Acid-free treatment process for alkali-minimization wastewater based on alkali precipitation coagulation-electrodialysis

Abstract

The invention discloses an alkali decrement wastewater acid-free treatment process based on alkali precipitation coagulation-electrodialysis. The alkali decrement wastewater acid-free treatment process is realized through alkali precipitation, coagulation, dehydration and electrodialysis in sequence. According to the treatment process, on the premise that acid is not added and pH is not adjusted, primary separation of organic matters and alkali in the alkali-minimization wastewater is realized through alkali precipitation, coagulation, dehydration and electrodialysis which are sequentially carried out, most of colloids and suspended solid particles with main components of terephthalic acid and oligomers thereof are removed, the quality of water before a membrane is improved, the treatment process can be used for recycling, and the treatment cost is reduced. More than 90% of NaOH is kept in the wastewater; meanwhile, the treatment process can also realize separation and recovery of NaOH and organic matters in the alkali-minimization wastewater, and the concentration of caustic alkali NaOH in concentrated water directly meets the requirement of splitting use concentration, so that the concentrated water can be directly used for splitting in a workshop; and the fresh water can be recycled for splitting after alkali supplementation, so that the purposes of zero emission of alkali-minimization wastewater, cost reduction and efficiency improvement are achieved.

Description

Technical Field

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

[0008] To this end, the technical solution of the present invention is as follows:

[0009] The invention discloses an acid-free treatment process for alkali-reduced wastewater based on alkali precipitation coagulation-electrodialysis, which is achieved through four steps of alkali precipitation, coagulation, sedimentation dehydration and electrodialysis performed in sequence.

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

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

[0012] In the alkali precipitation step of alkali reduction wastewater, Al in the alkali precipitation reagent 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:

[0013]

[0014] 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.

[0015] In the present invention, the alkali precipitation step is the key to recovering terephthalic acid. Compared with the acid precipitation method described in the background art, although the acid precipitation method has a high recovery rate for TA, the TA obtained during the acid precipitation process is microparticles with a slow sedimentation rate, and requires the use of a large amount of reagents (such as FeCl2, MgSO4, HPAM) to achieve coagulation assistance; at the same time, due to the strong water retention of the microparticles, the efficiency of the filter press is greatly reduced; while the alkali precipitation step of the present invention directly utilizes the precipitant aluminum sulfate and the Al in calcium chloride without adding acid (i.e., no pH adjustment is required) to recover terephthalic acid. 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, i.e., to form primary flocs. 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. However, the particles formed at this stage are still small, the sedimentation rate is slow, and the water holding capacity is large. Therefore, it is necessary to further improve the floc structure, increase the floc particle size, and reduce the water holding capacity.

[0016] Therefore, the second part performs coagulation treatment on the alkali reduction wastewater, and its specific operation is: first add coagulant I to the alkali reduction wastewater at a dosing flow rate of 0.8L / h~1.0L / h, and after the addition is completed, stir at a stirring rate of 250r / min~350r / min for 2min~4min; then add coagulant II to the alkali reduction wastewater, and stir at a stirring rate of 80r / min~120r / min for 4min~6min, and then stir at a stirring rate of 45r / min~55r / min for 4min~6min; wherein, coagulant I is a polyaluminum chloride aqueous solution, and its addition amount is 0.4kg~0.6kg polyaluminum chloride per ton of wastewater; coagulant II is a cationic polyacrylamide aqueous solution, and its addition amount is 15g~25g cationic polyacrylamide per ton of wastewater;

[0017] In the above steps, coagulant I is polyaluminium chloride PAC28, and the mass fraction of its aqueous solution is 4wt.% to 6wt.%; cationic polyacrylamide in coagulant II adopts 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.% to 6wt.%.

[0018] In the coagulation step of alkali-reduced wastewater, based on the previous alkali precipitation treatment results, coagulants are further added to the wastewater to form large-particle flocs by utilizing electrical neutralization, adsorption bridging, and sweeping effects, and then separated by gravity sedimentation; and the coagulation process is divided into two stages: mixing and reaction, and the mixing process requires rapid and intense mixing. Therefore, after adding two coagulants respectively, the stirring rate is 80r / min~120r / min, and the stirring is 4min~6min, and the agent and the precipitate are quickly and evenly mixed by hydraulic conditions; then in the reaction stage after the mixing is completed, the stirring speed is reduced, and the stirring reaction is continued for a sufficient time to allow the flocs to grow; however, since TA has been converted into insoluble high-valent metal salts in the previous alkali precipitation treatment, the amount of coagulant needs to be precisely controlled in the coagulation stage to ensure that the coagulant maintains the characteristics of strong positive charge in a high pH environment, thereby utilizing its stable electrical neutralization and bridging capabilities to quickly form stable flocs and improve the sedimentation effect.

[0019] Next, the third step is to use gravity sedimentation to settle the flocs formed in the wastewater obtained by the treatment in step S2, and then dehydrate them through a plate and frame filter press to recover white mud-like solids with a moisture content of 60% to 70%, thereby recovering terephthalate and its oligomers.

[0020] Finally, the water samples recovered after dehydration treatment are subjected to electrodialysis treatment, and by setting the concentration ratio of desalinated liquid: concentrated liquid (specifically NaOH) to 30:1, the final concentrated water obtained after treatment can directly meet the concentration requirements of fiber opening water, and the final fresh water can also be used as fiber opening water after adding alkali.

[0021] Compared with the prior art, the acid-free treatment process for alkali reduction wastewater based on the alkali precipitation-coagulation method realizes the preliminary separation of organic matter and alkali in the alkali reduction wastewater by sequentially performing alkali precipitation, coagulation, dehydration and electrodialysis without adding acid and adjusting pH, removes most of the colloids and suspended solid particles whose main components are terephthalic acid and its oligomers, improves the 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 the separation and recycling of NaOH and organic matter in the alkali reduction wastewater, the concentration of caustic soda NaOH in the concentrated water directly reaches the concentration requirement for fiber opening, and can be directly used for fiber opening in the workshop; the fresh water can also be reused for fiber opening after alkali supplementation, and the purpose of zero discharge of alkali reduction wastewater and cost reduction and efficiency improvement is achieved. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with specific examples, but the following examples are by no means intended to limit the present invention in any way.

[0023] The alkali reduction wastewater is treated by the acid-free treatment process for alkali reduction wastewater based on alkali precipitation coagulation-electrodialysis in this application. The dosage of the treatment agent and the 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 a specified dosing flow rate, and stir for 2 minutes at a stirring rate of V1; wherein the alkali precipitation reagent is a mixture of 15wt.% Al2(SO4)3 aqueous solution and 20wt.% CaCl2 aqueous solution;

[0025] S2, first add 5wt.% polyaluminium chloride aqueous solution to the alkali reduction wastewater at a specified dosing flow rate, and after the addition is completed, stir at a stirring rate of V1 for 2 minutes; then add 0.1wt.% cationic polyacrylamide aqueous solution to the alkali reduction wastewater, first stir at a stirring rate of V2 for 4 minutes, and then stir at a stirring rate of V3 for 4 minutes;

[0026] S3, using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2, and then dehydrating the wastewater through a plate and frame filter press to recover the white mud-like solids, i.e., terephthalate and its oligomers;

[0027] S4. Perform electrodialysis on the water sample obtained in step S3, and set the ratio of desalinated liquid to concentrated NaOH to 30:1.

[0028] Table 1:

[0029]

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

[0031] Table 2:

[0032]

[0033] From the test results shown in Table 2, we can see that:

[0034] According to Example 1, Example 2 and Example 3, when the dosage of PAC28 and cationic polyacrylamide is the same, as the dosage of the alkali precipitation agent increases, the alkali precipitation effect is gradually improved; specifically, the turbidity removal rate is increased from 51.88% to 76.28%, and the COD removal rate is increased from 29.02% to 35.72%; wherein, the turbidity removal rate is significantly improved, indicating that the alkali precipitation process mainly removes the hydrolysis of polyethylene terephthalate with a larger molecular weight; in addition, compared with Example 2 and Example 3, the treatment effect of Example 1 is poor and does not meet the treatment requirements, that is, the dosage of the alkali precipitation agent is too low; based on this, after experimental verification, the amount of aluminum sulfate added in the alkali precipitation agent is 1.3kg / T to 1.7kg / T, preferably 1.5kg / T; the amount of calcium chloride added is 2.3kg / T to 2.7kg / T, preferably 2.5kg / T;

[0035] According to Example 5 and Example 6, when the dosage of aluminum sulfate and calcium chloride is the same, at a higher dosage of coagulant, the turbidity removal rate and COD removal rate only increase slightly, and the change is not significant; taking the treatment effect of Example 5 and Example 6 as an example, the turbidity removal rate is only increased from 75.47% to 76.04%, and the COD removal rate is only increased from 37.67% to 39.24%; at the same time, the pH of the solution decreases with the increase in the dosage of the coagulant reagent. Since this will affect the recovery of the alkali in the wastewater, the dosage of the coagulant reagent needs to consider not only the flocculation effect, but also the consumption of the alkali in the wastewater; based on this, after experimental verification, the dosage of PAC28 in the coagulation step is set to 0.4kg / T~0.6kg / T, and the dosage of cationic polyacrylamide is set to 15g / T~25g / T, which has a better treatment effect; preferably, the dosage of PAC28 is 0.5kg / T, and the dosage of cationic polyacrylamide is 20g / T;

[0036] According to Example 6, Example 7 and Example 8, when the dosage of the alkali precipitation agent and the coagulation agent is kept the same, when the dosing rate is reduced from 0.9L / h to 0.6L / h, the coagulation reaction is too slow and microflocculation cannot be generated in time; and when the dosing rate is increased from 0.9L / h to 1.2L / h, it will cause the local coagulant concentration to be too high, resulting in unnecessary hydrolysis. Therefore, when the dosing rate is 0.6L / h and 1.2L / h, the COD removal rate and turbidity removal rate are significantly lower than the treatment effect when the dosing rate is 0.9L / h; Based on this, it has been experimentally verified that whether in the alkali precipitation step or the coagulation step, the dosing rate is set to 0.8L / h~1.0L / h, which has a better treatment effect;

[0037] When the amount of alkali precipitation reagent and coagulation reagent is the same and the dosing speed is the same, the stirring rate after each reagent is added also affects the treatment effect; wherein, Example 6, Example 9 and Example 10 are a comparison group, Example 6, Example 11 and Example 12 are a comparison group, and Example 6, Example 13 and Example 14 are a comparison group; based on the comparison of the treatment effects of the above three comparison groups, it can be seen that when the stirring rate is too low, the reagents are not mixed sufficiently, and when the stirring rate is too high, on the one hand, the flocculant degrades too quickly, and on the other hand, the flocs are easily broken by mechanical action, resulting in a decrease in the treatment effect of turbidity and COD; therefore, in this treatment process, V1 is set to 250r / min~350r / min, V2 is set to 80r / min~120r / min, and V3 is set to 45r / min~55r / min, which has a better treatment effect; more preferably, V1 is set to 300r / min, V2 is set to 100r / min, and V3 is set to 50r / min.

[0038] In addition, the white mud-like solid obtained by the above-mentioned embodiments was subjected to infrared spectrum test, and the infrared spectrum test spectrum was compared with the infrared spectrum of the standard product; the specific test comparison result was that the functional group region of the white mud-like solid was 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; the infrared spectrum of the TA recovered sample is basically consistent with that of the standard, indicating that the main components of the white mud recovered by this technology are terephthalic acid and its oligomers.

[0039] Further, the water sample obtained by dehydration by the plate and frame filter press in Example 2 is subjected to electrodialysis treatment in step S4. The electrodialyzer adopts existing equipment, and its main components are anion and cation exchange membranes, partitions and electrodes; the compartment formed by the partition is a channel for liquid flow, the compartment through which fresh water passes is a desalination chamber, and the compartment through which concentrated water passes is a concentration chamber, and the anion and cation exchange membranes and concentrated and fresh water partitions are arranged alternately and repeatedly stacked, and a pair of end electrodes are also configured; wherein, the cation exchange membrane adopts a cation exchange membrane with a sulfonic acid functional group, and the anion exchange membrane adopts an anion exchange membrane with a quaternary plating functional group; the partition adopts a hollow flow channel partition made of PP polytetrafluoro composite material, and the polar membrane adopts a perfluorosulfonic acid membrane.

[0040] In the electrodialysis step, 3000ml of alkali-reduced wastewater after coagulation is introduced into the desalination solution tank, and then a specified volume of deionized water is introduced into the concentrated solution tank to limit the concentration ratio, for example, 100mL of deionized water is introduced to make the concentration ratio 30:1, and 500mL of 2wt.% NaOH solution is introduced into the electrode liquid tank; the pole chamber pump, the feed liquid pump (desalination liquid pump) and the receiving liquid pump (concentrated liquid pump) are started, and the flow of the desalination liquid and the concentrated liquid is set not to exceed 40L / h, and the flow of the pole liquid is greater than 20L / h. At the same time, the flow is adjusted so that the inlet pressure of each solution remains basically the same, and then the electrodialysis operation is officially started. The trend of current and voltage changes is recorded every 5 minutes. The pH value, turbidity, COD and other water quality indicators of the final concentrated water and the final fresh water are measured by pH meter, turbidity meter and fully automatic COD measuring instrument respectively. The specific test results are shown in Table 3 below.

[0041] Table 3:

[0042]

[0043] From the test results shown in Table 3, it can be seen that in the electrodialysis concentration experiment, different concentration ratios were set during the electrodialysis process. wt％ It gradually increases with the increase of concentration ratio. When the concentration ratio is 30:1 for desalinated liquid: concentrated liquid, NaOH wt％ The concentration of NaOH was 73.56 kg / T, which met the requirements for water concentration for fiber opening. However, when the concentration ratio was further increased to 35:1, wt％ The concentration ratio of NaOH was 7.481%, and the turbidity and COD increased, the water quality decreased, and the energy consumption increased. When the concentration ratio was further reduced to 25:1, the wt％ It is 69.23kg / T. Although it is only a slight decrease, based on the water treatment cost, the concentration ratio of desalinated liquid: concentrated liquid is set to 30:1.

[0044] In the electrodialysis process, when the concentration ratio of desalinated liquid to concentrated liquid is set to 30:1, the NaOH content of the final concentrated water obtained by treatment is wt％ The concentration is 73.56 kg / T (i.e., the mass fraction of NaOH solution is greater than 7%), which meets the requirements for the reuse of fiber opening water and can be directly reused for fiber opening. wt％ The concentration is 4.20 kg / T (i.e., the mass fraction of the NaOH solution is less than 0.5%), and this concentration can be used for fiber opening after alkali supplementation.

[0045] In summary, the acid-free treatment process for alkali reduction wastewater based on alkali precipitation coagulation-electrodialysis can achieve the preliminary separation of organic matter and alkali in alkali reduction wastewater, remove most of the colloids and suspended solid particles whose main components are terephthalic acid and its oligomers, improve the water quality before the membrane, can be used for recycling, and keep more than 90% of NaOH in the wastewater; at the same time, the treatment process can also achieve the separation and recovery of NaOH and organic matter in alkali reduction wastewater, the mass concentration of caustic soda NaOH in concentrated water is greater than 7%, which meets the concentration requirements for fiber opening and can be directly used for fiber opening in the workshop; fresh water can also be reused for fiber opening after alkali supplementation, realizing zero discharge of alkali reduction wastewater.

Claims

1. A non-acid treatment process for alkali-reduced wastewater based on alkali precipitation coagulation-electrodialysis, characterized in that: The steps are: S1. Add alkali precipitation reagent to the alkali reduction wastewater at a dosing rate of 0.8L / h to 1.0L / h, and stir for 2min to 4min at a stirring rate of 250r / min to 350r / min; wherein the alkali precipitation reagent is a mixed aqueous solution of Al2(SO4)3 and CaCl2, the amount of Al2(SO4)3 added is 1.3kg to 1.7kg per ton of wastewater, and the amount of CaCl2 added is 2.3kg to 2.7kg per ton of wastewater; S2, at a dosing flow rate of 0.8L / h to 1.0L / h, first add coagulant I to the alkali-reduced wastewater, and after the addition is completed, stir at a stirring rate of 250r / min to 350r / min for 2min to 4min; then add coagulant II to the alkali-reduced wastewater, and stir at a stirring rate of 80r / min to 120r / min for 4min to 6min, and then stir at a stirring rate of 45r / min to 55r / min for 4min to 6min; wherein coagulant I is a polyaluminum chloride aqueous solution, and its addition amount is 0.4kg to 0.6kg polyaluminum chloride per ton of wastewater; coagulant II is a cationic polyacrylamide aqueous solution, and its addition amount is 15g to 25g cationic polyacrylamide per ton of wastewater; S3, using gravity sedimentation to settle the flocs formed in the wastewater treated in step S2, and then dehydrating the wastewater through a plate and frame filter press to recover the white mud-like solids, i.e., terephthalate and its oligomers; S4. Perform electrodialysis on the water sample obtained by the process in step S3, and set the concentration ratio of the desalinated liquid to the concentrated liquid to be 30:

1.

2. The acid-free treatment process for alkali-reduced wastewater based on alkali precipitation coagulation-electrodialysis according to claim 1, characterized in that: In step S1, the mass fraction of the Al2(SO4)3 aqueous solution is 14wt.% to 16wt.%, and the mass fraction of the CaCl2 aqueous solution is 19wt.% to 21wt.%.

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

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

Images