A polyaluminum chloride-based flocculant, its preparation method and application

Abstract

This invention discloses a polyaluminum chloride (PAC)-based flocculant, its preparation method, and its application, belonging to the field of flocculant preparation technology. It addresses the technical problem that the flocculation performance and stability of existing flocculants need further improvement. The method involves adjusting the surface charge of iron-based particles using sodium citrate, effectively avoiding particle aggregation and improving the uniformity of microgel coating and the specific surface area of ​​the material. Trimethylamine-modified chitosan introduces abundant cationic sites, significantly enhancing the electrostatic adsorption capacity of the microgel for anionic pollutants, while simultaneously improving the system's electrical properties and stability. The addition of sodium metasilicate further promotes the formation of a stable Al-O-Si bridging structure in the modified PAC, endowing the material with good three-dimensional skeleton strength and composite interface stability. These three elements synergistically play key roles in particle dispersibility, charge construction, and skeleton stability, respectively, ultimately constructing a highly efficient, stable, and storable composite flocculation system.

Description

Technical Field

[0001] This invention relates to the field of flocculant preparation technology, specifically to a polyaluminum chloride-based flocculant, its preparation method, and its application. Background Technology

[0002] Polyaluminum chloride (PAC) is an inorganic polymeric flocculant with superior technical performance compared to traditional aluminum salts. This is mainly due to its unique prepolymerized structure and high charge characteristics. During water treatment, PAC can quickly neutralize the negative charge on the surface of colloidal particles and promote particle aggregation through adsorption bridging, forming easily settling flocs. Compared to traditional flocculants such as aluminum sulfate, PAC has a stronger adaptability to pH changes, more stable flocculation effect, and lower residual aluminum content, reducing the risk of secondary pollution.

[0003] However, the stability of PAC is affected by alkalinity, storage conditions and chemical form. Too high or too low alkalinity will affect its flocculation activity. To improve stability, stabilizers can be added or it can be compounded with other metals to delay the conversion of its active components. Overall, PAC has become one of the important flocculants in the modern water treatment field due to its high efficiency in flocculation and relatively stable chemical properties.

[0004] For example, the prior art CN115925071B discloses a production process for a high-efficiency polyaluminum chloride-based composite flocculant. The process involves adding tourmaline to ethanol and ball milling it at low temperature to obtain fine tourmaline powder. Vermiculite is then ground at low temperature. Tourmaline, aluminum isopropoxide, and trichloromethylsilane are added sequentially and stirred. After dispersion and granulation, a pre-prepared vermiculite carrier is obtained. The pre-prepared vermiculite carrier is then allowed to stand for 1-2 hours, followed by constant temperature standing for 3-5 hours. It is then soaked in an alkaline solution for 20-30 minutes, ultrasonically treated for 5-10 minutes, filtered, and sintered to obtain porous mixed vermiculite. Dimethyl diallyl ammonium chloride and polyaluminum chloride are mixed evenly, then added to the porous mixed vermiculite and stirred evenly. The mixture is then microwave-treated for 1-2 hours. The porous mixed vermiculite particles are then removed, which is the pre-prepared flocculant. The pre-prepared flocculant is allowed to stand for 1-2 hours and then freeze-dried to obtain a high-efficiency flocculant.

[0005] However, the above invention utilizes vermiculite to enhance the sedimentation properties of flocculants, combined with the negative ion adsorption of tourmaline, to aggregate the flocs of dimethyl diallyl ammonium chloride and polyaluminum chloride, reducing the problem of floc dispersion and achieving the goal of adsorption, sedimentation and flocculation.

[0006] However, its particle dispersibility is poor, it lacks an effective surface charge regulation mechanism, and it is prone to agglomeration, which affects the specific surface area and adsorption efficiency. In addition, the cation sites are single and the charge density is insufficient, which limits the adsorption capacity for anionic pollutants.

[0007] Furthermore, its structural stability mainly relies on physical composite and sintering fixation, with loose interfacial coupling and lack of a strong supporting framework formed by covalent bridging. Finally, although the release of negative ions from tourmaline is introduced, its distribution is limited to the interior of the particles, lacking a charge conduction path that runs through the material system, resulting in weak overall adsorption synergy.

[0008] Therefore, a solution is proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a polyaluminum chloride-based flocculant, its preparation method, and its application, in order to solve the technical problem that the flocculation performance and stability of flocculants in the prior art need to be further improved.

[0010] The objective of this invention can be achieved through the following technical solution: a method for preparing a polyaluminum chloride-based flocculant, comprising the following steps:

[0011] S1. Add aluminum chloride and deionized water to the reaction vessel and stir. After the temperature of the reaction vessel is raised to 40-50℃, add the modification solution dropwise to the reaction vessel, keep it warm and stir for 20-24 min, continue to add an equal volume of modification solution, keep it warm and stir for 40-45 min to obtain the modified PAC precursor.

[0012] The reaction principle for preparing the modified PAC precursor is as follows: aluminum chloride dissolves in deionized water to form an aluminum ion hydrate system, followed by the addition of sodium hydroxide solution as a modifying solution. The alkaline hydrolysis process is controlled under low-temperature heating conditions, promoting the partial oxidation of Al₂O₃. 3+ It is converted into aluminum hydroxy complex, and then into polynuclear aluminum hydroxy complex, which provides reaction sites for subsequent multi-component synergistic assembly. By adding the modification solution in two drops and extending the temperature and stirring time, it is helpful to form a more uniform modified PAC precursor with higher reactivity.

[0013] S2. Add the modified PAC precursor to the reactor and stir. Raise the reactor temperature to 40-50℃ and add 5wt% sodium metasilicate aqueous solution and 5% ferric chloride aqueous solution to the reactor. After stirring for 18-20 min, add 1wt% tannic acid aqueous solution to the reactor and stir for 10 min. The modified PAC is obtained after post-treatment.

[0014] The reaction principle for preparing modified PAC is as follows: sodium metasilicate, ferric chloride, and tannic acid are sequentially introduced into the modified PAC precursor to construct a multi-component composite structure. The Si-OH groups in sodium metasilicate can form Al-O-Si bridging bonds with aluminum hydroxide complexes, enhancing the skeletal stability of PAC; ferric chloride provides Fe... 3+As the second cation center, it participates in the formation of bimetallic complex structure and enhances the flocculation reaction activity; tannic acid, as a polyphenolic natural organic ligand, further modifies the PAC molecule through coordination and hydrogen bonding, improving its adsorption capacity for various pollutants and its complexation capacity for organic matter, and finally prepares modified PAC.

[0015] S3. Add modified PAC, deionized water and composite microgel to the reactor, adjust the pH of the reaction system to 8-10 using saturated sodium hydroxide aqueous solution, stir at room temperature for 40-60 min, and then process to obtain polyaluminum chloride-based flocculant.

[0016] The reaction principle for preparing polyaluminum chloride-based flocculants is as follows: Under alkaline conditions, the polynuclear hydroxyaluminum complex in modified PAC can exist stably and has strong charge neutralization and bridging capabilities. The composite microgel is rich in various functional groups, such as carboxyl, hydroxyl, and amide groups, and has good adsorption performance and network structure support capabilities. When the two coexist in the same reaction system, the microgel undergoes a composite reaction with the active components in PAC through electrostatic adsorption, hydrogen bonding, and coordination bonding, thereby constructing an organic-inorganic synergistic composite structure at the molecular scale. This composite process enables the flocculant to have both the rapid sedimentation capability of inorganic components and the efficient capture capability of organic components, forming a structurally stable and highly adaptable flocculation system, ultimately preparing polyaluminum chloride-based flocculants.

[0017] Furthermore, in step S1, the ratio of aluminum chloride, deionized water, and modifying solution is 8-10g:100mL:20-24mL, wherein the modifying solution is a 10wt% sodium hydroxide aqueous solution.

[0018] Further, in step S2, the ratio of the modified PAC precursor, 5 wt% sodium metasilicate aqueous solution, 5% ferric chloride aqueous solution and 1 wt% tannic acid aqueous solution is 8-10 mL: 1-2 mL: 1-2 mL: 0.5-0.8 mL. The post-treatment includes: after the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution to collect the filter cake, wash the filter cake 3-5 times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain modified PAC.

[0019] Furthermore, in step S3, the ratio of modified PAC, deionized water, and composite microgel is 1-2g:10-12mL:1g. The post-treatment includes: after the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction liquid to collect the filter cake, wash the filter cake 3-5 times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain polyaluminum chloride-based flocculant.

[0020] Furthermore, the preparation method of the composite microgel is as follows: the composite microparticles and deionized water are added to a high-speed stirred tank, and the modified microgel dispersion is continuously added dropwise at a speed of 300-400 rpm for 10-15 min. Then, the high-speed stirred tank is sealed and the speed is increased to 8000-10000 rpm. The mixture is stirred continuously for 20-30 min, and the composite microgel is obtained after post-treatment.

[0021] The reaction principle for preparing composite microgels is as follows: under low-speed stirring conditions, the modified microgel dispersion is slowly added dropwise to the composite microparticle suspension system, which promotes the gradual adsorption of microgels onto the surface of microparticles. Then, by increasing the stirring speed, the loose aggregate state in the microgel structure is broken under strong shear conditions, which accelerates its spreading and dense coating on the surface of microparticles, thereby forming a stable core-shell composite microgel. The whole process relies on electrostatic adsorption, van der Waals forces and shear-induced interfacial rearrangement to achieve stable binding of microgels on the surface of microparticles, thus preparing composite microgels.

[0022] Furthermore, the ratio of composite microparticles, deionized water, and modified microgel dispersion is 1-2g:80-100mL:40-50mL. The modified microgel dispersion is obtained by mixing modified microgel and deionized water at a ratio of 2-3g:80mL. The post-treatment includes: after the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution to collect the filter cake, wash the filter cake 3-5 times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain composite microgel.

[0023] Furthermore, the preparation method of the modified microgel includes the following steps:

[0024] A1. Chitosan, acetic acid and deionized water are added to the reaction vessel. After the temperature of the reaction vessel is raised to 50-60℃, epichlorohydrin is added to the reaction vessel. The mixture is kept warm and stirred for 1-2 hours. Then, 25wt% trimethylamine aqueous solution is added to the reaction vessel and kept warm and stirred for 3-4 hours. The microgel precursor is obtained after post-treatment.

[0025] A2. Add the microgel precursor, acetic acid, and N,N-dimethylformamide to the reaction vessel. After raising the temperature of the reaction vessel to 50-60℃, add glutaraldehyde dropwise to the reaction vessel and keep the reaction at this temperature for 40-60 min. The modified microgel is then obtained through post-treatment.

[0026] The reaction principle for preparing modified microgels is as follows: under mild heating conditions, chitosan is dispersed in an acetic acid-water solution to form a homogeneous reaction system. Then, epichlorohydrin is introduced, and with the high reactivity of its epoxy groups, it undergoes a ring-opening etherification reaction with the hydroxyl and amino groups on the chitosan molecular chain to generate a chitosan derivative with chloropropyl side chains. Next, trimethylamine aqueous solution is added, and it undergoes a nucleophilic substitution or quaternization reaction with the derivative to form a stable cationic quaternary ammonium salt structure. This stage not only introduces positively charged functional groups into the molecular structure, enhancing the solubility and dispersibility of chitosan, but also provides a uniform reaction system and reaction sites for subsequent crosslinking reactions, thereby obtaining a stable microgel precursor.

[0027] Based on this, the obtained precursor was mixed with acetic acid and N,N-dimethylformamide to form a reaction medium, and glutaraldehyde was added dropwise to carry out a cross-linking reaction. The dialdehyde group in the glutaraldehyde molecule can undergo an aldehyde-amine condensation reaction with the free amino groups on the chitosan molecular chain to generate a -C=N- type Schiff base structure, thereby achieving effective cross-linking between chitosan molecules and constructing a stable three-dimensional network microgel structure. This cross-linking process regulates the particle size, mechanical strength and structural density of the microgel, and improves its stability and functionality in solution.

[0028] Further, in step A1, the ratio of chitosan, acetic acid, deionized water, epichlorohydrin and 25wt% trimethylamine aqueous solution is 4-5g:2-3g:40-50mL:2-3g:7-8g. The post-processing includes: after the reaction is completed, after the temperature of the reaction vessel drops to room temperature, the reaction solution is transferred to a rotary evaporator at 80℃ and distilled under reduced pressure until no liquid is collected, to obtain the microgel precursor.

[0029] Further, in step A2, the ratio of the microgel precursor, acetic acid, N,N-dimethylformamide and glutaraldehyde is 3-4g:1-2g:20-24mL:0.3-0.5g. The post-processing includes: after the reaction is completed, after the temperature of the reaction vessel drops to room temperature, the reaction solution is transferred to a rotary evaporator at a temperature of 80℃ and distilled under reduced pressure until no liquid is collected, thus obtaining the modified microgel.

[0030] Furthermore, the preparation method of the composite microparticles includes the following steps:

[0031] B1. Add citric acid, urea and deionized water to a high-pressure reactor. After sealing the high-pressure reactor, raise the reactor temperature to 160-180℃ and keep it at that temperature for 5-6 hours to obtain a composite solution.

[0032] B2. Add the composite solution to the reaction vessel and stir. After the temperature of the reaction vessel is raised to 70-80℃, adjust the pH of the reaction system to 9-10 with saturated sodium hydroxide aqueous solution. Then add ferric chloride and ferrous chloride to the reaction vessel, keep it warm and stir for 40-60 min, add sodium citrate to the reaction vessel, keep it warm and stir for 1 h, and then process to obtain composite microparticles.

[0033] The reaction principle for preparing composite microparticles is as follows: Under high-temperature and sealed conditions, citric acid and urea react in deionized water, undergoing a series of thermally induced reactions such as dehydration condensation, amidation, and carbon-nitrogen bond construction, forming a stable composite organic solution containing functional groups such as carboxyl and amino groups. This solution is also rich in potential coordination sites, a C / N elemental framework, and a certain reducing environment, laying the foundation for subsequent complexation and fixation of metal ions, particle size control, and structural uniformity. Next, under heating and stirring conditions, the pH of the precursor solution is adjusted to alkaline, and a mixed ferric salt system of ferric chloride and ferrous chloride is introduced. At this point, Fe... 3+ and Fe 2 + Hydrolysis occurs in an alkaline environment, and some Fe... 3+ The iron ions are oxidized and co-precipitated to form iron-based particles such as Fe3O4 and FeOOH. At the same time, there are a large number of organic ligand groups in the system. The precipitation process of these iron ions is regulated by complexation and can gradually self-assemble into dense and well-dispersed composite particles under the action of the organic framework. In order to further improve the stability and functionality of the particles, sodium citrate is introduced as a synergistic ligand and dispersant in the later stage of the reaction. It forms a surface complex layer with the residual iron ions, inhibits agglomeration, and optimizes the surface charge state of the particles, thereby forming stable composite particles.

[0034] Furthermore, in step B1, the ratio of citric acid, urea, and deionized water is 1-2g:1-2g:20-30mL.

[0035] Furthermore, in step B2, the ratio of the composite solution, ferric chloride, ferrous chloride, and sodium citrate is 20-30 mL: 2-3 g: 2-3 g: 0.5 g. The post-treatment includes: after the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution to collect the filter cake, wash the filter cake 3-5 times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain composite microparticles.

[0036] The present invention also proposes a polyaluminum chloride-based flocculant, which is prepared by the above-mentioned method for preparing a polyaluminum chloride-based flocculant.

[0037] This invention also proposes an application of a polyaluminum chloride-based flocculant, wherein the polyaluminum chloride-based flocculant prepared by the above-mentioned method is applied to wastewater treatment.

[0038] The present invention has the following beneficial effects:

[0039] 1. The modified PAC prepared in this invention incorporates sodium metasilicate, ferric chloride, and tannic acid to construct a stable Al-O-Si bridging structure and bimetallic complex center, enhancing its charge neutralization and floc formation capabilities. Simultaneously, the resulting composite microgel possesses a superior three-dimensional network structure, rich in various functional groups such as carboxyl, hydroxyl, and amide groups. This allows it to efficiently bind with pollutant particles not only through electrostatic adsorption, hydrogen bonding, and coordination bonds, but also to form molecular-level composite structures with the active components in the PAC. During the reaction, the microgel coats the surface of the composite particles and undergoes interfacial rearrangement with the modified PAC, ultimately constructing a synergistic system where the inorganic flocculation framework and the organic flexible network are tightly nested. This synergistic structure improves the density and stability of the flocs, enhances particle bridging capabilities, and promotes rapid aggregation and sedimentation of pollutant particles, thereby achieving higher flocculation efficiency and shorter sedimentation time.

[0040] 2. The composite microgel prepared by this invention forms a stable three-dimensional network structure by cross-linking chitosan derivatives with glutaraldehyde, exhibiting excellent thermal stability. Even at high temperatures, it is not easily degraded or precipitated, ensuring the integrity of functional groups and the durability of flocculation performance during storage. Secondly, the Al-O-Si and Fe-O bonds constructed by sodium metasilicate and ferric chloride introduced into the modified PAC have high pyrolysis temperatures and structural stability, effectively avoiding the performance degradation of conventional polyaluminum chloride under high-temperature storage conditions due to loose structure, hydrolysis reaction, or decreased activity. Furthermore, the introduction of tannic acid not only enhances the complexation ability of the flocculant for organic pollutants but also strengthens the antioxidant and anti-caking capabilities of the entire system through its polyhydroxy structure. Ultimately, the synergistic stabilizing effect of the organic-inorganic composite structure significantly improves the stability of the polyaluminum chloride-based flocculant.

[0041] 3. The polynuclear aluminum hydroxy complex in the modified PAC prepared in this invention and Fe 3+ Together they provide a high charge density, which can effectively neutralize phosphate (PO4) in water. 3- ) and chromate (CrO4) 2-The presence of anions such as phosphorus and chromium enables charge neutralization and precipitation reactions. Phosphorus can form aluminum phosphate and iron phosphate precipitates, while hexavalent chromium can be reduced to Cr(III) under the weak reducing environment of organic components, thus forming Cr(OH)3 precipitate. Simultaneously, the composite microparticles provide a high specific surface area and porous structure, enhancing the physical adsorption capacity for phosphorus and chromium. The modified microgel is rich in carboxyl, amino, and amide groups, which can form multi-point complexes with phosphorus and chromium, further improving removal efficiency. The introduction of sodium metasilicate and tannic acid stabilizes the flocculation framework and enhances the complexation capacity of metal ions, enabling the overall system to maintain high flocculation performance under conditions of acid-base fluctuations and complex water quality, ultimately achieving deep removal of phosphorus and chromium pollutants. Detailed Implementation

[0042] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1

[0044] This embodiment provides a method for preparing composite microparticles for preparing polyaluminum chloride-based flocculants, including the following steps:

[0045] Step ①: Preparation of composite solution

[0046] Weigh out 10.0g of citric acid, 10.0g of urea and 200.0mL of deionized water and add them to a high-pressure reactor. After sealing the high-pressure reactor, raise the temperature of the reactor to 160℃ and keep it at that temperature for 5 hours to obtain a composite solution.

[0047] Step 2: Preparation of composite microparticles

[0048] Weigh 200.0 mL of the composite solution and add it to the reaction vessel. After stirring, adjust the pH of the reaction system to 9 using a saturated sodium hydroxide aqueous solution when the temperature of the reaction vessel rises to 70℃. Then, add 20.0 g of ferric chloride and 20.0 g of ferrous chloride to the reaction vessel and stir for 40 min. After stirring for 1 h, add 5.0 g of sodium citrate to the reaction vessel and stir for 1 h. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake three times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain composite microparticles.

[0049] Example 2

[0050] This embodiment provides a method for preparing composite microparticles for preparing polyaluminum chloride-based flocculants, including the following steps:

[0051] Step ①: Preparation of composite solution

[0052] Weigh out 20.0g of citric acid, 20.0g of urea and 300.0mL of deionized water and add them to a high-pressure reactor. After sealing the high-pressure reactor, raise the temperature of the reactor to 180℃ and keep it at that temperature for 6 hours to obtain a composite solution.

[0053] Step 2: Preparation of composite microparticles

[0054] Weigh 300.0 mL of the composite solution and add it to the reaction vessel. After stirring, raise the temperature of the reaction vessel to 80°C and adjust the pH of the reaction system to 10 using a saturated sodium hydroxide aqueous solution. Then, add 30.0 g of ferric chloride and 30.0 g of ferrous chloride to the reaction vessel. Keep the mixture warm and stir for 60 min. Then, add 5.0 g of sodium citrate to the reaction vessel and keep the mixture warm and stir for 1 h. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature. Filter the reaction solution and collect the filter cake. Wash the filter cake 5 times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain composite microparticles.

[0055] Example 3

[0056] This embodiment provides a method for preparing composite microparticles for preparing polyaluminum chloride-based flocculants, including the following steps:

[0057] Step ①: Preparation of composite solution

[0058] Weigh out 16.0g of citric acid, 16.0g of urea and 240.0mL of deionized water and add them to a high-pressure reactor. After sealing the high-pressure reactor, raise the temperature of the reactor to 180℃ and keep it at that temperature for 6 hours to obtain a composite solution.

[0059] Step 2: Preparation of composite microparticles

[0060] Weigh 270.0 mL of the composite solution and add it to the reaction vessel. After stirring, raise the temperature of the reaction vessel to 80°C and adjust the pH of the reaction system to 10 using a saturated sodium hydroxide aqueous solution. Then, add 25.0 g of ferric chloride and 25.0 g of ferrous chloride to the reaction vessel and stir for 50 min. After stirring for 1 h, add 5.0 g of sodium citrate to the reaction vessel and stir for 1 h. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake four times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain composite microparticles.

[0061] Example 4

[0062] This embodiment provides a method for preparing composite microgels for preparing polyaluminum chloride-based flocculants, including the following steps:

[0063] Step I: Preparation of microgel precursors

[0064] Weigh out 40.0 g chitosan, 20.0 g acetic acid and 400.0 mL deionized water and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 50℃, add 20.0 g epichlorohydrin and stir for 1 h. Then add 70.0 g 25 wt% trimethylamine aqueous solution and stir for 3 h. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, then transfer the reaction solution to a rotary evaporator at 80℃ and distill under reduced pressure until no liquid is collected to obtain the microgel precursor.

[0065] Step II: Preparation of modified microgels

[0066] Weigh out 30.0 g of microgel precursor, 10.0 g of acetic acid and 200.0 mL of N,N-dimethylformamide and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 50℃, add 3.0 g of glutaraldehyde dropwise to the reaction vessel and keep the reaction at this temperature for 40 min. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, and then transfer the reaction solution to a rotary evaporator at 80℃. Distill under reduced pressure until no liquid is collected to obtain the modified microgel.

[0067] Step III: Preparation of composite microgels

[0068] Weigh out 20.0 g of modified microgel and mix it with 800.0 mL of deionized water to obtain a modified microgel dispersion;

[0069] Weigh 10.0g of the composite microparticles prepared in Example 1 and 800.0mL of deionized water and add them to a high-speed stirred tank. Add 400.0mL of modified microgel dispersion dropwise at 300rpm for 10min. Then seal the high-speed stirred tank and increase the speed to 8000rpm. Stir continuously for 20min. After the reaction is complete, wait for the temperature of the reaction tank to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake three times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain the composite microgel.

[0070] Example 5

[0071] This embodiment provides a method for preparing composite microgels for preparing polyaluminum chloride-based flocculants, including the following steps:

[0072] Step I: Preparation of microgel precursors

[0073] Weigh out 50.0 g chitosan, 30.0 g acetic acid and 500.0 mL deionized water and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 60℃, add 30.0 g epichlorohydrin and stir for 2 h. Then add 80.0 g 25 wt% trimethylamine aqueous solution and stir for 4 h. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, then transfer the reaction solution to a rotary evaporator at 80℃ and distill under reduced pressure until no liquid is collected to obtain the microgel precursor.

[0074] Step II: Preparation of modified microgels

[0075] Weigh out 40.0 g of microgel precursor, 20.0 g of acetic acid and 240.0 mL of N,N-dimethylformamide and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 60℃, add 5.0 g of glutaraldehyde dropwise to the reaction vessel and keep the reaction at this temperature for 60 min. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, and then transfer the reaction solution to a rotary evaporator at 80℃. Distill under reduced pressure until no liquid is collected to obtain the modified microgel.

[0076] Step III: Preparation of composite microgels

[0077] Weigh out 30.0 g of modified microgel and mix it with 800.0 mL of deionized water to obtain a modified microgel dispersion;

[0078] Weigh 20.0g of the composite microparticles prepared in Example 2 and 1000.0mL of deionized water and add them to a high-speed stirred tank. Add 500.0mL of modified microgel dispersion dropwise at 400rpm for 15min. Then seal the high-speed stirred tank and increase the speed to 10000rpm. Stir continuously for 30min. After the reaction is complete, wait for the temperature of the reaction tank to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake 5 times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain the composite microgel.

[0079] Example 6

[0080] This embodiment provides a method for preparing composite microgels for preparing polyaluminum chloride-based flocculants, including the following steps:

[0081] Step I: Preparation of microgel precursors

[0082] Weigh out 18.0 g chitosan, 24.0 g acetic acid and 450.0 mL deionized water and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 54℃, add 25.0 g epichlorohydrin and stir for 2 h. Then add 72.0 g 25 wt% trimethylamine aqueous solution and stir for 6 h. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, then transfer the reaction solution to a rotary evaporator at 80℃ and distill under reduced pressure until no liquid is collected to obtain the microgel precursor.

[0083] Step II: Preparation of modified microgels

[0084] Weigh out 36.0 g of microgel precursor, 16.0 g of acetic acid and 210.0 mL of N,N-dimethylformamide and add them to the reaction vessel. After the temperature of the reaction vessel is raised to 54℃, 4.0 g of glutaraldehyde is added dropwise to the reaction vessel. The reaction is kept at this temperature for 50 min. After the reaction is completed, the temperature of the reaction vessel is lowered to room temperature. The reaction solution is then transferred to a rotary evaporator at 80℃ and distilled under reduced pressure until no liquid is collected, thus obtaining the modified microgel.

[0085] Step III: Preparation of composite microgels

[0086] Weigh out 25.0 g of modified microgel and mix it with 800.0 mL of deionized water to obtain a modified microgel dispersion;

[0087] Weigh 15.0g of the composite microparticles prepared in Example 3 and 900.0mL of deionized water and add them to a high-speed stirred tank. Add 450.0mL of modified microgel dispersion dropwise at 360rpm for 12min. Then seal the high-speed stirred tank and increase the speed to 9000rpm. Stir continuously for 25min. After the reaction is complete, wait for the temperature of the reaction tank to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake four times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain the composite microgel.

[0088] Example 7

[0089] This embodiment provides a method for preparing a polyaluminum chloride-based flocculant, including the following steps:

[0090] Step 1: Preparation of modified PAC precursor

[0091] Weigh out 80.0 g of aluminum chloride and 1000.0 mL of deionized water and add them to the reaction vessel. Stir the mixture. After the temperature of the reaction vessel is raised to 40°C, add 100.0 mL of 10 wt% sodium hydroxide aqueous solution dropwise to the reaction vessel. Keep the mixture warm and stir for 20 min. Continue to add an equal volume of 10 wt% sodium hydroxide aqueous solution and keep the mixture warm and stir for 40 min to obtain the modified PAC precursor.

[0092] Step 2: Preparation of modified PAC

[0093] Weigh 80.0 mL of the modified PAC precursor and add it to the reaction vessel. Stir the mixture, raise the temperature of the reaction vessel to 40°C, and add 10.0 mL of 5 wt% sodium metasilicate aqueous solution and 10.0 mL of 5% ferric chloride aqueous solution. Keep the mixture warm and stir for 18 min, then add 2.0 mL of 1 wt% tannic acid aqueous solution and keep the mixture warm and stir for 10 min. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake four times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain modified PAC.

[0094] Step 3: Preparation of polyaluminum chloride-based flocculant

[0095] Weigh out 10.0g of modified PAC, 100.0mL of deionized water and 10.0g of the composite microgel prepared in Example 4 and add them to the reaction vessel. Adjust the pH of the reaction system to 8 using saturated sodium hydroxide aqueous solution. Stir at room temperature for 40min. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction liquid and collect the filter cake. Wash the filter cake three times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain polyaluminum chloride-based flocculant.

[0096] Example 8

[0097] This embodiment provides a method for preparing a polyaluminum chloride-based flocculant, including the following steps:

[0098] Step 1: Preparation of modified PAC precursor

[0099] Weigh out 100.0g of aluminum chloride and 1000.0mL of deionized water and add them to the reaction vessel. Stir the reaction vessel and raise the temperature to 50℃. Then add 120.0mL of 10wt% sodium hydroxide aqueous solution dropwise to the reaction vessel. Keep the temperature and stir for 24min. Continue to add an equal volume of 10wt% sodium hydroxide aqueous solution and keep the temperature and stir for 45min to obtain the modified PAC precursor.

[0100] Step 2: Preparation of modified PAC

[0101] Weigh 100.0 mL of modified PAC precursor and add it to the reaction vessel. Stir the mixture, raise the temperature of the reaction vessel to 50°C, and add 20.0 mL of 5 wt% sodium metasilicate aqueous solution and 20.0 mL of 5% ferric chloride aqueous solution. Keep the mixture warm and stir for 20 min, then add 8.0 mL of 1 wt% tannic acid aqueous solution and keep the mixture warm and stir for 10 min. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake 5 times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain modified PAC.

[0102] Step 3: Preparation of polyaluminum chloride-based flocculant

[0103] Weigh out 20.0g of modified PAC, 120.0mL of deionized water and 10.0g of the composite microgel prepared in Example 5 and add them to the reaction vessel. Adjust the pH of the reaction system to 10 using saturated sodium hydroxide aqueous solution. Stir at room temperature for 60min. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction liquid and collect the filter cake. Wash the filter cake 5 times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain polyaluminum chloride-based flocculant.

[0104] Example 9

[0105] This embodiment provides a method for preparing a polyaluminum chloride-based flocculant, including the following steps:

[0106] Step 1: Preparation of modified PAC precursor

[0107] Weigh out 96.0 g of aluminum chloride and 1000.0 mL of deionized water and add them to the reaction vessel. Stir the mixture. After the temperature of the reaction vessel is raised to 45°C, add 120.0 mL of 10 wt% sodium hydroxide aqueous solution dropwise to the reaction vessel. Keep the mixture warm and stir for 21 min. Continue to add an equal volume of 10 wt% sodium hydroxide aqueous solution and keep the mixture warm and stir for 45 min to obtain the modified PAC precursor.

[0108] Step 2: Preparation of modified PAC

[0109] Weigh 90.0 mL of the modified PAC precursor and add it to the reaction vessel. Stir the mixture, raise the temperature of the reaction vessel to 45°C, and add 16.0 mL of 5 wt% sodium metasilicate aqueous solution and 16.0 mL of 5% ferric chloride aqueous solution. After stirring for 20 min, add 7.2 mL of 1 wt% tannic acid aqueous solution to the reaction vessel and stir for 10 min. After the reaction is complete, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction solution and collect the filter cake. Wash the filter cake four times with anhydrous ethanol and deionized water, and then transfer the filter cake to a drying oven at 80°C and vacuum dry it to constant weight to obtain modified PAC.

[0110] Step 3: Preparation of polyaluminum chloride-based flocculant

[0111] Weigh out 16.0g of modified PAC, 108.0mL of deionized water and 10.0g of the composite microgel prepared in Example 6 and add them to the reaction vessel. Adjust the pH of the reaction system to 9 using saturated sodium hydroxide aqueous solution. Stir at room temperature for 50min. After the reaction is completed, wait for the temperature of the reaction vessel to drop to room temperature, filter the reaction liquid and collect the filter cake. Wash the filter cake 4 times with anhydrous ethanol and deionized water. Transfer the filter cake to a drying oven at 80℃ and vacuum dry it to constant weight to obtain polyaluminum chloride-based flocculant.

[0112] Comparative Example 1

[0113] The difference between this comparative example and Example 9 is that the composite microgel used in this example uses composite microparticles, and sodium citrate is omitted in step ② of the preparation process.

[0114] Comparative Example 2

[0115] The difference between this comparative example and Example 9 is that the composite microgel used omits the use of 25wt% trimethylamine aqueous solution in step I of the preparation process.

[0116] Comparative Example 3

[0117] The difference between this comparative example and Example 9 is that the sodium metasilicate aqueous solution is omitted in step two.

[0118] Performance testing:

[0119] The turbidity reduction rate and settling time reduction rate of the polyaluminum chloride-based flocculants prepared in Examples 7-9 and Comparative Examples 1-3 were tested in accordance with the standard T / CECS10356-2024 "Flocculants for Wastewater Treatment of Washed Sand and Gravel Aggregates".

[0120] The stability of the polyaluminum chloride-based flocculants prepared in Examples 7-9 and Comparative Examples 1-3 was evaluated in accordance with the standard YS / T 802-2012 "Flocculants for Alumina Production".

[0121] The total phosphorus and total chromium content of the wastewater discharged from the wastewater treatment plant after flocculation by the polyaluminum chloride-based flocculants prepared in Examples 7-9 and Comparative Examples 1-3 were compared with the standard GB 18918-2002 "Discharge Standard of Pollutants for Urban Wastewater Treatment Plants". The specific data are shown in Table 1.

[0122] Table 1 - Performance Test Data for Each Sample

[0123]

[0124] Data Analysis:

[0125] Comparative analysis of the data in Table 1 reveals that, compared to flocculation without flocculant, the polyaluminum chloride-based flocculant prepared in this invention exhibits a 98% reduction in turbidity, an 86% reduction in settling time, and stable storage for 13 months, while the total phosphorus content of the flocculated wastewater is 0.2 mg·L⁻¹. -1 The total chromium content is 0.06 mg·L⁻¹. -1 All data points are better than the comparative data, indicating that:

[0126] In Comparative Example 1, no sodium citrate was added during the preparation of the composite microparticles. Although iron-based particles such as Fe3O4 could still be generated, the lack of terminal ligands to regulate the surface charge led to significant particle aggregation and decreased dispersibility. This, in turn, affected the uniformity and stability of the subsequent microgel coating. The resulting composite microgel had an incomplete surface structure and a reduced overall specific surface area, resulting in a significant decrease in the adsorption and complexation capacity for pollutants such as phosphorus and chromium. This was manifested as insufficient interfacial synergy of the composite material.

[0127] Comparative Example 2 skipped the quaternization modification step of trimethylamine, resulting in insufficient surface charge of the microgel precursor. This led to weak electrical properties and poor solution stability of the microgel formed after subsequent cross-linking. At the same time, due to the lack of cation sites provided by the quaternary ammonium salt structure, the microgel's adsorption capacity for anionic pollutants (such as phosphate and chromate) was significantly reduced, ultimately affecting the composite adsorption and sedimentation performance of the flocculant. This demonstrates the necessity of this scheme in terms of positive charge introduction and structural regulation.

[0128] In Comparative Example 3, without the introduction of sodium metasilicate during the preparation of modified PAC, the Al-O-Si bridging framework could not be formed inside the PAC, resulting in a decrease in its three-dimensional structural stability. In the subsequent composite process, the modified PAC was prone to depolymerization or loose structure, affecting its ability to form a synergistic interface with microgels. Such flocculants exhibited the disadvantage of fast short-term adsorption rate but easy structural disintegration and decreased capture capacity during water treatment, reflecting the key role of sodium metasilicate in enhancing structural stability and synergistic adsorption.

[0129] In conclusion, this scheme effectively avoids particle aggregation by adjusting the surface charge of iron-based particles with sodium citrate, thereby improving the uniformity of microgel coating and the specific surface area of ​​the material. Trimethylamine-modified chitosan introduces abundant cationic sites, significantly enhancing the electrostatic adsorption capacity of the microgel for anionic pollutants, while also improving the electrical properties and stability of the system. The addition of sodium metasilicate further promotes the formation of a stable Al-O-Si bridging structure in modified PAC, endowing the material with good three-dimensional skeleton strength and composite interface stability. These three factors work synergistically, playing key roles in particle dispersibility, charge construction, and skeleton stability, respectively, ultimately constructing a highly efficient, stable, and storable composite flocculation system.

[0130] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for producing a polyaluminum chloride-based flocculant, characterized by, It comprises the following steps: S1, aluminum chloride and deionized water are added to the reaction kettle and stirred, the temperature of the reaction kettle is raised to 40-50℃, then the modification solution is added dropwise to the reaction kettle, and the mixture is stirred for 20-24 min, then an equal volume of modification solution is added, and the mixture is stirred for 40-45 min to obtain a modified PAC precursor, wherein the modification solution is 10wt% sodium hydroxide solution; S2, the modified PAC precursor is added to the reaction kettle and stirred, the temperature of the reaction kettle is raised to 40-50℃, and 5wt% sodium metasilicate solution and 5% ferric chloride solution are added to the reaction kettle, and the mixture is stirred for 18-20 min, then 1wt% tannic acid solution is added to the reaction kettle, and the mixture is stirred for 10 min, and then treated to obtain modified PAC; S3, the modified PAC, deionized water and composite microgel are added to the reaction kettle, the pH of the reaction system is adjusted to 8-10 using saturated sodium hydroxide solution, and the mixture is stirred at room temperature for 40-60 min, and then treated to obtain polyaluminum chloride-based flocculant; The preparation method of the composite microgel comprises the following steps: A1, chitosan, acetic acid and deionized water are added to the reaction kettle, the temperature of the reaction kettle is raised to 50-60℃, then epichlorohydrin is added to the reaction kettle, and the mixture is stirred for 1-2h, then 25wt% trimethylamine solution is added to the reaction kettle, and the mixture is stirred for 3-4h, and then treated to obtain microgel precursor; A2, the microgel precursor, acetic acid and N,N-dimethylformamide are added to the reaction kettle, the temperature of the reaction kettle is raised to 50-60℃, then glutaraldehyde is added dropwise to the reaction kettle, and the mixture is reacted for 40-60 min, and then treated to obtain modified microgel; The preparation method of the composite microgel comprises the following steps: B1, citric acid, urea and deionized water are added to the high-pressure reaction kettle, the high-pressure reaction kettle is closed, the temperature of the reaction kettle is raised to 160-180℃, and the mixture is treated for 5-6h to obtain a composite solution; B2, the composite solution is added to the reaction kettle and stirred, the temperature of the reaction kettle is raised to 70-80℃, then the pH of the reaction system is adjusted to 9-10 using saturated sodium hydroxide solution, then ferric chloride and ferrous chloride are added to the reaction kettle, and the mixture is stirred for 40-60 min, then sodium citrate is added to the reaction kettle, and the mixture is stirred for 1h, and then treated to obtain composite microgel. ​ 2. The method for preparing a polyaluminum chloride-based flocculant according to claim 1, characterized in that, The use amount ratio of the aluminum chloride, deionized water and modification liquid in step S1 is 8-10g:100mL:20-24mL; the use amount ratio of the modified PAC precursor, 5wt% sodium metasilicate aqueous solution, 5% ferric trichloride aqueous solution and 1wt% tannic acid aqueous solution in step S2 is 8-10mL:1-2mL:1-2mL:0.5-0.8mL; the use amount ratio of the modified PAC, deionized water and composite microgel in step S3 is 1-2g:10-12mL:1g.

3. The method for preparing a polyaluminum chloride-based flocculant according to claim 1, characterized in that, The use amount ratio of the composite microparticle, deionized water and modified microgel dispersion liquid is 1-2g:80-100mL:40-50mL, and the modified microgel dispersion liquid is obtained by mixing the modified microgel and deionized water in a use amount ratio of 2-3g:80mL.

4. The method for preparing a polyaluminum chloride-based flocculant according to claim 1, characterized in that, The use amount ratio of the chitosan, acetic acid, deionized water, epichlorohydrin and 25wt% trimethylamine aqueous solution in step A1 is 4-5g:2-3g:40-50mL:2-3g:7-8g; the use amount ratio of the microgel precursor, acetic acid, N,N-dimethylformamide and glutaraldehyde in step A2 is 3-4g:1-2g:20-24mL:0.3-0.5g.

5. The method for preparing a polyaluminum chloride-based flocculant according to claim 1, characterized in that, The use amount ratio of the citric acid, urea and deionized water in step B1 is 1-2g:1-2g:20-30mL; the use amount ratio of the composite solution, ferric trichloride, ferrous chloride and sodium citrate in step B2 is 20-30mL:2-3g:2-3g:0.5g.

6. A polyaluminum chloride-based flocculant characterized by, The polyaluminum chloride-based flocculant is prepared by the preparation method of the polyaluminum chloride-based flocculant in any one of claims 1-5.

7. Use of a polyaluminum chloride-based flocculant, characterized in that The polyaluminum chloride-based flocculant prepared by the preparation method of the polyaluminum chloride-based flocculant in any one of claims 1-5 is applied to sewage treatment.

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