A process for the preparation of a high molecular weight cationic polyacrylamide

Abstract

The application belongs to the technical field of high-molecular polymer material preparation, and particularly relates to a preparation method of high-molecular-weight cationic polyacrylamide. 85%-95% of the total amount of acrylamide and 50%-70% of the total amount of acryloyloxyethyl trimethyl ammonium chloride are prepared into initial kettle liquid, and the remaining acrylamide and the remaining acryloyloxyethyl trimethyl ammonium chloride are prepared into subsequent feed liquid, which are cooled and deoxygenated respectively. A water-soluble azo initiator, a persulfate and a reducing agent are added to the initial kettle liquid to initiate the reaction, and when the temperature difference and the moving average heating rate of the reaction system reach the specified range at the same time, the subsequent feed is started. The feeding speed is adjusted according to the moving average heating rate, and the feeding is completed before visible local gel is generated in the reaction system. After the feeding is completed, the heating rate of the main polymerization temperature zone and the polymerization peak temperature are controlled, and then aging, granulation, drying, crushing and screening are carried out to obtain the high-molecular-weight cationic polyacrylamide.

Description

Technical Field

[0001] This invention belongs to the field of polymer material preparation technology, specifically relating to a method for preparing high molecular weight cationic polyacrylamide. Background Technology

[0002] Cationic polyacrylamide possesses charge neutralization and adsorption capabilities, and is widely used in sludge dewatering, wastewater flocculation, and papermaking filtration aids. Generally, under similar cationicity, solubility, and dosing conditions, a higher relative molecular mass is usually beneficial for enhancing adsorption bridging. Currently, the industrial preparation of high molecular weight cationic polyacrylamide mainly employs aqueous solution gel polymerization with acrylamide and acryloyloxyethyltrimethylammonium chloride as the main monomers. In existing high molecular weight polyacrylamide gel polymerization processes, fluctuations in monomer composition, initiation state, and heat transfer conditions can easily lead to batch-to-batch instability in molecular weight, residual monomers, and water-insoluble matter. Therefore, it is necessary to improve the controllability and repeatability of the polymerization process. However, the following problems still exist in the preparation process: First, current technologies typically involve a one-time, full-volume mixing of acryloyloxyethyltrimethylammonium chloride and acrylamide, which increases the initial concentration of cationic monomers and may increase fluctuations in system viscosity, local composition, and exothermic state, hindering stable control of the polymerization process. Second, to ensure reliable start-up under low-temperature conditions, some processes employ high initiator dosages or composite initiation systems containing multiple oxidants and reductants. Excessive initiator dosage may lead to excessively high initial free radical concentrations, increasing the probability of bimolecular termination and concentrating the exothermic reaction, which is detrimental to the stable control of relative molecular mass and temperature trajectory. Finally, existing gel polymerization processes rely primarily on the system's own temperature rise, lacking active intervention in the temperature trajectory and making it difficult to actively adjust the exothermic polymerization trajectory. As the polymerization reaction proceeds, the large amount of heat accumulated in the system cannot be dissipated in time, leading to a rapid temperature rise in the later stages of the reaction. Higher temperatures accelerate initiator decomposition, increase instantaneous free radical concentration, and may increase chain transfer, bimolecular termination, and local overheating, thus hindering the acquisition of high molecular weight products.

[0003] Therefore, there is a need for a preparation method that can determine the feeding time based on the actual reaction state and control the feed composition, feeding rate and heat transfer trajectory in a coordinated manner. Summary of the Invention

[0004] To address at least one of the above problems, the present invention provides a method for preparing high molecular weight cationic polyacrylamide, comprising the following steps: S1. Using acrylamide and acryloyloxyethyltrimethylammonium chloride as polymerization monomers, prepare an initial reactor solution with 85%-95% of the total acrylamide and 50%-70% of the total acryloyloxyethyltrimethylammonium chloride; prepare the remaining acrylamide and remaining acryloyloxyethyltrimethylammonium chloride as the subsequent feed solution; wherein, based on the total effective monomer mass in the final polymerization system, acrylamide is 68-78%, acryloyloxyethyltrimethylammonium chloride is 22%-32%, the final effective monomer mass concentration is 27.5%-30%, and the molar fraction of acryloyloxyethyltrimethylammonium chloride in the subsequent feed solution is 25%-55%; adjust the pH of the initial reactor solution to 5.2-5.9 with acid; S2. Cool the initial reactor liquid and the subsequent feed liquid to 2-5℃ respectively, and deoxygenate them respectively until the dissolved oxygen is no higher than 0.3mg / L. After deoxygenation, maintain nitrogen protection. S3. Add water-soluble azo initiator, persulfate and reducing agent to the initial reactor liquid to start the polymerization reaction at 2-5℃; S4. Take the average temperature of the initial reactor solution within 180 seconds before adding the reducing agent as the reference temperature T0, and take the difference between the reaction temperature T and the reference temperature T0 at any given time as the temperature difference ΔT. Calculate the 180-second moving average heating rate R using the following formula: R = (T... t -T t-180 ) / 3; where T t T represents the current temperature. t-180 The temperature is 180s ago, and the unit of R is ℃ / min; when ΔT is 0.4-0.8℃ and R is 0.02-0.08℃ / min, and both conditions are maintained continuously for 30-90s, the subsequent feed liquid is added. S5. Add the subsequent feed liquid within 8-30 minutes, and adjust the feed rate according to the moving average heating rate R; S6. After feeding is completed, continue polymerization, and control the maximum heating rate of the reaction system in the temperature range of 15-45℃ to 0.3-0.7℃ / min, and control the peak polymerization temperature to 58-65℃. S7. After reaching the peak temperature, the mixture is kept warm and cured to obtain a polymeric gel. After granulation, drying, crushing, and sieving, cationic polyacrylamide is obtained.

[0005] In step (1), the initial reactor solution contains 85%-95% of the total acrylamide and 50%-70% of the total acryloyloxyethyltrimethylammonium chloride. If the initial acrylamide ratio is less than 85%, the effective monomer concentration and acrylamide concentration in the initial reactor solution may be too low, resulting in slow polymerization start-up and insufficient effective chain growth. If it is higher than 95%, the amount of acrylamide in the subsequent feed solution will be too low, and its inhibitory effect on the local enrichment of acryloyloxyethyltrimethylammonium chloride will be significantly weakened. If the initial acryloyloxyethyltrimethylammonium chloride ratio is less than 50%, the acryloyloxyethyltrimethylammonium chloride ratio in the subsequent feed solution will be too high, which can easily lead to a sudden change in ionic strength in the feed area. If it is higher than 70%, the subsequent feed solution will have limited effect on the regulation of the initial cationic monomer load, making it difficult to effectively reduce the local enrichment of the initial cationic monomer in the system.

[0006] The molar fraction of acryloyloxyethyltrimethylammonium chloride in the subsequent feed solution is set at 25%-55%, so that the feed solution contains enough acryloyloxyethyltrimethylammonium chloride while retaining enough acrylamide to participate in subsequent chain growth.

[0007] In step (2), oxygen can capture carbon-centered free radicals and form less reactive peroxy radicals. If only the initial reactor liquid is deoxygenated, but the feed liquid is not, oxygen will be reintroduced into the reaction system during the feeding process, causing deactivation of active chains and product batch fluctuations. Therefore, this invention deoxygenates both liquids separately and maintains nitrogen sealing during the reaction process.

[0008] In step (5), the subsequent addition of feed liquid not only introduces the monomer to be polymerized, but also changes the heat capacity and instantaneous monomer concentration of the reaction system. When R is in a low range, the feed rate is maintained or moderately increased; when R increases, the feed rate is reduced or suspended to reduce the possibility of concentrated monomer addition and concentrated exothermic polymerization.

[0009] In step (6), the 15-45℃ range is the main chain growth and reaction acceleration zone. Too rapid a temperature increase will promote rapid decomposition of the initiator and increase bimolecular termination; too slow a temperature increase may lead to insufficient conversion. Therefore, the heating rate is limited to 0.3-0.7℃ / min, and the peak temperature is limited to 58-65℃.

[0010] In step (7), ripening is used to consume residual monomers. Low-shear granulation and lower-temperature drying are used to reduce mechanical chain scission and thermal degradation.

[0011] Furthermore, the initial reactor solution contains 88%-92% of the total acrylamide and 55%-65% of the total acryloyloxyethyltrimethylammonium chloride; the subsequent feed solution contains 30%-45% of acryloyloxyethyltrimethylammonium chloride, resulting in a final effective monomer concentration of 28%-29.5%, with acryloyloxyethyltrimethylammonium chloride accounting for 26%-30% of the final effective monomer mass. During this process, when the effective monomer concentration is below 28%, the probability of collision between the monomer and the active chain decreases, which is unfavorable for forming longer molecular chains; above 29.5%, the system viscosity and polymerization exothermics increase, raising the risks of mixing and heat transfer.

[0012] Furthermore, in step S1, urea at 1%-2.2% of the total effective monomer mass and a complexing agent at 2-12 ppm of the total effective monomer mass are added to the initial reactor solution. Urea is beneficial for improving the post-treatment of the polymer and the solubility of the product, but excessive dosage will increase the content of small molecules and the subsequent drying burden. The complexing agent is used to complex transition metal ions such as Fe and Cu that may cause abnormal decomposition or chain termination of the initiator.

[0013] Furthermore, the complexing agent is one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and their water-soluble salts.

[0014] Further, the water-soluble azo initiator is 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis(2-methylpropanediamine) dihydrochloride, or a combination thereof, and its addition amount is 0.012%-0.035% of the total mass of effective monomers, wherein 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride is preferred, which decomposes slowly at low temperatures and gradually decomposes and maintains polymerization in the middle and later stages as the temperature increases. The persulfate is one or more of potassium persulfate, ammonium persulfate, and sodium persulfate, and its addition amount is 5-15 ppm of the total effective monomer mass. When the amount of persulfate added is less than 5 ppm, the number of initial free radicals generated by the redox initiation system may be insufficient, and the repeatability of polymerization initiation will be reduced. When it is higher than 15 ppm, the free radical concentration may be too high, thereby increasing the probability of bimolecular termination and reducing the average molecular chain length. The reducing agent is one of sodium bisulfite, sodium sulfite, and sodium formaldehyde bisulfite, and its addition amount is 3-10 ppm of the total effective monomer mass. When the amount of reducing agent added is less than 3 ppm, low-temperature initiation may be unstable. When it is higher than 10 ppm, it may lead to excessively rapid initial free radical generation and an increased exothermic rate.

[0015] Furthermore, the initial feed rate of the subsequent feed liquid is 4%-8% / min of the total mass of the subsequent feed liquid, and is adjusted as follows: When R is below 0.03℃ / min, increase the feed rate; When R is 0.03-0.08℃ / min, maintain the feed rate; When R is higher than 0.08℃ / min but not higher than 0.12℃ / min, reduce the feed rate; When R is higher than 0.12℃ / min or the reaction system temperature reaches 10℃, the feed is stopped, and the feed is resumed when R is lower than 0.08℃ / min and the system temperature is lower than 9.5℃.

[0016] Furthermore, after feeding, the heating rate is controlled at 0.4-0.6℃ / min within the 15-45℃ temperature range, and the peak polymerization temperature is controlled at 60-63℃. After reaching the peak temperature, it is matured at 55-65℃ for 60-120 minutes. When the peak temperature is below 58℃, the residual monomer may increase; when the peak temperature is above 65℃, the azo decomposition rate and the probability of bimolecular termination increase, and 60-63℃ is the preferred range.

[0017] Further, the polymer is cut into 3-5mm wet granules, dried and pulverized at 45-60℃, and then sieved to obtain 18-80 mesh products.

[0018] A high molecular weight cationic polyacrylamide is prepared by the preparation method of a high molecular weight cationic polyacrylamide described in any of the above technical solutions.

[0019] Application of a high molecular weight cationic polyacrylamide in sludge dewatering, wastewater flocculation, or papermaking retention and filtration aids.

[0020] The present invention has the following beneficial effects: Compared to existing processes that add all monomers at once, this invention provides a higher acrylamide ratio environment for early chain growth by reducing the initial content of acryloyloxyethyltrimethylammonium chloride. Compared to processes that add acryloyloxyethyltrimethylammonium chloride at a fixed time, the subsequent feed solution prepared in this invention contains both acrylamide and acryloyloxyethyltrimethylammonium chloride, which can reduce the instantaneous local concentration and ionic strength abrupt changes of acryloyloxyethyltrimethylammonium chloride near the feed point. Compared to processes that start feeding at a fixed time, the dual thresholds of ΔT and moving average heating rate can reflect the actual start-up state of each batch of polymerization system. Furthermore, this invention uses a linkage between feed rate and moving average heating rate, which helps to reduce exothermic peaks and batch-to-batch variations in raw materials. Detailed Implementation

[0021] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Raw materials used: Acrylamide aqueous solution with a mass concentration of 30%; Acryloyloxyethyltrimethylammonium chloride aqueous solution with a mass concentration of 78%; Urea (analytical grade); Phosphoric acid: prepared as a 20% aqueous solution; Ethylenediaminetetraacetic acid: prepared as a 1% aqueous solution; 2,2'-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride: prepared as a 1% aqueous solution; 2,2'-Azobis(2-methylpropanediamine) dihydrochloride: prepared as a 1% aqueous solution; Potassium persulfate: prepared as a 0.1% aqueous solution; Ammonium persulfate: prepared as a 0.1% aqueous solution; Sodium bisulfite: prepared as a 0.1% aqueous solution; Sodium sulfite: prepared as a 0.1% aqueous solution; Sodium formaldehyde bisulfite: prepared as a 0.1% aqueous solution; Deionized water: conductivity not higher than 10 μS / cm; Nitrogen gas: purity not lower than 99.99%.

[0023] Process control methods: Temperature reference and dual threshold calculation method The average temperature of the initial solution in the reactor within 180 seconds before the addition of the reducing agent was used as the reference temperature T0.

[0024] The temperature difference ΔT is defined as the difference between the reaction temperature T and the reference temperature T0 at any given time: ΔT = T - T0.

[0025] 180s moving average temperature rise rate R: R = (T t -T t-180 ) / 3; where T t T represents the current temperature. t-180 R represents the temperature 180 seconds ago, and the unit of R is °C / min.

[0026] When ΔT and R simultaneously enter the specified range and remain there for 60 seconds, the subsequent feeding is started.

[0027] Subsequent feeding methods Unless otherwise stated in the embodiments, the initial feed rate of the subsequent feed liquid is set to 4%-8% / min of the total mass of the subsequent feed liquid. The control system calculates R every 30 seconds and adjusts it as follows: When R is below 0.03℃ / min, the current pump speed is increased by 10%; When R is 0.03-0.08℃ / min, the pump speed remains constant; When R is higher than 0.08℃ / min but not higher than 0.12℃ / min, the current pump speed is reduced by 15%; When R is higher than 0.12℃ / min or the reaction system temperature reaches 10℃, the feed is stopped, and the feed is resumed when R is lower than 0.08℃ / min and the system temperature is lower than 9.5℃.

[0028] Feeding must be completed before visible localized clumps appear in the system. If a large amount of subsequent feed liquid is not added after reaching the above upper limit, the batch is considered a failure and no additional feed should be forced.

[0029] Example 1 A method for preparing high molecular weight cationic polyacrylamide includes the following steps: Preparation of initial reactor solution: Add 610.2g of 30% acrylamide aqueous solution, 63.6g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution, 5g of urea and 211g of deionized water to the reactor, stir at 100r / min, and after the urea is completely dissolved, add 0.2g of 1% ethylenediaminetetraacetic acid solution, and adjust the pH to 5.5-5.7 with 20% phosphoric acid to obtain the initial reactor solution.

[0030] Preparation of subsequent feed solution: Mix 67.8g of 30% acrylamide aqueous solution and 42.4g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution to obtain the subsequent feed solution; The initial reactor liquid and the subsequent feed liquid were cooled to 3-5℃ respectively, and nitrogen was purged to remove oxygen until the dissolved oxygen in both was no higher than 0.2mg / L. After deoxygenation, nitrogen protection was maintained. Add 8g of 1% 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride solution to the initial reactor solution, stir for 2 min, add 2.5g of 0.1% potassium persulfate solution, stir for 1 min; record the average temperature of the reducing agent in the first 180s as T0, and then add 1.5g of 0.1% sodium bisulfite solution; When ΔT is 0.4-0.8℃ and R is 0.02-0.08℃ / min, and both conditions are maintained continuously for 60s, the subsequent feed liquid is added. The initial pump speed is set to 7 g / min, and all feed is completed within 12-20 min. The feed rate is adjusted according to the moving average heating rate R to maintain the moving average heating rate at 0.05-0.08℃ / min and the reaction system temperature at no higher than 10℃ during the feeding period. After feeding is completed, stop stirring when the system can no longer flow normally. Control the heating rate in the 15-45℃ temperature range to 0.4-0.6℃ / min through the jacket, and control the peak temperature to 60-63℃. After reaching the peak temperature, the polymer is cured at 60±2℃ for 90 minutes. The polymer is then removed, cut into 3-5mm wet granules, dried at 50±2℃ until the moisture content is no more than 10%, pulverized under low shear, and sieved to obtain a product of 18-80 mesh, which is high molecular weight cationic polyacrylamide.

[0031] Example 2 This embodiment differs from Embodiment 1 in the following ways: In step (1), during the preparation of the initial reactor solution, the mass of the 30% acrylamide aqueous solution is 596.6g, the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 58.3g, and the pH is adjusted to 5.4-5.6 with 20% phosphoric acid; during the preparation of the subsequent feed solution, the mass of the 30% acrylamide aqueous solution is 81.4g, and the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 47.7g; the initial reactor solution and the subsequent feed solution are purged with nitrogen to remove oxygen until the dissolved oxygen in both is not higher than 0.15mg / L.

[0032] In step (2), 7g of 1% 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride solution, 2g of 0.1% potassium persulfate solution, and 1.2g of 0.1% sodium bisulfite solution are added to the initial reactor solution; In step (3), the triggering range of ΔT is 0.4-0.6℃ and the triggering range of R is 0.02-0.05℃ / min.

[0033] In step (4), the feeding time is 18-22 min.

[0034] In step (5), the heating rate in the 15-45℃ temperature range is controlled to be 0.4-0.5℃ / min, and the peak temperature is 60-62℃.

[0035] In step (6), the curing conditions are: curing at 60±2℃ for 100 min.

[0036] Example 3 This embodiment differs from Embodiment 1 in the following ways: In step (1), during the preparation of the initial reactor solution, the mass of the 30% acrylamide aqueous solution is 623.8g, the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 68.9g, and the pH is adjusted to 5.7-5.9 with 20% phosphoric acid; during the preparation of the subsequent feed solution, the mass of the 30% acrylamide aqueous solution is 54.2g, and the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 37.1g; the initial reactor solution and the subsequent feed solution are purged with nitrogen to remove oxygen until the dissolved oxygen in both is not higher than 0.3mg / L.

[0037] In step (2), 9g of 1% 2,2'-azobis(2-methylpropanediamine) dihydrochloride solution, 3g of 0.1% ammonium persulfate solution, and 1.8g of 0.1% sodium formaldehyde sulfoxylate solution were added to the initial reactor solution. In step (3), the triggering range of ΔT is 0.6-0.8℃ and the triggering range of R is 0.05-0.08℃ / min.

[0038] In step (4), the feeding time is 10-14 min.

[0039] In step (5), the heating rate in the 15-45℃ temperature range is controlled to be 0.5-0.7℃ / min, and the peak temperature is 62-65℃.

[0040] In step (6), the curing conditions are: curing at 62±2℃ for 80 min.

[0041] Example 4 This embodiment differs from Embodiment 1 in the following ways: In step (1), during the preparation of the initial reactor solution, the mass of the 30% acrylamide aqueous solution is 644.1g, the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 74.2g, and the pH is adjusted to 5.8-5.9 with 20% phosphoric acid; during the preparation of the subsequent feed solution, the mass of the 30% acrylamide aqueous solution is 33.9g, and the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 31.8g; the initial reactor solution and the subsequent feed solution are purged with nitrogen to remove oxygen until the dissolved oxygen in both is not higher than 0.3mg / L.

[0042] In step (2), 10g of 1% 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride solution, 4g of 0.1% potassium persulfate solution, and 2.8g of 0.1% sodium bisulfite solution are added to the initial reactor solution; In step (3), the triggering range of ΔT is 0.7-0.8℃ and the triggering range of R is 0.06-0.08℃ / min.

[0043] In step (4), the total mass of the subsequent feed liquid is 65.7g, and the initial pump speed is set to 7.0g / min, which is equivalent to about 10.7% / min of the total mass of the subsequent feed liquid. The feed speed is adjusted according to the moving average heating rate R, and all feed is completed within 8-10min.

[0044] In step (5), the peak temperature is controlled to be 63-65℃.

[0045] In step (6), the curing conditions are: curing at 63±2℃ for 70 min.

[0046] Example 5 This embodiment differs from Embodiment 1 in the following ways: In step (1), during the preparation of the initial reactor solution, the mass of the 30% acrylamide aqueous solution is 576.3g, the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 53g, and the pH is adjusted to 5.2-5.4 with 20% phosphoric acid; during the preparation of the subsequent feed solution, the mass of the 30% acrylamide aqueous solution is 101.7g, and the mass of the 78% acryloyloxyethyltrimethylammonium chloride aqueous solution is 53g; the initial reactor solution and the subsequent feed solution are purged with nitrogen to remove oxygen until the dissolved oxygen in both is not higher than 0.2mg / L.

[0047] In step (2), 3.5g of 1% 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride solution, 1.5g of 0.1% potassium persulfate solution, and 0.9g of 0.1% sodium bisulfite solution are added to the initial reactor solution; In step (3), the triggering range of ΔT is 0.4-0.58℃ and the triggering range of R is 0.02-0.04℃ / min.

[0048] In step (4), the feeding time is 25-30 min.

[0049] In step (5), the peak temperature is controlled to be 58-60℃.

[0050] In step (6), the curing conditions are: curing at 60±2℃ for 120 min.

[0051] Comparative Example 1 This comparative example provides a method for preparing cationic polyacrylamide, specifically including the following steps: Add 678g of 30% acrylamide solution and 106g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution to the reaction vessel, then add 5g of urea and 211g of deionized water, stir until the urea is completely dissolved, adjust the pH of the solution to 4-6 with 20% phosphoric acid, and cool to 3-5℃. After purging with nitrogen for 10 minutes, add 0.3 g of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride and 0.1 g of 1% ethylenediaminetetraacetic acid solution; continue purging with nitrogen for 10 minutes, then add 0.1 g of 1% sodium formate aqueous solution, followed by 0.4 g of 1% potassium persulfate aqueous solution after 5 minutes, then 0.3 g of 1% sodium bisulfite aqueous solution after 2 minutes, and finally 0.4 g of 1% formaldehyde sodium bisulfite aqueous solution after 1 minute. Once the material shows significant viscosity increase and a continuous temperature rise, stop purging with nitrogen into the liquid, maintain nitrogen protection in the reactor and seal the reaction. After the reaction temperature reaches its peak of approximately 73°C and stops rising, continue sealing and aging for 1 hour. After aging, remove the colloid, granulate it, and dry it at 50°C until the moisture content is no more than 10%. Then, pulverize and sieve 18-80 mesh particles to obtain the cationic polyacrylamide product.

[0052] Comparative Example 2 Compared to Example 1, in this comparative example, 678g of 30% acrylamide aqueous solution and 106g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution were added entirely to the initial reactor liquid, and no subsequent feed liquid was provided. All other aspects were the same as in Example 1.

[0053] Comparative Example 3 This comparative example differs from Example 1 in the following ways: In the initial reactor solution preparation, 678g of acrylamide aqueous solution, 63.6g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution, 5g of urea, and 211g of deionized water were added. The mixture was stirred at 100r / min until the urea was completely dissolved. Then, 0.2g of 1% ethylenediaminetetraacetic acid solution was added, and the pH was adjusted to 5.5-5.7 with 20% phosphoric acid. In the subsequent feed solution preparation, only 42.4g of 78% acryloyloxyethyltrimethylammonium chloride aqueous solution was added, without any acrylamide aqueous solution.

[0054] After adding the reducing agent, the subsequent feeding is started after a fixed 15-minute period, and the addition is completed within 15 minutes at a constant rate of 2.83 g / min. The dual thresholds of ΔT and R are not used, and the pump speed is not adjusted according to R.

[0055] The rest are the same as in Example 1.

[0056] Comparative Example 4 Compared with Example 1, this comparative example started subsequent feeding 15 minutes after the addition of the reducing agent, adding 110.2g of subsequent feed liquid at a constant rate of 7.35g / min over 15 minutes, without using the dual thresholds of ΔT and R, and without adjusting the pump speed according to R. All other procedures were the same as in Example 1.

[0057] Comparative Example 5 Compared with Example 1, this comparative example uses the dual thresholds ΔT and R of Example 1 to determine the starting point of the feed, but after starting the feed, it is added at a fixed rate of 7.35 g / min, and the pump speed is not adjusted according to R. All other aspects are the same as in Example 1.

[0058] Comparative Example 6 Compared to Example 1, in this comparative example, active cooling was turned off after feeding, and only the natural temperature rise process was recorded. The peak temperature was allowed to rise to 70-75°C. If the temperature exceeded 75°C, emergency cooling was immediately initiated and the test was terminated. All other aspects were the same as in Example 1.

[0059] Related tests: All sample tests should be performed in accordance with the relevant standards in GB / T 31246—2025.

[0060] Sample pretreatment: After drying, pulverizing and sieving the polymer to 18-80 mesh, the sample is reduced to no less than 100g by quartering. The sample should be placed in a sealed moisture-proof bag and placed at 23±2℃ for 24h before testing.

[0061] Dry basis correction: This involves calculating indicators such as relative molecular mass, cationicity, and residual monomers, and requires correction based on the solid content determination results according to dry basis mass. The formula for calculating dry basis mass is: m d =m s × (S / 100); where: m d The dry weight of the sample is in g; m s The actual mass of the sample taken is in grams; S is the solid content of the sample, in grams.

[0062] Solid content test: After drying the clean weighing bottle at 105±2℃ for 1 hour, place it in a desiccator to cool for 30 minutes and weigh it, recording the empty bottle mass as m0; add about 1.0000g of sample to the weighing bottle and weigh it, recording it as m1; place the weighing bottle in a drying oven at 105±2℃ for 2 hours, remove it and place it in a desiccator to cool for 30 minutes and weigh it; dry for another 30 minutes, cool and weigh it, until the difference between two adjacent weighings is no greater than 0.2mg, and the final mass is recorded as m2; then the solid content S=[(m2-m0) / (m1-m0)]×100%, where S is the solid content.

[0063] Relative molecular mass determination: A 1.00 mol / L sodium chloride solution was used as the solvent to shield the cationic charge on the polymer chain and reduce the polyelectrolyte effect. 0.4000 g of sample was weighed on a dry basis and approximately 800 mL of 1.00 mol / L NaCl solution was added to a 1000 mL beaker. The sample was slowly added over 60-90 s with stirring at 200 rpm, and stirred at 30 °C until completely dissolved. The solution was then transferred to a 1000 mL volumetric flask and diluted to volume with 1.00 mol / L NaCl solution to obtain a 0.0400 g / dL mother liquor. The solution was allowed to stand for 30 min to defoam.

[0064] Prepare test solutions with concentrations of 0.0100 g / dL, 0.0200 g / dL, 0.0300 g / dL, and 0.0400 g / dL by serial dilution. Use an Ubbelohde capillary viscometer (solvent outflow time controlled between 100-130 s) to measure the outflow time of each concentration solution and solvent in a 30℃ constant temperature water bath. Take three measurements each time and average the results (error not exceeding 0.2 s).

[0065] Linear regression extrapolation was performed using the Huggins equation (specific viscosity versus concentration) and the Kraemer equation (specific log viscosity versus concentration) until the concentration c=0. The average of the two intercepts was taken as the intrinsic viscosity [η]. The Mark-Houwink equation [η]=KM was then used. η a Where, [η] is the intrinsic viscosity, in mL / g; M η The value represents the viscosity-average molecular weight; K = 4.75 × 10⁻⁶. -3 mL / g; a=0.80; calculate the relative molecular mass.

[0066] Cationicity test: Weigh 0.1000 g of the dry sample and dissolve it in deionized water, then dilute to 200 mL to obtain an approximately 0.0500% sample solution. Transfer 10.00 mL of the solution to 90.0 mL of deionized water, adjust the pH to 6.0 ± 0.2, and titrate with a 0.0025 mol / L potassium polyvinyl sulfate (PVSK) standard solution. Monitor the current signal using a particle charge detector, and use the endpoint of the titration when the current signal returns to zero and remains stable within 30 s. Record the volume of PVSK consumed to calculate the charge density and convert it to cation concentration.

[0067] Residual acrylamide test: A C18 column (4.6 mm × 250 mm, 5 μm) was used with a UV detector wavelength of 202 nm. The column temperature was 30 °C, and the mobile phase was water / acetonitrile (98 / 2 v / v) at a flow rate of 0.8 mL / min. 1.0000 g of dry sample was added to 100 mL of deionized water and extracted at 25 °C and 200 rpm for 2 h. 5.00 mL of the extract was added to 20.0 mL of acetonitrile to precipitate the polymer. After vortexing, the mixture was centrifuged at 10000 rpm for 10 min. The supernatant was filtered through a 0.22 μm filter before injection for analysis.

[0068] Water-insoluble matter test: Weigh 1.0000 g of sample (dry basis) and add it to 1000 mL of deionized water. Stir at 25 °C and 300 rpm for 2 h. After stirring, pass the entire solution through a pre-washed, dried, and constant-weighted 100 μm nylon filter. Wash the soluble polymer on the filter with a fixed volume of deionized water and dry the filter at 105 °C to constant weight. Calculate the mass fraction of water-insoluble matter based on the mass difference of the filter before and after filtration.

[0069] The test results are shown in Table 1. Table 1 Product Performance Results

[0070] The test results above show that the cationicity of the products obtained in Examples 1 to 5 is basically similar, but there are differences in viscosity-average molecular weight, residual acrylamide, and water-insoluble matter. Example 2 has the highest molecular weight, indicating that a lower free radical generation rate and a gentler temperature rise process can reduce the number of newly formed active chains and bimolecular termination, allowing more monomers to continue growing on existing molecular chains, thus resulting in the highest molecular weight. Example 3, using a higher initiator dosage, faster feed, and a higher peak temperature, reduced the molecular weight to 16.48 million, but the residual acrylamide was only 0.024%, indicating that faster free radical generation is beneficial for improving conversion rate but not for forming longer molecular chains.

[0071] Dual-threshold feed monitoring: Real-time acquisition of reaction system temperature and dissolved oxygen data, with temperature sampling intervals not exceeding 5 s and dissolved oxygen sampling intervals not exceeding 10 s. For the embodiments and comparative examples using dual thresholds to determine the feed start point, the temperature difference ΔT at the start of feed, the average heating rate R within the 180-s rolling time window, and the continuous maintenance time for ΔT and R to simultaneously meet the corresponding thresholds are recorded. Batches that do not simultaneously meet the specified dual thresholds and continuous maintenance time requirements at the start of feed are not included in the embodiments of this invention. The polymerization process test results are shown in Table 2.

[0072] Table 2. Polymerization process test characterization

[0073] As can be seen from Table 2, in Examples 2 and 5, due to the lower amount of initiator, the triggering R was relatively low and the induction time was longer; in Examples 3 and 4, due to the higher amount of initiator, the triggering ΔT and R were relatively high and the triggering time was shorter.

[0074] Sludge dewatering performance test: Use the same batch of municipal sludge, and strictly control the sludge solids content within the range of 1.5%-3.0%. If it exceeds this range, adjust it using the supernatant from the same batch of sludge. Prepare a 0.100% polymer aqueous solution on a dry basis, completely dissolve it, and let it stand for 30 minutes to mature. Ensure that the solution is used up immediately after preparation.

[0075] Flocculation process: Accurately measure 500 mL of pretreated sludge and place it in a 1000 mL beaker. Calculate the polymer dosage based on the dry solids mass of the sludge (set at 4 kg activated polymer / t dry sludge). After adding the corresponding amount of polymer solution, stir rapidly at 250 r / min for 20 s, then stir slowly at 50 r / min for 60 s, and stop stirring. Let it stand for 5 min, observe and record the floc state and settling velocity.

[0076] Capillary absorption time (CST) test: The same model of CST tester and the same batch of filter paper were used for the test.

[0077] Filtrate turbidity test: Extract the supernatant after standing or filtration and measure it using a calibrated turbidity meter (unit: NTU).

[0078] Filter cake moisture content: The flocculated sludge was placed under a gauge pressure of -(0.060±0.005) MPa and filtered for 5 minutes. The filter cake was collected and its wet mass m was measured. w The filter cake was then placed in a constant temperature drying oven at 105℃ and dried until constant weight was achieved. The dry mass m was then measured. d The filter cake moisture content (MC) is calculated using the following formula: MC = [(m w -m d ) / m w ]×100%.

[0079] The test results are shown in Table 3. Table 3. Sludge dewatering test results

[0080] The test results above show that Example 2 has the highest molecular weight, therefore it has the lowest CST and a higher settling velocity. Compared with Comparative Example 1, Example 1 shows a decrease in CST, filter cake moisture content, and filtrate turbidity, and an increase in settling velocity, indicating that the polymer obtained in Example 1 has a longer effective molecular chain and better solubility, and can form larger, denser sludge flocs with certain shear resistance.

[0081] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Those skilled in the art can make corresponding adjustments or equivalent substitutions to the described technical solutions without departing from the inventive concept. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for preparing high molecular weight cationic polyacrylamide, characterized in that, Includes the following steps: S1. Using acrylamide and acryloyloxyethyltrimethylammonium chloride as polymerization monomers, prepare an initial reactor solution with 85%-95% of the total acrylamide and 50%-70% of the total acryloyloxyethyltrimethylammonium chloride; prepare the remaining acrylamide and remaining acryloyloxyethyltrimethylammonium chloride as the subsequent feed solution; wherein, based on the total effective monomer mass in the final polymerization system, acrylamide is 68-78%, acryloyloxyethyltrimethylammonium chloride is 22%-32%, the final effective monomer mass concentration is 27.5%-30%, and the molar fraction of acryloyloxyethyltrimethylammonium chloride in the subsequent feed solution is 25%-55%; adjust the pH of the initial reactor solution to 5.2-5.9 with acid; S2. Cool the initial reactor liquid and the subsequent feed liquid to 2-5℃ respectively, and deoxygenate them respectively until the dissolved oxygen is no higher than 0.3mg / L. After deoxygenation is completed, maintain nitrogen protection. S3. Add water-soluble azo initiator, persulfate and reducing agent to the initial reactor liquid to start the polymerization reaction at 2-5℃; S4. Take the average temperature of the initial reactor solution within 180 seconds before adding the reducing agent as the reference temperature T0, and take the difference between the reaction temperature T and the reference temperature T0 at any given time as the temperature difference ΔT. Calculate the 180-second moving average heating rate R using the following formula: R = (T... t -T t-180 ) / 3; where T t T represents the current temperature. t-180 The temperature is 180s ago, and the unit of R is ℃ / min; when ΔT is 0.4-0.8℃ and R is 0.02-0.08℃ / min, and both conditions are maintained continuously for 30-90s, the subsequent feed liquid is added. S5. Add the subsequent feed liquid within 8-30 minutes, and adjust the feed rate according to the moving average heating rate R; S6. After feeding is completed, continue polymerization, and control the maximum heating rate of the reaction system in the 15-45℃ temperature range to 0.3-0.7℃ / min, and control the peak polymerization temperature to 58-65℃. S7. After reaching the peak temperature, heat preservation and curing are carried out to obtain polymerized glue, which is then granulated, dried, crushed and sieved to obtain cationic polyacrylamide.

2. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, The initial reactor solution contains 88%-92% of the total acrylamide and 55%-65% of the total acryloyloxyethyltrimethylammonium chloride; the subsequent feed solution contains 30%-45% of acryloyloxyethyltrimethylammonium chloride, and the final effective monomer mass concentration is 28%-29.5%, with acryloyloxyethyltrimethylammonium chloride accounting for 26%-30% of the final effective monomer mass.

3. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, In step S1, urea, accounting for 1%-2.2% of the total mass of effective monomers, and a complexing agent, accounting for 2-12 ppm of the total mass of effective monomers, are also added to the initial reactor solution.

4. The method for preparing a high molecular weight cationic polyacrylamide according to claim 3, characterized in that, The complexing agent is one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and their water-soluble salts.

5. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, The water-soluble azo initiator is 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis(2-methylpropanediamine) dihydrochloride, or a combination thereof, and its addition amount is 0.012%-0.035% of the total effective monomer mass; the persulfate is one or more of potassium persulfate, ammonium persulfate, and sodium persulfate, and its addition amount is 5-15 ppm of the total effective monomer mass; the reducing agent is one of sodium bisulfite, sodium sulfite, and sodium formaldehyde bisulfite, and its addition amount is 3-10 ppm of the total effective monomer mass.

6. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, The initial feed rate of the subsequent feed solution is 4%-8% / min of the total mass of the subsequent feed solution, and is adjusted as follows: When R is below 0.03℃ / min, increase the feed rate; When R is 0.03-0.08℃ / min, maintain the feed rate; When R is higher than 0.08℃ / min but not higher than 0.12℃ / min, reduce the feed rate; When R is higher than 0.12℃ / min or the reaction system temperature reaches 10℃, the feed is stopped, and the feed is resumed when R is lower than 0.08℃ / min and the system temperature is lower than 9.5℃.

7. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, After feeding, the heating rate is controlled at 0.4-0.6℃ / min within the temperature range of 15-45℃, and the peak polymerization temperature is controlled at 60-63℃; after reaching the peak temperature, it is matured at 55-65℃ for 60-120min.

8. The method for preparing a high molecular weight cationic polyacrylamide according to claim 1, characterized in that, The polymer is cut into 3-5mm wet granules, dried and pulverized at 45-60℃, and then sieved to obtain 18-80 mesh products.

9. A high molecular weight cationic polyacrylamide, characterized in that, It is prepared by the method for preparing a high molecular weight cationic polyacrylamide according to any one of claims 1-8.

10. The application of a high molecular weight cationic polyacrylamide as described in claim 9 in sludge dewatering, wastewater flocculation, or papermaking retention and filtration aids.

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