction wastewater B, determine the dosage of coagulant I to be 0.5 kg / T, and prepare a 5 wt.% PAC aqueous solution; add the aqueous solution of coagulant I to the wastewater treated in step S1, and then continue stirring for 3 minutes; then, based on the weight of the alkali reduction wastewater B, determine the dosage of coagulant II to be 15 g / T, and prepare a 0.1 wt.% mixed aqueous solution; continue adding the aqueous solution of coagulant II to the wastewater, and then continue stirring for 5 minutes; wherein, coagulant I is PAC28, and coagulant II is a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the degree of ionization of cationic polyacrylamide is 30 and the molecular weight is 13 million Da, and the degree of substitution of cationic starch is 0.035 and the molecular weight is 2 million Da;

[0043] S3. Utilize gravity settling to allow the flocs formed in the wastewater treated in step S2 to settle; use a plate and frame filter press for dewatering to recover the white mud-like solids.

[0044] As a comparative example 2, the amount of CaCl2 added in the above treatment process was replaced with 2.1 kg / T, 2.9 kg / T and 5.0 kg / T respectively, while the other steps were the same.

[0045] The treatment results of alkali reduction wastewater B using the methods of Example 2 and Comparative Example 2 are shown in Table 2.

[0046] Table 2:

[0047]

[0048] The processing results in Table 2 show that:

[0049] When the dosage of CaCl2 gradually increased from 2.1 kg / T to 5.0 kg / T, while other conditions remained the same, although the COD removal rate, transmittance, and white mud yield all gradually increased, it indicates that increasing the dosage of CaCl2 helps to reduce wastewater pollution indicators, improve water quality, and increase TA recovery rate. Similarly, when the dosage of CaCl2 is <2.3 kg / T, the water treatment effect does not meet the requirements, and the C(OH)2 content in the wastewater increases. - The addition of CaCl2 begins to decrease significantly after exceeding 2.7 kg / T, resulting in a substantial reduction in the retention rate of NaOH in the wastewater and causing alkali loss. This is detrimental to the subsequent recovery of TA salts and their oligomers, hindering the recovery of alkali from the remaining water sample or the recycling of wastewater. It also raises the cost of chemical treatment. Therefore, considering factors such as COD removal rate, light transmittance, white mud yield, chemical treatment cost, and alkali recovery efficiency, the appropriate addition of CaCl2 is 2.3 kg / T to 2.7 kg / T.

[0050] Compared with the water quality test results of alkali reduction wastewater B, when the CaCl2 dosage is 2.3 kg / T to 2.7 kg / T, the COD removal rate is 32.3 wt.% to 38.7 wt.%, the transmittance increases by 35.6 times to 53.2 times, the white mud yield is 6.8 g / L to 7.79 g / L, and the caustic alkali retention rate is 78.9% to 84.2%.

[0051] Furthermore, the applicant also discovered that during the alkaline precipitation process of highly alkaline (pH≥13.5) fiber-opening wastewater, the precipitation of Al(OH)3 and Ca(OH)2 when Al2(SO4)3 and CaCl2 aqueous solutions are a problem that must be considered. As supplementary experiments to Examples 1 and 2: when pH=13.5, a large amount of precipitation occurred when aluminum sulfate and calcium chloride were added alone, indicating that the two precipitants, when used alone, would react with hydroxide ions to generate a large amount of hydroxides, which is not suitable for the treatment of highly alkaline wastewater; however, when aluminum sulfate and calcium chloride were added simultaneously, the amount of precipitate was significantly reduced, which was attributed to the salt effect, that is, when two different types of strong electrolytes coexist in water, they can respectively inhibit the precipitation of their respective hydroxides.

[0052] It is evident that in the alkaline precipitation step of this invention, the alkaline precipitation reagent is a compound of aluminum sulfate and calcium chloride, which can not only effectively inhibit the formation of Al(OH)3 and Ca(OH)2 precipitates and improve the alkaline precipitation efficiency of TA, but also maintain the concentration of hydroxide ions in the wastewater to the maximum extent, ensuring the recovery rate of NaOH and the reuse of fiber opening wastewater. This is also the key to the smooth progress of the alkaline precipitation method.

[0053] Example 3

[0054] The following steps describe the treatment of alkali reduction wastewater C using an alkali precipitation-coagulation treatment agent:

[0055] S1. Alkali precipitation: Based on the weight of the alkali reduction wastewater C, determine the dosage of Al2(SO4)3 and CaCl2; the dosage of Al2(SO4)3 is 1.7 kg / T, and a 14 wt.% Al2(SO4)3 aqueous solution is prepared; the dosage of CaCl2 is 2.3 kg / T, and a 21 wt.% CaCl2 aqueous solution is prepared; the mixture of Al2(SO4)3 aqueous solution and CaCl2 aqueous solution prepared by the above method is added to the alkali reduction wastewater C, and then stirred for 2 minutes to allow for full reaction, generating aluminum and calcium salt precipitates of TA and its oligomers, forming primary flocs;

[0056] S2. Coagulation: Based on the weight of the alkali reduction wastewater C, determine the dosage of coagulant I as 0.4 kg / T, 0.5 kg / T, and 0.6 kg / T, and prepare a 4 wt.% PAC aqueous solution; add the aqueous solution of coagulant I to the wastewater treated in step S1, and then continue stirring for 3 min; then, based on the weight of the alkali reduction wastewater C, determine the dosage of coagulant II as 25 g / T, and prepare a 0.1 wt.% mixed aqueous solution; continue adding the aqueous solution of coagulant II to the wastewater, and then continue stirring for 6 min; wherein, coagulant I is PAC28, and coagulant II is a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the degree of ionization of cationic polyacrylamide is 30, the molecular weight is 13 million Da, and the degree of substitution of cationic starch is 0.035, the molecular weight is 2 million Da;

[0057] S3. Utilize gravity settling to allow the flocs formed in the wastewater treated in step S2 to settle; use a plate and frame filter press for dewatering to recover the white mud-like solids.

[0058] As a comparative example 3, the amount of coagulant I added in the above treatment process was replaced with 0.3 kg / T, 0.7 kg / T and 0.8 kg / T respectively, while the other steps were the same.

[0059] The treatment results of alkali reduction wastewater C using the methods of Example 3 and Comparative Example 3 are shown in Table 3.

[0060] Table 3:

[0061]

[0062] Example 4

[0063] The following steps describe the treatment of alkali reduction wastewater D using an alkali precipitation-coagulation treatment agent:

[0064] S1. Alkali precipitation: Based on the weight of the alkali reduction wastewater D, determine the amounts of Al2(SO4)3 and CaCl2. The amount of Al2(SO4)3 added is 1.3 kg / T, and a 14 wt.% Al2(SO4)3 aqueous solution is prepared. The amount of CaCl2 added is 2.7 kg / T, and a 19 wt.% CaCl2 aqueous solution is prepared. The mixture of the Al2(SO4)3 aqueous solution and CaCl2 aqueous solution prepared using the above method is added to the alkali reduction wastewater D, and then stirred for 2 minutes to allow for a complete reaction, generating aluminum and calcium salt precipitates of TA and its oligomers, forming primary flocs.

[0065] S2. Coagulation: Based on the weight of the alkali reduction wastewater D, determine the dosage of coagulant I to be 0.5 kg / T, and prepare a 6 wt.% PAC aqueous solution; add the aqueous solution of coagulant I to the wastewater treated in step S1, and then continue stirring for 3 minutes; then, based on the weight of the alkali reduction wastewater D, determine the dosage of coagulant II to be 20 g / T and 25 g / T, and prepare a 0.12 wt.% mixed aqueous solution; continue adding the aqueous solution of coagulant II to the wastewater, and then continue stirring for 6 minutes; wherein, coagulant I is PAC28, and coagulant II is a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the degree of ionization of cationic polyacrylamide is 30 and the molecular weight is 13 million Da, and the degree of substitution of cationic starch is 0.035 and the molecular weight is 2 million Da;

[0066] S3. Utilize gravity settling to allow the flocs formed in the wastewater treated in step S2 to settle; use a plate and frame filter press for dewatering to recover the white mud-like solids.

[0067] As a comparative example 4, the dosage of coagulant II in the above treatment process was replaced with 30 kg / T and 65 kg / T respectively, while the other steps remained the same.

[0068] The treatment results of alkali reduction wastewater D using the methods of Example 4 and Comparative Example 4 are shown in Table 4.

[0069] Table 4:

[0070]

[0071] As shown in Tables 3 and 4, the treatment results indicate that, regardless of whether the dosage of coagulant I increases while other conditions remain constant, or regardless of whether the dosage of coagulant II increases while other conditions remain constant, the pollutants such as COD and turbidity show a non-significant decreasing trend. This suggests that TA salts and their oligomers in the wastewater are primarily removed through the first step of alkaline precipitation, while the second step of coagulation mainly promotes floc growth and accelerates floc settling. In the experimental tests, the water treated in Examples 3 and 4 achieved rapid clarification without a significant increase in viscosity. However, while the water treated in Comparative Examples 3 and 4 also achieved rapid clarification, the viscosity increased significantly, which is not conducive to the subsequent concentration treatment with caustic alkali. Furthermore, the higher price of the flocculant increases the cost of chemical treatment.

[0072] Compared with the raw water quality test results, when the dosage of PAC28 is 0.4 kg / T to 0.6 kg / T and the dosage of composite polymer is 15 g / T to 20 g / T, the COD removal rate is 26.8% to 34.0%, the transmittance is increased by 25.1 times to 26.6 times, the yield of solid matter (white mud) is 6.99 g / L to 8.27 g / L, and the recovery rate of caustic alkali (NaOH) is 82.1% to 95.2%.

[0073] Furthermore, the applicant discovered that after the alkaline precipitation step, a coagulant needs to be added immediately for coagulation to ensure timely reaction with the TA salt and its oligomers, forming large flocs that then settle and separate. Coagulant I and Coagulant II can be added sequentially or simultaneously; there is no significant difference in treatment effect between the two methods.

[0074] Example 5

[0075] The following steps describe the treatment of alkali reduction wastewater E using an alkali precipitation-coagulation treatment agent:

[0076] S1. Alkali precipitation: Based on the weight of the alkali reduction wastewater E, determine the dosage of Al2(SO4)3 and CaCl2; the dosage of Al2(SO4)3 is 1.3 kg / T, and a 14 wt.% Al2(SO4)3 aqueous solution is prepared; the dosage of CaCl2 is 2.7 kg / T, and a 19 wt.% CaCl2 aqueous solution is prepared; the mixture of the prepared Al2(SO4)3 aqueous solution and CaCl2 aqueous solution is added to the alkali reduction wastewater E at a dosing flow rate of 0.8 L / h, and stirred at a stirring rate of 350 r / min for 2 min to ensure sufficient reaction, generating aluminum and calcium salt precipitates of TA and its oligomers, forming primary flocs;

[0077] S2. Coagulation: Based on the weight of the alkali reduction wastewater E, determine the dosage of coagulant I to be 0.5 kg / T, and prepare a 6 wt.% PAC aqueous solution; add the aqueous solution of coagulant I to the wastewater treated in step S1, and then continue stirring for 3 min; then, based on the weight of the alkali reduction wastewater E, determine the dosage of coagulant II to be 25 g / T, and prepare a 0.08 wt.% mixed aqueous solution; continue adding the aqueous solution of coagulant II to the wastewater, and then continue stirring for 6 min; wherein, coagulant I is PAC28, and coagulant II is a mixture of cationic polyacrylamide and cationic starch in weight ratios of 4:6, 5:5, and 6:4, respectively. The cationic polyacrylamide has an ionic degree of 30 and a molecular weight of 13 million Da, and the cationic starch has a degree of substitution of 0.035 and a molecular weight of 2 million Da;

[0078] S3. Utilize gravity settling to allow the flocs formed in the wastewater treated in step S2 to settle; use a plate and frame filter press for dewatering to recover the white mud-like solids.

[0079] As a comparative example 5, the total ratio of cationic polyacrylamide and cationic starch in coagulant II in the above treatment process was replaced with 0:10, 7:3 and 10:0, while the other steps were the same.

[0080] The treatment results of alkali reduction wastewater E using the methods of Example 5 and Comparative Example 5 are shown in Table 5.

[0081] Table 5:

[0082]

[0083] In coagulant II, cationic starch CS and cationic polyacrylamide CPAM have a synergistic effect, which is key to the effective flocculation of TA precipitates in wastewater. Specifically, under strongly alkaline conditions (pH > 13.5), most nonionic and anionic coagulants lose their charge neutralization effect due to charge conversion (deprotonation), leading to a significant decrease in flocculation efficiency or even failure. Since both CS and CPAM are cationic polymeric electrolytes, their macromolecular backbones contain strongly alkaline quaternary ammonium groups, which can maintain sufficient positive charge in strongly alkaline environments. This neutralizes and destabilizes the charge on the surface of TA colloids and particles in the water, thereby achieving effective flocculation of TA precipitates. The bridging and trapping effects of the macromolecular backbone form stable large-particle flocs, which are removed by gravity sedimentation. Among them, CS has a relatively low molecular weight, with a molecular weight of approximately 2 million DA, but it has a high positive charge density (cationic substitution degree DS > 0.035), which can exert a strong charge neutralization effect and adsorb a large amount of TA colloids and suspended particles, thus destabilizing them. CPAM has a lower charge density than CS, but a higher molecular weight, with a molecular weight of approximately 13 million DA, which can fully exert its bridging and trapping effects. Therefore, through the effective combination of the two and their complementary performance, the particle size and mechanical strength of TA flocs can be effectively improved, the floc settling speed can be accelerated, the water holding capacity of the flocs can be reduced, the pressure filter dewatering performance can be improved, and the production efficiency can be effectively increased.

[0084] As shown in Table 5, flocculation experiments were conducted using CS and CPAM in different ratios. Regarding pollutant removal rates, turbidity removal increased (wastewater transmittance increased) with increasing CS ratio, but COD and TA removal rates gradually decreased after exceeding a certain ratio (CS:CPAM > 6:4). Simultaneously, observing the floc structure, the floc particle size gradually decreased, the structure became looser, and settling properties deteriorated with increasing cationic starch CS usage. Furthermore, the effect of using CS or CPAM alone was worse than that of their combined use. Based on this, considering both flocculation performance and reagent costs, the optimal CPAM:CS ratio is 4:6 to 6:4, with 5:5 being more preferable.

[0085] Furthermore, the white mud samples recovered through Examples 1-5 were subjected to infrared testing, and their infrared spectra were compared with those of terephthalic acid standard.

[0086] like Figure 1 The image shown is a comparison of the infrared spectrum of one of the white clay samples recovered in Example 1 and the infrared spectrum of the terephthalic acid standard. The comparison reveals that the functional group region of the white clay sample is between 1640 and 1560 cm⁻¹. -1 The part is the carboxylate group (COO) in terephthalic acid. － Absorption of stretching vibration, 1450cm -1 The area near the benzene ring vibration absorbs at 1345 cm⁻¹. -1 The nearby absorption peak is due to the bending vibration of methylene CH2, indicating that the infrared spectrum of the recovered white mud sample is basically consistent with that of the standard. The infrared spectral results of the recovered white mud samples in other embodiments also show similarities. Figure 1 The same characteristics prove that the main components of the recovered white mud are terephthalic acid and its oligomers. That is, the alkali reduction wastewater treatment agent and treatment method of this application can effectively achieve the pollution treatment of alkali reduction wastewater and the recovery of terephthalic acid and its oligomers.

Claims

1. A method for treating alkali reduction wastewater using an alkali precipitation-coagulation treatment agent, characterized in that, The alkali precipitation-coagulation treatment agent consists of alkali precipitation treatment agent I, alkali precipitation treatment agent II, coagulation treatment agent I, and coagulation treatment agent II; wherein, alkali precipitation treatment agent I is a 14wt%~16wt% aqueous solution of Al2(SO4)3, alkali precipitation treatment agent II is a 19wt%~21wt% aqueous solution of CaCl2, coagulation treatment agent I is a 4wt%~6wt% aqueous solution of polyaluminum chloride, and coagulation treatment agent II is a 0.08wt%~0.12wt% aqueous solution of a composite polymer, wherein the composite polymer is composed of cationic starch and cationic polyacrylamide in a weight ratio of 4:6~6:4; The processing steps are as follows: S1. Add alkali precipitation agent I and alkali precipitation agent II to the alkali reduction wastewater simultaneously and stir to react evenly to obtain alkali precipitation treated solution; wherein, the addition amount of alkali precipitation agent I is: 1.3kg~1.7kg of Al2(SO4)3 per ton of wastewater, and the addition amount of alkali precipitation agent II is: 2.3kg~2.7kg of CaCl2 per ton of wastewater; S2. First, add coagulant I to the alkaline precipitation treatment liquid. After stirring and reacting evenly, add coagulant II and stir and react evenly. The dosage of coagulant I is 0.4 kg to 0.6 kg of polyaluminum chloride per ton of wastewater, and the dosage of coagulant II is 15 g to 25 g of composite polymer per ton of wastewater. S3. Utilize gravity sedimentation to allow the flocs formed in the wastewater obtained from step S2 to settle; use a plate and frame filter press to dewater and recover the white mud-like solids, namely terephthalate and its oligomers.

2. The processing method according to claim 1, characterized in that, Polyaluminum chloride uses PAC28.

3. The processing method according to claim 1, characterized in that, The cationic polyacrylamide used is a cationic polyacrylamide with an ionicity of 30 and a molecular weight of 13 million Da.

4. The processing method according to claim 1, characterized in that, Cationic starch is a cationic starch with a degree of substitution of 0.035 and a molecular weight of 2 million Da.

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