Alkali precipitation-coagulation treatment agent for alkali-minimization wastewater and alkali-minimization wastewater treatment method

Abstract

The invention discloses an alkali precipitation-coagulation treatment agent for alkali-minimization wastewater and a treatment method thereof, the alkali precipitation-coagulation treatment agent comprises 14 wt.%-16 wt.% of an Al2 (SO4) 3 aqueous solution of an alkali precipitation treatment agent I, 19 wt.%-21 wt.% of a CaCl2 aqueous solution of an alkali precipitation treatment agent II, 4 wt.%-6 wt.% of a polyaluminum chloride aqueous solution of a coagulation treatment agent I, and 0.08 wt.%-0.12 wt.% of a composite polymer aqueous solution of the coagulation treatment agent II, the weight ratio of the cationic starch to the cationic polyacrylamide is (4: 6)-(6: 4); the alkali precipitation-coagulation treating agent is used for sequentially carrying out alkali precipitation, coagulation and filter pressing dehydration on alkali-minimization wastewater to realize wastewater treatment, pollutants in the alkali-minimization wastewater are treated and TA in the wastewater is recycled on the premise of not adding acid and not adjusting pH, and meanwhile, caustic alkali in the wastewater can be reserved to the greatest extent.

Description

Technical Field

[0001] The invention relates to the technical field of water treatment, and in particular to an alkali precipitation-coagulation treatment agent for alkali reduction wastewater and an alkali reduction wastewater treatment method. Background Art

[0002] Alkali weight reduction is a technology to improve the performance of polyester fiber fabrics. Specifically, it refers to soaking polyester in a high-concentration solution of caustic soda NaOH at a certain temperature to cause a hydrolysis reaction, so that some substances on the surface of the fiber are peeled off and dissolved, thereby achieving the purpose of weight reduction. In the above process, the polyester PEG macromolecules are hydrolyzed, the ester bonds are broken, and soluble polycondensates with different polymerization degrees are continuously formed, and finally the sodium salt of terephthalic acid (TA) and ethylene glycol EG are generated. After the alkali solution erosion and weight reduction, the polyester fiber becomes thinner, softer and lighter in total weight, and the polyester fabric has the softness and wrinkle resistance of silk. The specific chemical reaction formula is:

[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 biochemical treatment, and large changes in water quality and quantity, are becoming increasingly prominent, making its treatment increasingly difficult.

[0005] At present, the commonly used treatment methods for alkali reduction wastewater include acid precipitation, activated carbon adsorption, coagulation and alkali precipitation-coagulation. The specific operation of the acid precipitation method is to produce water-insoluble TA salt after polyester hydrolysis, add acid to the alkali reduction wastewater for neutralization, and adjust the pH of the wastewater to a certain value to precipitate TA; although this method has a simple process, high TA recovery rate, low cost and convenient operation, it has the disadvantages of large acid consumption, large amount of salts, high corrosion resistance requirements for treatment equipment, certain dangers, and many impurities in the recovered product, poor sedimentation performance, poor dehydration, and poor filterability. The specific operation of the activated carbon adsorption method is to mix powdered adsorbents such as activated carbon with wastewater, or let the wastewater pass through the filter bed, so that the pollutants in the wastewater are adsorbed on the surface of the activated carbon and filtered out; although this method has a simple process, high COD and turbidity removal rates, and can be operated continuously; however, it has high requirements for equipment, the activated carbon adsorbent needs to be replaced or regenerated regularly, the operating cost is high, and it is only suitable for the treatment of low-concentration alkali reduction wastewater. The specific operation of the coagulation method is to use coagulants such as ferrous chloride and magnesium sulfate. The hydrolysis or polymerization products thereof are adsorbed and bridged to aggregate large particle flocs in the wastewater, 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, the precipitate is easy to dehydrate; however, with the change of the properties of the alkali reduction wastewater, the flocculation treatment effect is prone to fluctuations, the process also needs to be changed frequently, and the actual production management is difficult.

[0006] Based on this, in view of the defects existing in the above-mentioned 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 high-efficiency operation method that can directly achieve flocculation and particle precipitation without adjusting the pH. Summary of the invention

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

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

[0009] To this end, the technical solution of the present invention is as follows:

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

[0011] Preferably, polyaluminium chloride is PAC28.

[0012] Preferably, the cationic polyacrylamide uses 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 for alkali reduction wastewater, comprising the following steps:

[0015] S1. Adding alkali precipitation treatment agent I and alkali precipitation treatment agent II to alkali reduction wastewater at the same time, and mixing and stirring to obtain alkali precipitation treatment solution; wherein, the amount of alkali precipitation treatment agent I is satisfied: 1.3kg to 1.7kg of Al2(SO4)3 is added per ton of wastewater, and the amount of alkali precipitation treatment agent II is satisfied: 2.3kg to 2.7kg of CaCl2 is added per ton of wastewater;

[0016] S2, first add coagulant treatment agent I to the alkali precipitation treatment liquid, and after mixing and stirring, add coagulant treatment agent II, and mix and stir evenly; wherein, the amount of coagulant treatment agent I is satisfied: 0.4kg-0.6kg of polyaluminium chloride is added per ton of wastewater, and the amount of coagulant treatment agent II is satisfied: 15g-25g of composite polymer is added per ton of wastewater;

[0017] S3, utilizing gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids, namely, terephthalate and its oligomers.

[0018] In the above-mentioned alkali reduction wastewater treatment steps, step S1 is an alkali precipitation treatment step, which is the key to recovering terephthalic acid in alkali reduction wastewater; compared with the acid precipitation method described in the background technology, although the acid precipitation method has a high recovery rate for TA, the TA obtained in the acid precipitation process is a microparticle with a slow sedimentation rate, and a large amount of reagents (such as FeCl2, MgSO4, HPAM) are required to achieve coagulation; at the same time, due to the strong water retention of the microparticles, the efficiency of the filter press is greatly reduced; and the alkali precipitation step of the present invention directly utilizes the precipitant aluminum sulfate and the Al in the calcium chloride without adding acid (i.e., no pH adjustment is required) 3+ , Ca 2+ These two high-valent ions react with terephthalate in the form of ion exchange to generate insoluble aluminum salt and calcium salt, thus forming primary floccules. However, the problem that still exists after the alkali precipitation step is that the precipitate particles are small and difficult to settle. Therefore, further treatment is required to allow the precipitate to be fully settled.

[0019] In the alkali precipitation step, Al 3+ With Ca 2+ The two ions react with TA in the wastewater, converting it into water-insoluble aluminum salt TAAl and calcium salt TACa, which precipitate from the water; the chemical reaction formula of this chemical reaction is:

[0020]

[0021] At the same time, the low molecular weight homopolymer of TA will also react with the above ions to generate insoluble high-valent metal salts and precipitate together, that is, what is finally precipitated is the salt of TA and its oligomers.

[0022] In the above-mentioned alkali reduction wastewater treatment step, step S2 is a coagulation treatment step, which is a subsequent step of alkali precipitation. Although the particles of the insoluble calcium salt of terephthalic acid and its oligomers and aluminum salt obtained after alkali precipitation are larger than those of the acid precipitation method, the particle size is still smaller, the sedimentation velocity is slower, and the water holding capacity is larger. In order to further improve the floc structure, increase the floc particle size, and reduce the water holding capacity, it is necessary to further add polyaluminum chloride PAC, cationic acrylamide CPAM, and cationic starch CS to form an inorganic-organic coagulant, improve the floc density, enhance the sedimentation performance, and improve the separation efficiency. In addition, in the coagulation step, in the alkali precipitation stage, TA has been converted into an insoluble high-valent metal salt, so in the coagulation stage, the amount of coagulant should be as little as possible to ensure that it is under a high pH environment, by carrying a strong positive charge, to maintain stable electrical neutralization and bridging ability, and quickly forming a stable floc to improve the floc structure and improve the sedimentation performance.

[0023] Compared with the prior art, when the alkali precipitation-coagulation treatment agent is used 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 alkali precipitation-coagulation treatment agent is used to treat alkali reduction wastewater to retain caustic soda to the greatest extent, so that the water sample after the recovery of TA salt and its oligomers can further recover the caustic soda or reuse the water body. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is an infrared analysis comparison spectrum of the white mud sample recovered by Example 1 of the present invention and the TA standard product. DETAILED DESCRIPTION

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

[0026] Example 1

[0027] The alkali reduction wastewater A is treated with an alkali precipitation-coagulation treatment agent for alkali reduction wastewater, and the steps are as follows:

[0028] S1, alkali precipitation: according to the weight of alkali reduction wastewater A, determine the dosage of Al2(SO4)3 and CaCl2; wherein, the dosage of Al2(SO4)3 is 1.3kg / T, 1.5kg / T, 1.7kg / T, and is prepared as a 15wt.% Al2(SO4)3 aqueous solution; the dosage of CaCl2 is 2.5kg / T, and is prepared as a 20wt.% CaCl2 aqueous solution; the mixed solution of the Al2(SO4)3 aqueous solution and the CaCl2 aqueous solution prepared by the above method is added to the alkali reduction wastewater A, and then continued to stir for 3min to fully react, generate aluminum salt and calcium salt precipitates of TA and its oligomers, and form primary flocs;

[0029] S2, coagulation: according to the weight of the alkali-reduced wastewater A, the amount of coagulant I is determined to be 0.5kg / T, and a 5wt.% PAC aqueous solution is prepared; the aqueous solution of coagulant I is added to the wastewater treated in step S1, and then stirring is continued for 3 minutes; then according to the weight of the alkali-reduced wastewater A, the amount of coagulant II is determined to be 20g / T, and a 0.1wt.% mixed aqueous solution is prepared; the aqueous solution of coagulant II is continued to be added to the wastewater, and then stirring is continued for 5 minutes; wherein, coagulant I adopts PAC28, and coagulant II adopts a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the ionicity of the cationic polyacrylamide is 30, the molecular weight is 13 million Da, and the substitution degree of the cationic starch is 0.035, and the molecular weight is 2 million Da;

[0030] S3. Using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids.

[0031] As comparative example 1, the amount of Al2(SO4)3 added in the above treatment process was replaced with 1.2kg / T, 1.8kg / T and 5.0kg / T respectively, and 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] From the processing results in Table 1, we can see that:

[0036] When the addition amount of Al2(SO4)3 gradually increased from 1.2kg / T to 5.0kg / T, while other conditions were the same, COD removal rate, transmittance and white mud yield gradually increased, indicating that the increase in the addition of Al2(SO4)3 was helpful to reduce the pollution index of fiber opening wastewater, improve water quality, and increase TA recovery rate; among them, when the addition amount of Al2(SO4)3 was less than 1.3kg / T, the wastewater treatment effect did not meet the qualified standard (COD removal rate <25%); when the addition amount of Al2(SO4)3 exceeded 1.7kg / T, C(OH - ) shows a sharp decline. This is because excessive aluminum sulfate will react with hydroxide in water, which will greatly reduce the retention rate of NaOH in wastewater and cause alkali loss, which is not conducive to the recovery or recycling of alkali in the remaining water samples after the later recovery of TA salts and their oligomers. At the same time, there is also the problem of higher drug costs. Based on this, considering COD removal rate, transmittance, white mud yield, drug cost and alkali recovery efficiency, the appropriate amount of Al2(SO4)3 is

[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~1.7kg / T, the COD removal rate is 43.5wt.%~44.8wt.%, the transmittance is increased by 56.4 times~95 times, the white mud yield is 10.04g / L~12.08g / L, and the caustic soda retention rate is 85.2%~92.6%.

[0039] Example 2

[0040] The alkali reduction wastewater B is treated with an alkali precipitation-coagulation treatment agent for alkali reduction wastewater, and the steps are as follows:

[0041] S1, alkali precipitation: according to the weight of alkali reduction wastewater B, determine the dosage of Al2(SO4)3 and CaCl2; wherein, the dosage of Al2(SO4)3 is 1.5kg / T, and is prepared as a 15wt.% Al2(SO4)3 aqueous solution; the dosage of CaCl2 is 2.3kg / T, 2.5kg / T and 2.7kg / T, and is prepared as a 20wt.% CaCl2 aqueous solution; the mixed solution of the Al2(SO4)3 aqueous solution and the CaCl2 aqueous solution prepared by the above method is added to the alkali reduction wastewater B, and then continued to stir for 3min to fully react, generate aluminum salt and calcium salt precipitates of TA and its oligomers, and form primary flocs;

[0042] S2, coagulation: according to the weight of the alkali-reduced wastewater B, the amount of coagulant I is determined to be 0.5kg / T, and a 5wt.% PAC aqueous solution is prepared; the aqueous solution of coagulant I is added to the wastewater treated in step S1, and then stirring is continued for 3 minutes; then according to the weight of the alkali-reduced wastewater B, the amount of coagulant II is determined to be 15g / T, and a 0.1wt.% mixed aqueous solution is prepared; the aqueous solution of coagulant II is continued to be added to the wastewater, and then stirring is continued for 5 minutes; wherein, coagulant I adopts PAC28, and coagulant II adopts a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the cationic polyacrylamide has an ionicity 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;

[0043] S3. Using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids.

[0044] As 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, and 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] From the processing results in Table 2, we can see that:

[0049] When the amount of CaCl2 was gradually increased from 2.1kg / T to 5.0kg / T, while other conditions were the same, although COD removal rate, transmittance and white mud yield were gradually improved, it showed that the increase of CaCl2 addition was helpful to reduce wastewater pollution index, improve water quality and increase TA recovery rate; similarly, when the amount of CaCl2 was less than 2.3kg / T, the water treatment effect did not meet the requirements, and C(OH - ) also begins to decline sharply after the addition amount of CaCl2 exceeds 2.7kg / T, which greatly reduces the retention rate of NaOH in the wastewater and causes the loss of alkali, which is not conducive to the recovery of alkali in the remaining water samples or the recycling of wastewater after the later recovery of TA salts and their oligomers. At the same time, there is also the problem of higher drug cost. Based on this, considering COD removal rate, transmittance, white mud yield, drug cost and alkali recovery efficiency, the appropriate addition amount of CaCl2 is 2.3kg / T~2.7kg / T.

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

[0051] In addition, the applicant also found that during the alkali precipitation of high alkaline (pH ≥ 13.5) fiber opening wastewater, when adding Al2(SO4)3 and CaCl2 aqueous solutions, the precipitation of Al(OH)3 and Ca(OH)2 is a problem that must be considered. As a supplementary experiment for Example 1 and Example 2: When pH = 13.5, when aluminum sulfate and calcium chloride are added alone, a large amount of precipitation is precipitated, indicating that the above two precipitants, when used alone, will react with hydroxide to generate a large amount of hydroxide, which is not suitable for the treatment of high alkaline wastewater; and when aluminum sulfate and calcium chloride are added at the same time, the precipitate is significantly reduced, which is analyzed to be caused by the salt effect, that is, when two different types of strong electrolytes coexist in water, they can inhibit the precipitation of their respective hydroxides.

[0052] It can be seen that in the alkali precipitation step of the present invention, the alkali precipitation reagent adopts a compound of aluminum sulfate and calcium chloride, which can not only effectively inhibit the production of Al(OH)3 and Ca(OH)2 precipitates and improve the alkali precipitation efficiency of TA; but also can maintain the concentration of hydroxide in the wastewater to the greatest extent, ensure the subsequent recovery rate of NaOH and the reuse of fiber opening wastewater, which is also the key to the smooth implementation of the alkali precipitation method.

[0053] Example 3

[0054] The alkali reduction wastewater C is treated with an alkali precipitation-coagulation treatment agent for alkali reduction wastewater, and the steps are as follows:

[0055] S1, alkali precipitation: according to the weight of the alkali reduction wastewater C, determine the dosage of Al2(SO4)3 and CaCl2; wherein, the dosage of Al2(SO4)3 is 1.7kg / T, and is prepared as a 14wt.% Al2(SO4)3 aqueous solution; the dosage of CaCl2 is 2.3kg / T, and is prepared as a 21wt.% CaCl2 aqueous solution; the mixed solution of the Al2(SO4)3 aqueous solution and the CaCl2 aqueous solution prepared by the above method is added to the alkali reduction wastewater C, and then continued to stir for 2min to fully react, generate aluminum salt and calcium salt precipitates of TA and its oligomers, and form primary flocs;

[0056] S2, coagulation: according to the weight of the alkali-reduced wastewater C, the amount of coagulant I is determined to be 0.4kg / T, 0.5kg / T and 0.6kg / T, and a 4wt.% PAC aqueous solution is prepared; the aqueous solution of coagulant I is added to the wastewater treated in step S1, and then stirring is continued for 3min; according to the weight of the alkali-reduced wastewater C, the amount of coagulant II is determined to be 25g / T, and a 0.1wt.% mixed aqueous solution is prepared; the aqueous solution of coagulant II is continued to be added to the wastewater, and then stirring is continued for 6min; wherein, coagulant I adopts PAC28, and coagulant II adopts a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the ionicity of the cationic polyacrylamide is 30, the molecular weight is 13 million Da, and the substitution degree of the cationic starch is 0.035, and the molecular weight is 2 million Da;

[0057] S3. Using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids.

[0058] As comparative example 3, the dosage of coagulant I in the above treatment process was replaced with 0.3 kg / T, 0.7 kg / T and 0.8 kg / T respectively, and 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 alkali reduction wastewater D is treated with an alkali precipitation-coagulation treatment agent for alkali reduction wastewater, and the steps are as follows:

[0064] S1, alkali precipitation: according to the weight of alkali reduction wastewater D, determine the dosage of Al2(SO4)3 and CaCl2; wherein, the dosage of Al2(SO4)3 is 1.3kg / T, and is prepared as a 14wt.% Al2(SO4)3 aqueous solution; the dosage of CaCl2 is 2.7kg / T, and is prepared as a 19wt.% CaCl2 aqueous solution; the mixed solution of the Al2(SO4)3 aqueous solution and the CaCl2 aqueous solution prepared by the above method is added to the alkali reduction wastewater D, and then continued to stir for 2min to fully react, generate aluminum salt and calcium salt precipitates of TA and its oligomers, and form primary flocs;

[0065] S2, coagulation: according to the weight of the alkali-reduced wastewater D, the amount of coagulant I is determined to be 0.5kg / T, and a 6wt.% PAC aqueous solution is prepared; the aqueous solution of coagulant I is added to the wastewater treated in step S1, and then stirring is continued for 3min; according to the weight of the alkali-reduced wastewater D, the amount of coagulant II is determined to be 20g / T and 25g / T, and a 0.12wt.% mixed aqueous solution is prepared; the aqueous solution of coagulant II is continued to be added to the wastewater, and then stirring is continued for 6min; wherein, coagulant I adopts PAC28, and coagulant II adopts a mixture of cationic polyacrylamide and cationic starch in a weight ratio of 1:1, the ionicity of the cationic polyacrylamide is 30, the molecular weight is 13 million Da, and the substitution degree of the cationic starch is 0.035, and the molecular weight is 2 million Da;

[0066] S3. Using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids.

[0067] As comparative example 4, the dosage of coagulant II in the above treatment process was replaced with 30 kg / T and 65 kg / T respectively, and the other steps were 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] From the treatment results shown in Table 3 and Table 4, it can be seen that, whether with the increase of coagulant I and other conditions remain unchanged, or with the increase of the dosage of coagulant II and other conditions remain unchanged, the pollutants such as COD and turbidity show an insignificant downward trend, indicating that TA salts and oligomers thereof in the wastewater are basically removed by the first step of alkali precipitation, and the second step of coagulation mainly plays a role in promoting the growth of floc particles and accelerating the sedimentation of flocs; in the experimental test, the water body can be quickly clarified after the treatment of Example 3 and Example 4, and the viscosity of the water body does not increase significantly, while the water body can also be quickly clarified after the treatment of Comparative Example 3 and Comparative Example 4, but the viscosity of the water body increases significantly, which is not conducive to the later concentration treatment of caustic soda, and the price of flocculants is relatively high, which increases the cost of medication.

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

[0073] In addition, the applicant also found that after the alkali precipitation step, a coagulant needs to be added immediately for the coagulation step in order to react with TA salt and its oligomers in time to form large particle flocs and settle and separate. Coagulant I and coagulant II can be added sequentially or simultaneously. There is no obvious difference in the flocculation treatment effect of the wastewater between the two addition methods.

[0074] Example 5

[0075] The alkali reduction wastewater E is treated with an alkali precipitation-coagulation treatment agent for alkali reduction wastewater, and the steps are as follows:

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

[0077] S2, coagulation: according to the weight of the alkali-reduced wastewater E, the amount of coagulant I is determined to be 0.5kg / T, and a 6wt.% PAC aqueous solution is prepared; the aqueous solution of coagulant I is added to the wastewater treated in step S1, and then stirring is continued for 3min; then according to the weight of the alkali-reduced wastewater E, the amount of coagulant II is determined to be 25g / T, and a 0.08wt.% mixed aqueous solution is prepared; the aqueous solution of coagulant II is continued to be added to the wastewater, and then stirring is continued for 6min; wherein, coagulant I adopts PAC28, and coagulant II adopts a mixture of cationic polyacrylamide and cationic starch in weight ratios of 4:6, 5:5, and 6:4, respectively, the ionicity of the cationic polyacrylamide is 30, the molecular weight is 13 million Da, and the substitution degree of the cationic starch is 0.035, and the molecular weight is 2 million Da;

[0078] S3. Using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids.

[0079] As comparative example 5, the total amount 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, and 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 the key to achieving effective flocculation of TA precipitates in wastewater; specifically, under strong alkaline conditions (pH>13.5), most nonionic and anionic coagulants will lose their electrical neutralization effect due to charge conversion (deprotonation), resulting in a serious decrease in flocculation effect or even failure; since CS and CPAM are both cationic polymer electrolytes, their macromolecular main chains carry strongly alkaline quaternary ammonium groups, which can still maintain sufficient positive charges in a strong alkaline environment, play a role in the electrical neutralization and destabilization of the TA colloids and particle surfaces in the water, and then through The bridging and netting effects of the macromolecular main chain form stable large-particle flocs, which are removed by gravity sedimentation. Among them, CS has a lower molecular weight, Mw of about 2 million DA, but it has a higher positive charge density (cationic substitution degree DS>0.035), which can play a strong electrical neutralization role, adsorb a large amount of TA colloids and suspended particles to destabilize them; while CPAM has a lower charge density than CS, but has a higher molecular weight, Mw of about 13 million DA, which can give full play to the bridging and netting effects. Furthermore, through the effective cooperation of the two and the complementary performance, the particle size of TA flocs and the mechanical strength of flocs can be effectively improved, the floc sedimentation rate can be accelerated, and at the same time, the water holding rate of flocs can be reduced, the filter press dehydration performance can be improved, and the production efficiency can be effectively improved.

[0084] From the treatment results in Table 5, it can be seen that when CS and CPAM are compounded in different proportions for flocculation tests, from the perspective of pollutant removal rate, as the CS ratio increases, the turbidity removal rate increases (wastewater transmittance increases), but after exceeding a certain ratio (CS:CPAM>6:4), the COD and TA removal rates gradually decrease; at the same time, from the perspective of floc structure, as the proportion of cationic starch CS increases, the floc particle size gradually decreases, the structure tends to be loose, and the sedimentation property becomes worse; and the effect of using CS alone or CPAM alone is worse than the effect of using the two in combination; based on this, considering the flocculation performance and the cost of using the agent, the compounding ratio of CPAM and CS is 4:6-6:4, and more preferably 5:5.

[0085] Furthermore, infrared testing was performed on the white mud samples recovered from Examples 1 to 5, and the infrared spectra were compared with those of terephthalic acid standards.

[0086] like Figure 1 The figure shows the infrared spectrum test diagram of one of the white mud samples recovered in Example 1 and the comparison diagram of the infrared spectrum of terephthalic acid standard. From the infrared spectrum comparison, it can be seen that the functional group area of ​​the white mud sample is 1640~1560cm -1 The COO in terephthalic acid － Absorption of stretching vibration, 1450cm -1 Nearby is the benzene ring vibration absorption, 1345cm -1 Nearby is the absorption peak of methylene CH2 bending vibration. It can be seen that the infrared spectrum of the recovered white mud sample is basically consistent with that of the standard product; the infrared spectrum test results of the white mud samples recovered in other embodiments are consistent with 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 precipitation-coagulation treatment agent for alkali reduction wastewater and the treatment method thereof of the present application can effectively achieve the pollution treatment of alkali reduction wastewater and the recovery of terephthalic acid and its oligomers.

Claims

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

4.

2. The alkali precipitation-coagulation treatment agent for alkali-reduced wastewater according to claim 1, characterized in that: The polyaluminium chloride used is PAC28.

3. The alkali precipitation-coagulation treatment agent for alkali-reduced wastewater according to claim 1, characterized in that: Cationic polyacrylamide uses cationic polyacrylamide with an ionicity of 30 and a molecular weight of 13 million Da.

4. The alkali precipitation-coagulation treatment agent for alkali-reduced wastewater according to claim 1, characterized in that: The cationic starch has a degree of substitution of 0.035 and a molecular weight of 2 million Da.

5. A method for treating alkali-reduced wastewater using the alkali precipitation-coagulation treatment agent for alkali-reduced wastewater according to any one of claims 1 to 4, characterized in that: The steps are: S1. Adding alkali precipitation treatment agent I and alkali precipitation treatment agent II to alkali reduction wastewater at the same time, and stirring the reaction evenly to obtain alkali precipitation treatment liquid; wherein, the amount of alkali precipitation treatment agent I is satisfied: 1.3kg to 1.7kg of Al2(SO4)3 is added per ton of wastewater, and the amount of alkali precipitation treatment agent II is satisfied: 2.3kg to 2.7kg of CaCl2 is added per ton of wastewater; S2, first add coagulant treatment agent I to the alkali precipitation treatment liquid, and after stirring and reacting evenly, add coagulant treatment agent II, and stir and react evenly; wherein, the amount of coagulant treatment agent I is satisfied: 0.4kg-0.6kg of polyaluminium chloride is added per ton of wastewater, and the amount of coagulant treatment agent II is satisfied: 15g-25g of composite polymer is added per ton of wastewater; S3, utilizing gravity sedimentation to settle the flocs formed in the wastewater treated in step S2; using a plate and frame filter press to dehydrate and recover the white mud-like solids, namely, terephthalate and its oligomers.

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