Alkaline-reduction wastewater acid-free treatment process based on alkali precipitation coagulation-electrodialysis

Abstract

The application discloses an alkali-reduction wastewater acid-free treatment process based on alkali precipitation coagulation-electrodialysis, which is realized by sequentially performing alkali precipitation, coagulation, dehydration and electrodialysis; under the premise of not adding acid and not adjusting pH, the treatment process realizes preliminary separation of organic matters and alkali in alkali-reduction wastewater by sequentially performing alkali precipitation, coagulation, dehydration and electrodialysis, removes most of colloid and suspended solid particles with terephthalic acid and its oligomers as main components, improves water quality before the membrane, can be used for recycling, and keeps more than 90% of NaOH in the wastewater; at the same time, the treatment process can also realize separation and recycling of NaOH and organic matters in alkali-reduction wastewater, the concentration of caustic alkali NaOH in concentrated water directly reaches the concentration requirement of opening fiber use, and can be directly used for opening fiber in the workshop; after alkali supplement, the fresh water can also be reused for opening fiber, and the purposes of zero discharge and cost reduction and benefit increase of alkali-reduction wastewater are realized.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and in particular to an acid-free treatment process for alkali-reduced wastewater based on alkali precipitation coagulation-electrodialysis. 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 terephthalate (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 acid-free wastewater treatment process based on alkali precipitation coagulation-electrodialysis to solve technical problems.

[0008] Therefore, the technical solution of the present invention is as follows:

[0009] An acid-free wastewater treatment process based on alkali precipitation coagulation-electrodialysis is proposed, which is achieved through four steps: alkali precipitation, coagulation, sedimentation dewatering, and electrodialysis.

[0010] The first step is to treat the alkali reduction wastewater with alkali precipitation. The specific operation is as follows: add alkali precipitation reagent to the alkali reduction wastewater at a dosing flow rate of 0.8L / h to 1.0L / h, and stir at a stirring rate of 250r / min to 350r / min for 2min to 4min.

[0011] In the above steps, the alkaline precipitation reagent is a mixed aqueous solution of Al2(SO4)3 and CaCl2. The amount of Al2(SO4)3 added is 1.3 kg to 1.7 kg per ton of wastewater, and the amount of CaCl2 added is 2.3 kg to 2.7 kg per ton of wastewater. Specifically, the mass fraction of the Al2(SO4)3 aqueous solution is 14 wt.% to 16 wt.%, and the mass fraction of the CaCl2 aqueous solution is 19 wt.% to 21 wt.%.

[0012] In the alkaline precipitation step of the alkaline reduction wastewater treatment process, the Al in the alkaline precipitation reagent... 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:

[0013]

[0014] 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.

[0015] In this invention, the alkaline precipitation step is crucial for the recovery of terephthalic acid (TA). Compared to the acid precipitation method described in the background art, although the acid precipitation method has a high TA recovery rate, the TA obtained during the acid precipitation process is in the form of particulates 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; while the alkaline precipitation step of this invention, without the addition of acid (i.e., without pH adjustment), directly utilizes the precipitant aluminum sulfate and the Al in calcium chloride. 3+ Ca 2+ These two high-valence ions react with terephthalate ions via ion exchange to form insoluble aluminum and calcium salts, thus forming primary flocs. However, a problem remains after the alkaline precipitation step: the precipitate particles are too small to settle easily. Therefore, further treatment is needed to ensure sufficient settling of the precipitate. However, the particles formed at this stage are still small, the settling rate is slow, and the water holding capacity is large. Further improvements to the floc structure are needed to increase the floc particle size and reduce the water holding capacity.

[0016] Therefore, the second step involves coagulation treatment of the alkali reduction wastewater. The specific operation is as follows: Coagulant I is first added to the alkali reduction wastewater at a dosing rate of 0.8 L / h to 1.0 L / h. After addition, the mixture is stirred at a stirring rate of 250 r / min to 350 r / min for 2 to 4 minutes. Then, coagulant II is added to the alkali reduction wastewater, and the mixture is stirred sequentially at a stirring rate of 80 r / min to 120 r / min for 4 to 6 minutes, followed by a stirring rate of 45 r / min to 55 r / min for 4 to 6 minutes. Coagulant I is an aqueous solution of polyaluminum chloride, added at a dosage of 0.4 kg to 0.6 kg of polyaluminum chloride per ton of wastewater; coagulant II is an aqueous solution of cationic polyacrylamide, added at a dosage of 15 g to 25 g of cationic polyacrylamide per ton of wastewater.

[0017] In the above steps, coagulant I is polyaluminum chloride PAC28, and its aqueous solution has a mass fraction of 4 wt.% to 6 wt.%; the cationic polyacrylamide in coagulant II is cationic polyacrylamide with an ionicity of 30 and a molecular weight of 13 million Da, and its aqueous solution has a mass fraction of 4 wt.% to 6 wt.%.

[0018] In the coagulation step of alkali reduction wastewater treatment, based on the results of the prior alkali precipitation treatment, coagulants are further added to the wastewater to form large flocs through charge neutralization, adsorption bridging, and sweeping effects, which are then separated by gravity sedimentation. The coagulation process is divided into two stages: mixing and reaction. The mixing process requires rapid and vigorous mixing. Therefore, after adding two coagulants, the mixture is stirred at a speed of 80-120 rpm for 4-6 minutes to rapidly and uniformly mix the reagents and precipitates using hydraulic conditions. Following the mixing, in the reaction stage, the stirring speed is reduced, but stirring continues for a sufficient time to allow floc growth. However, since TA has been converted into insoluble high-valence metal salts in the previous alkali precipitation treatment, the dosage of coagulant needs to be precisely controlled during the coagulation stage to ensure that it maintains its strong positive charge characteristics in a high pH environment. This allows it to utilize its stable charge neutralization and bridging capabilities to quickly form stable flocs and improve sedimentation efficiency.

[0019] Next, the third step is to settle the flocs formed in the wastewater obtained after step S2 by gravity sedimentation, and then dewater them by plate and frame filter press to recover the white mud-like solids with a water content of 60% to 70%, thereby enabling the recovery and treatment of phthalates and their oligomers.

[0020] Finally, the water sample recovered after dehydration is subjected to electrodialysis. By setting the concentration ratio of desalination solution to concentrate (specifically NaOH) to 30:1, the final concentrated water obtained after treatment directly meets the concentration requirements for fiber opening water. The final desalination water can also be used as fiber opening water after adding alkali.

[0021] Compared with existing technologies, this acid-free treatment process for alkali reduction wastewater based on alkali precipitation-coagulation achieves preliminary separation of organic matter and alkali in alkali reduction wastewater without adding acid or adjusting pH. It involves sequential alkali precipitation, coagulation, dewatering, and electrodialysis, removing most colloidal and suspended solid particles, primarily composed of terephthalic acid and its oligomers, improving the water quality before membrane treatment, and allowing for recycling. It also maintains over 90% NaOH retention in the wastewater. Simultaneously, this process separates and recovers NaOH from organic matter in the alkali reduction wastewater. The concentration of caustic alkali NaOH in the concentrated wastewater directly meets the requirements for fiber opening and can be directly used in the fiber opening workshop. The desalinated water, after alkali replenishment, can also be reused for fiber opening, achieving zero discharge of alkali reduction wastewater and cost reduction and efficiency improvement. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0023] The acid-free treatment process for alkali reduction wastewater based on alkali precipitation coagulation-electrodialysis, as described in this application, is used to treat alkali reduction wastewater. The dosage of the treatment agent and key process parameters in the specific process are shown in Table 1 below.

[0024] S1. Add alkali precipitation reagent to the alkali reduction wastewater at the specified dosing flow rate and stir at the stirring rate of V1 for 2 minutes; wherein, the alkali precipitation reagent is a mixture of 15 wt.% Al2(SO4)3 aqueous solution and 20 wt.% CaCl2 aqueous solution.

[0025] S2. At the specified dosing flow rate, first add 5 wt.% of polyaluminum chloride aqueous solution to the alkali reduction wastewater. After the addition is complete, stir at the stirring rate of V1 for 2 min. Then add 0.1 wt.% of cationic polyacrylamide aqueous solution to the alkali reduction wastewater. First stir at the stirring rate of V2 for 4 min, and then stir at the stirring rate of V3 for 4 min.

[0026] S3. Utilize gravity settling to allow the flocs formed in the wastewater obtained from step S2 to settle, and then dewater them using a plate and frame filter press to recover the white mud-like solids, namely terephthalate and its oligomers.

[0027] S4. The water sample obtained after step S3 is subjected to electrodialysis treatment, with the ratio of desalination solution to concentrated NaOH solution set at 30:1.

[0028] Table 1:

[0029]

[0030] Table 2 below shows the test results of pH, COD and turbidity of the treated water samples obtained after treating the alkali reduction wastewater using the dosage of the treatment agent and key process parameters shown in Table 1 through steps S1 to S3.

[0031] Table 2:

[0032]

[0033] The test results shown in Table 2 indicate that:

[0034] As shown in Examples 1, 2, and 3, when the amounts of PAC28 and cationic polyacrylamide are the same, the alkaline precipitation effect gradually improves with the increase of the amount of alkaline precipitation reagent. Specifically, the turbidity removal rate increases from 51.88% to 76.28%, and the COD removal rate increases from 29.02% to 35.72%. The improvement in turbidity removal rate is more significant, indicating that the alkaline precipitation process mainly removes the hydrolyzed polyterephthalic acid with a larger molecular weight. In addition, compared with Examples 2 and 3, the treatment effect of Example 1 is poor and does not meet the treatment requirements, i.e., the amount of alkaline precipitation reagent is too low. Based on this, after experimental verification, the amount of aluminum sulfate added in the alkaline precipitation reagent is 1.3 kg / T to 1.7 kg / T, preferably 1.5 kg / T; the amount of calcium chloride added is 2.3 kg / T to 2.7 kg / T, preferably 2.5 kg / T.

[0035] As shown in Examples 5 and 6, with the same dosage of aluminum sulfate and calcium chloride, the turbidity removal rate and COD removal rate only slightly increased with higher coagulant dosage, and the changes were not significant. Taking the treatment effects of Examples 5 and 6 as examples, the turbidity removal rate only increased from 75.47% to 76.04%, and the COD removal rate only increased from 37.67% to 39.24%. At the same time, the pH of the solution decreased with the increase of coagulant dosage. Since this will affect the recovery of alkali in the wastewater, the dosage of coagulant needs to consider not only the flocculation effect but also the consumption of alkali in the wastewater. Based on this, after experimental verification, a better treatment effect is achieved when the dosage of PAC28 in the coagulation step is set to 0.4 kg / T to 0.6 kg / T and the dosage of cationic polyacrylamide is set to 15 g / T to 25 g / T. Preferably, the dosage of PAC28 is 0.5 kg / T and the dosage of cationic polyacrylamide is 20 g / T.

[0036] According to Examples 6, 7, and 8, when the dosage of alkali precipitation reagent and coagulation reagent remains the same, when the dosing rate is reduced from 0.9 L / h to 0.6 L / h, the coagulation reaction is too slow and micro-flocs cannot be generated in time. Conversely, when the dosing rate is increased from 0.9 L / h to 1.2 L / h, the local coagulant concentration becomes too high, leading to unnecessary hydrolysis. Therefore, the COD removal rate and turbidity removal rate are significantly lower at dosing rates of 0.6 L / h and 1.2 L / h than at a dosing rate of 0.9 L / h. Based on this, experimental verification shows that a dosing rate of 0.8 L / h to 1.0 L / h provides better treatment results in both the alkali precipitation and coagulation steps.

[0037] When the dosage and dosing rate of the alkali precipitation reagent and coagulation reagent are the same, the stirring rate after each reagent is added also affects the treatment effect. Examples 6, 9, and 10 form one control group; Examples 6, 11, and 12 form another control group; and Examples 6, 13, and 14 form yet another control group. Based on the comparison of the treatment effects of these three control groups, it can be seen that when the stirring rate is too low, the reagents are not mixed sufficiently. Conversely, when the stirring rate is too high, the flocculant degrades too quickly, and the flocs are easily broken by mechanical action, resulting in a decrease in both turbidity and COD treatment effects. Therefore, in this treatment process, setting V1 to 250 r / min to 350 r / min, V2 to 80 r / min to 120 r / min, and V3 to 45 r / min to 55 r / min yields better treatment effects. More preferably, V1 is set to 300 r / min, V2 to 100 r / min, and V3 to 50 r / min.

[0038] In addition, infrared spectroscopy was performed on the white clay-like solids obtained after the above-described embodiments, and the infrared spectra were compared with those of the standard. Specifically, the functional group region of the white clay-like solids was 1640–1560 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 peaks are due to the bending vibrations of methylene CH2; the infrared spectra of the TA recovered sample and the standard are basically consistent, indicating that the main components of the white mud recovered by this technology are terephthalic acid and its oligomers.

[0039] Furthermore, the water sample obtained from the plate and frame filter press in Example 2 was subjected to electrodialysis treatment in step S4. The electrodialysis unit used was existing equipment, its main components being anion and cation exchange membranes, separators, and electrodes. The compartments formed by the separators served as channels for liquid flow; the compartment through which the desalination water passed was the desalination chamber, and the compartment through which the concentrated water passed was the concentration chamber. The anion and cation exchange membranes and the concentrated and desalination water separators were arranged alternately and repeatedly stacked. In addition, a pair of end electrodes were provided. The cation exchange membrane was a cation exchange membrane with sulfonic acid functional groups, and the anion exchange membrane was an anion exchange membrane with quaternary ammonium functional groups. The separators were hollow flow channel separators made of PP polytetrafluoroethylene composite material, and the electrode membranes were perfluorosulfonic acid membranes.

[0040] In the electrodialysis step, 3000 ml of coagulated alkali-reducing wastewater was introduced into the desalination solution tank, followed by a specified volume of deionized water into the concentration solution tank to define the concentration ratio (e.g., 100 mL of deionized water to achieve a concentration ratio of 30:1). Meanwhile, 500 mL of 2 wt.% NaOH solution was introduced into the electrode solution tank. The electrode chamber pump, feed pump (desalination solution pump), and receiving liquid pump (concentrate pump) were started, with the flow rates of the desalination and concentrate set to no more than 40 L / h, and the electrode solution flow rate greater than 20 L / h. The flow rates were adjusted to maintain a relatively consistent inlet pressure for each solution stream before the electrodialysis operation officially began. Current and voltage changes were recorded every 5 minutes. The pH, turbidity, and COD of the final concentrate and desalination water were measured using a pH meter, turbidity meter, and fully automatic COD analyzer, respectively. The specific test results are shown in Table 3 below.

[0041] Table 3:

[0042]

[0043] As shown in Table 3, the test results indicate that in electrodialysis concentration experiments with different concentration ratios, the concentration of NaOH in the concentrate... wt％ The concentration gradually increases with the increase of the concentration ratio. When the concentration ratio is 30:1 for desalinated solution and concentrated solution, the concentration of NaOH... wt％ The concentration was 73.56 kg / t, meeting the requirements for water concentration in fiber opening; however, when the concentration ratio was further increased to 35:1, the NaOH concentration... wt％ The concentration ratio was 7.481%, and both turbidity and COD showed an increasing trend, indicating a decline in water quality and an increase in energy consumption. Furthermore, when the concentration ratio was further reduced to 25:1, the NaOH concentration... wt％ The concentration ratio is 69.23 kg / T, which is only slightly lower than the previous value. However, considering the cost of water treatment, the concentration ratio of desalination liquid to concentrate is set at 30:1.

[0044] In the electrodialysis process, when the concentration ratio of desalination solution to concentrate is set to 30:1, the final concentrated water obtained after treatment contains NaOH. wt％ The concentration is 73.56 kg / T (i.e., the mass fraction of NaOH solution is >7%), which meets the requirements for the reuse of water from fiber opening and can be directly reused for fiber opening; while the final freshwater contains NaOH... wt％ The concentration is 4.20 kg / T (i.e., the mass fraction of NaOH solution is <0.5%), which is sufficient for the fiber opening process after alkali replenishment.

[0045] In summary, this acid-free treatment process for alkali reduction wastewater based on alkali precipitation coagulation-electrodialysis can achieve preliminary separation of organic matter and alkali in alkali reduction wastewater, removing most colloidal and suspended solid particles whose main components are terephthalic acid and its oligomers, improving the water quality before membrane treatment, and allowing for recycling, while maintaining more than 90% of NaOH retained in the wastewater. Simultaneously, this process can also separate and recover NaOH and organic matter from the alkali reduction wastewater, with the concentrated caustic alkali NaOH concentration >7%, meeting the concentration requirements for fiber opening and allowing direct use in the workshop. The desalinated water can also be reused for fiber opening after alkali replenishment, achieving zero discharge of alkali reduction wastewater.

Claims

1. A zero-acid wastewater treatment process based on alkali precipitation coagulation-electrodialysis, characterized in that, The steps are as follows: S1. Add the alkali precipitation reagent to the alkali reduction wastewater at a dosing flow rate of 0.8 L / h to 1.0 L / h, and stir at a stirring rate of 250 r / min to 350 r / min for 2 min to 4 min. The alkali precipitation reagent is a mixed aqueous solution of Al2(SO4)3 and CaCl2. The dosage of Al2(SO4)3 is 1.3 kg to 1.7 kg per ton of wastewater, and the dosage of CaCl2 is 2.3 kg to 2.7 kg per ton of wastewater. The mass fraction of the Al2(SO4)3 aqueous solution is 14 wt% to 16 wt%, and the mass fraction of the CaCl2 aqueous solution is 19 wt% to 21 wt%. S2. Add coagulant I to the alkali reduction wastewater at a dosing rate of 0.8 L / h to 1.0 L / h. After addition, stir at a stirring rate of 250 r / min to 350 r / min for 2 min to 4 min. Then add coagulant II to the alkali reduction wastewater and stir at a stirring rate of 80 r / min to 120 r / min for 4 min to 6 min, and then at a stirring rate of 45 r / min to 55 r / min for 4 min to 6 min. Coagulant I is an aqueous solution of polyaluminum chloride, and its dosage is 0.4 kg to 0.6 kg of polyaluminum chloride per ton of wastewater. Coagulant II is an aqueous solution of cationic polyacrylamide, and its dosage is 15 g to 25 g of cationic polyacrylamide per ton of wastewater. S3. Utilize gravity settling to allow the flocs formed in the wastewater obtained from step S2 to settle, and then dewater them using a plate and frame filter press to recover the white mud-like solids, namely terephthalate and its oligomers. S4. The water sample obtained after step S3 is subjected to electrodialysis treatment, and the concentration ratio of desalination solution to concentrate is set to 30:

1.

2. The acid-free wastewater treatment process based on alkali precipitation coagulation-electrodialysis according to claim 1, characterized in that, In step S2, the polyaluminum chloride is PAC28, and its aqueous solution has a mass fraction of 4wt%~6wt%.

3. The acid-free wastewater treatment process based on alkali precipitation coagulation-electrodialysis according to claim 1, characterized in that, In step S2, the cationic polyacrylamide used is a cationic polyacrylamide with an ionicity of 30 and a molecular weight of 13 million Da, and the mass fraction of its aqueous solution is 4wt%~6wt%.

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