Method for on-line continuous reaction of nanosilica sol and its application in papermaking polymeric retention and drainage

Abstract

This invention belongs to the field of nanomaterial preparation and papermaking additive application technology, and discloses a method for online continuous reaction of nano-silica sol and its application in multi-element retention and filtration aids in papermaking. The method includes seven steps: raw material pretreatment and precise metering and delivery; tubular micro-mixing for rapid nucleation reaction; segmented gradient temperature control for crystal nucleus growth; continuous countercurrent ion exchange purification; tangential flow ultrafiltration concentration and particle size classification; online addition of composite stabilizers and precise pH control; and high-shear homogeneous dispersion and direct online delivery. This invention achieves precise control and stable quality control of nano-silica sol particle size through a fully continuous process. The resulting product has a narrow particle size distribution, high purity, and good colloidal activity, and can be directly delivered online to the wet end of papermaking. Combining it with cationic polyacrylamide and modified bentonite to form a ternary retention and filtration aid system can significantly improve the retention rate of fine fibers and fillers, accelerate pulp filtration, and improve paper quality, making it suitable for continuous production on high-speed paper machines.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation and papermaking additive application technology, and more specifically, it is a method for online continuous reaction of nano-silica sol and its application in multi-element retention and filtration aids in papermaking. Background Technology

[0002] Nano-silica sol, as a highly dispersed nano-silica gel system, has become a core component of multi-element retention and filtration aid systems in the wet end of papermaking due to its large specific surface area, high surface activity, and excellent charge adsorption properties. During the papermaking process, nano-silica sol can effectively capture fine fibers, fillers, and colloidal particles through electrostatic adsorption and bridging, in conjunction with cationic polymers, bentonite, and other additives. This significantly improves pulp retention and filtration speed, while also enhancing paper uniformity and physical strength.

[0003] Currently, the industrial preparation of nano-silica sol mainly employs a batch reactor process, using sodium silicate and inorganic acids as raw materials. Silica sol is generated through a neutralization reaction, followed by aging, purification, and concentration steps to obtain the final product. This process suffers from a core and difficult-to-solve problem: poor batch-to-batch product quality stability, a wide and uncontrollable particle size distribution, and an inability to precisely adapt to the dynamic conditions of continuous papermaking production. Specifically, uneven material mixing and large fluctuations in temperature and concentration fields during batch reactions result in an average particle size deviation of 10-20 nm between different batches of silica sol, with a particle size distribution index (PDI) > 0.3. The product requires storage and transportation, during which particle agglomeration and gelation easily occur, leading to a decrease in surface activity. On-site dilution and preparation are necessary during use, increasing process complexity and causing fluctuations in wet-end charge due to uneven dilution. Ultimately, this results in unstable retention and filtration effects, with fine fiber retention rates fluctuating by 8-12% and filtration rates differing by more than 15%, severely impacting the operating efficiency of the paper machine and the consistency of paper quality. In addition, traditional intermittent processes have long production cycles (12-24 hours per batch), high energy consumption, and cannot dynamically adjust the performance parameters of silica sol according to the real-time operating conditions of the paper production line, making it difficult to meet the continuous and intelligent production needs of modern high-speed paper machines. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for online continuous reaction of nano-silica sol and its application in multi-element retention and filtration aids in papermaking, thereby solving the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for online continuous reaction of nano-silica sol, the method being as follows: Step 1: Raw material pretreatment and precise metering and conveying Industrial-grade sodium silicate solution is filtered through a plate and frame filter press to remove insoluble impurities, resulting in a clarified sodium silicate solution with a silica mass fraction of 8-12% and a modulus of 3.2-3.4. 98% industrial concentrated sulfuric acid is diluted with deionized water at a volume ratio of 1:5 to obtain a dilute sulfuric acid solution. The clarified sodium silicate solution and the dilute sulfuric acid solution are then separately fed into high-precision diaphragm metering pumps, delivered at a constant flow rate according to a silica to sulfuric acid molar ratio of 1:0.85-0.95, with metering accuracy controlled within ±0.2%. Step 2: Rapid nucleation reaction using tubular micromixing The metered sodium silicate solution and dilute sulfuric acid solution are simultaneously introduced into a tubular micro-mixing reactor. The reactor is equipped with three layers of staggered sinusoidal mixing channels with an inner diameter of 2-3 mm. The reaction temperature is controlled at 45-55℃, the reaction pressure at 0.3-0.5 MPa, and the residence time of the materials in the reactor is 10-15 seconds. Through high-intensity micro-mixing, an instantaneous and uniform neutralization reaction is achieved, generating silica sol crystal nuclei with uniform particle size. The average particle size of the crystal nuclei is controlled at 3-5 nm. Step 3: Segmented gradient temperature control of crystal nucleus growth The reaction solution containing crystal nuclei is continuously fed into a segmented tubular growth reactor, which is divided into three temperature-controlled sections connected in series: the first temperature-controlled section has a temperature of 60-70℃ and a residence time of 20-30 minutes; the second temperature-controlled section has a temperature of 75-85℃ and a residence time of 30-40 minutes; and the third temperature-controlled section has a temperature of 90-95℃ and a residence time of 15-20 minutes. By controlling the growth rate of crystal nuclei through gradient heating, abnormal particle growth is avoided, and primary silica sol with an average particle size of 15-25 nm is obtained. Step 4: Continuous countercurrent ion exchange purification The primary silica sol was continuously fed into a series of cation exchange resin columns and anion exchange resin columns at a flow rate of 1.5-2.5 L / min, and ion exchange was carried out in a countercurrent contact manner. The cation exchange resin was a strongly acidic styrene-based cation exchange resin, and the anion exchange resin was a weakly basic acrylic-based anion exchange resin. The sodium sulfate and metal ion impurities such as potassium, calcium, and magnesium generated in the reaction were removed, and the total metal ion content in the purified silica sol was controlled to be less than 50 ppm. Step 5: Tangential flow ultrafiltration concentration and particle size fractionation The purified silica sol was passed into a tangential flow ultrafiltration system. A polyethersulfone ultrafiltration membrane with a molecular weight cutoff of 50 kDa was selected, and the transmembrane pressure was controlled at 0.2-0.4 MPa and the feed flow rate was 3-5 m / s. Small molecule impurities and excess water in the system were removed by ultrafiltration, while agglomerated particles with a particle size greater than 30 nm were retained. The silica sol was concentrated to a solid content of 18-22 wt%, resulting in narrow-distribution nano-silica sol with a particle size distribution index (PDI) < 0.15. Step 6: Online addition of composite stabilizer and precise pH control The concentrated silica sol is passed into a static mixer, and a composite stabilizer is continuously added to the mixer using a metering pump. The amount of composite stabilizer added is 0.3-0.6% of the silica sol mass. After mixing evenly, a 10% sodium hydroxide solution is added dropwise to the silica sol using an automatic potentiometric titration system to precisely adjust the pH value of the silica sol to 8.8-9.2, with the titration accuracy controlled within ±0.05 pH. Step 7: High-shear homogeneous dispersion and online direct delivery The pH-adjusted silica sol is fed into a high-shear homogenizer and continuously homogenized at a speed of 15,000-18,000 rpm for 2-3 minutes. After homogenization, the nano-silica sol is directly transported to the wet end addition point of the paper production line through a closed pipeline. During the transport process, the temperature is maintained at 25-30℃ and the pressure is maintained at 0.2-0.3MPa.

[0006] As a preferred embodiment of the present invention: the mixing channel surface of the tubular micro-mixing reactor in step 2 is coated with polytetrafluoroethylene coating, and pressure sensors and temperature sensors are installed at the reactor inlet and outlet to monitor the pressure and temperature changes during the reaction process in real time. When the pressure fluctuation exceeds ±0.05MPa, the delivery flow rate of the metering pump is automatically adjusted.

[0007] As a preferred embodiment of the present invention: each temperature control section of the segmented tubular growth reactor described in step 3 is equipped with an online dynamic light scattering particle size analyzer to detect the average particle size and particle size distribution of the silica sol in real time, and automatically adjust the temperature and material residence time of the corresponding temperature control section according to the detection results.

[0008] In a preferred embodiment of the present invention, both the cation exchange resin column and the anion exchange resin column in step 4 are equipped with regeneration bypasses. When the adsorption capacity of the ion exchange resin reaches saturation, the system automatically switches to the standby resin column for operation, while simultaneously regenerating the saturated resin column online, thereby achieving continuous and uninterrupted operation of the ion exchange process.

[0009] As a preferred embodiment of the present invention: the tangential flow ultrafiltration system in step 5 is equipped with a concentrate reflux pipeline, which refluxes the intercepted large-diameter particles to the first temperature control section in step 3, so as to participate in the subsequent crystal nucleus growth process as seed crystals, thereby improving the utilization rate of raw materials. The reflux ratio is controlled at 1:8-1:10.

[0010] As a preferred embodiment of the present invention: the composite stabilizer in step 6 is composed of polyvinylpyrrolidone and hydroxyethyl cellulose in a mass ratio of 2:1. The two stabilizers are added separately through independent metering pumps, and the addition ratio can be dynamically adjusted according to the pulp properties of the papermaking production line.

[0011] As a preferred embodiment of the present invention: the stator and rotor gap of the high shear homogenizer in step 7 is 0.1-0.2mm, and the temperature of the silica sol is controlled to not exceed 40°C by jacket cooling water during the homogenization process to prevent the silica sol particles from agglomerating due to high temperature.

[0012] Specifically, an online continuous reaction method for nano-silica sol is applied in the production of papermaking multi-component retention and filtration aids. The application is as follows: Online-prepared nano-silica sol is combined with cationic polyacrylamide and modified bentonite to form a ternary retention and filtration aid system, which is then applied to the production process of cultural paper. First, cationic polyacrylamide is added to the inlet of the pulp pump of the paper machine at a rate of 0.03-0.05% of the oven-dry pulp mass. After uniform mixing, modified bentonite is added to the inlet of the pressure screen at a rate of 0.1-0.2% of the oven-dry pulp mass. Finally, online-prepared nano-silica sol is added to the inlet of the wire mesh box at a rate of 0.2-0.4% of the oven-dry pulp mass. The nano-silica sol forms flocs with the fine fibers and fillers that have adsorbed cationic polyacrylamide through electrostatic adsorption. Simultaneously, bentonite is interspersed between the flocs, forming a dense and relatively strong network structure, improving the retention rate of fine fibers and fillers, and accelerating the filtration speed of the pulp. This application is suitable for machine speeds of 800-1500 m / min. The high-speed cultural paper machine can effectively reduce white water concentration, reduce fiber loss, and improve the operating efficiency of the paper machine.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] The online continuous reaction method for nano-silica sol provided by this invention achieves precise control of silica sol particle size through tubular micro-mixing rapid nucleation and segmented gradient temperature growth technology. The resulting product has an average particle size deviation of less than ±2 nm and a particle size distribution index (PDI) of <0.15, which is far superior to the product quality stability of traditional batch processes. The combination of online continuous ion exchange and tangential flow ultrafiltration technology not only effectively removes impurity ions from the system but also achieves silica sol concentration and particle size classification, with a total metal ion content of less than 50 ppm, avoiding interference of impurities on the charge balance of the wet end of papermaking.

[0015] This method employs a fully continuous production process, shortening the production cycle to 2-3 hours and increasing production efficiency by 4-6 times compared to traditional intermittent processes, while reducing energy consumption by more than 30%. The online addition of composite stabilizers and precise pH control technology significantly improve the storage stability and colloidal activity of silica sol. Combined with high-shear homogeneous dispersion and direct online delivery, it completely avoids particle agglomeration and gelation problems during storage and transportation, ensuring that the silica sol maintains optimal surface activity when entering the wet end of papermaking.

[0016] In the application of multi-element retention and filtration aids in papermaking, the nano-silica sol prepared by this invention exhibits excellent synergistic effects with cationic polyacrylamide and bentonite, increasing the retention rate of fine fibers to over 85%, the filler retention rate to over 70%, and the filtration speed by 20-30%. Simultaneously, it effectively improves paper uniformity and physical strength, increasing tensile strength by 8-10% and tear strength by 5-7%. Furthermore, this method allows for dynamic adjustment of the silica sol's performance parameters according to the real-time operating conditions of the papermaking production line, achieving precise adaptation to continuous papermaking production and significantly improving the consistency of paper quality, making it suitable for widespread application on modern high-speed papermaking machines. Attached Figure Description

[0017] Figure 1 This is a flowchart of the method described in this invention;

[0018] Figure 2 This invention provides a process for the application of nano-silica sol in papermaking as a ternary retention and filtration aid. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0020] Basic Implementation Example (Optimal Process Implementation Example)

[0021] I. Raw Materials and Reagents

[0022] Industrial grade sodium silicate solution (SiO2 mass fraction 28%, modulus 3.3)

[0023] Industrial concentrated sulfuric acid (98% by mass)

[0024] Strongly acidic styrene-based cation exchange resin (Type 001×7)

[0025] Weakly basic acrylic anion exchange resin (D311 type)

[0026] Polyethersulfone ultrafiltration membrane (molecular weight cutoff 50 kDa)

[0027] Polyvinylpyrrolidone (PVP-K30)

[0028] Hydroxyethyl cellulose (HEC, viscosity 3000-5000 mPa·s)

[0029] Sodium hydroxide solution (10% by mass)

[0030] Cationic polyacrylamide (CPAM, molecular weight 8 million, cationicity 30%)

[0031] Modified bentonite (sodium-based bentonite, particle size 200 mesh)

[0032] Bleached sulfate hardwood pulp (beating degree 35°SR)

[0033] Light calcium carbonate (GCC, particle size 2-5μm)

[0034] II. Preparation Method

[0035] Raw material pretreatment and precise metering and conveying

[0036] Industrial-grade sodium silicate solution was filtered through a plate and frame filter press (filtration accuracy 5μm) to remove insoluble impurities, and then diluted with deionized water to a SiO2 mass fraction of 10% while maintaining a modulus of 3.3. Concentrated sulfuric acid was diluted with deionized water at a volume ratio of 1:5 to obtain a dilute sulfuric acid solution.

[0037] The clarified sodium silicate solution and dilute sulfuric acid solution were separately fed into a high-precision diaphragm metering pump and delivered to the tubular micro-mixing reactor at a constant flow rate according to the SiO2 to H2SO4 molar ratio of 1:0.9, with the metering accuracy controlled within ±0.2%.

[0038] Technical Principle: Precise metering is crucial for ensuring a stable stoichiometric ratio in the neutralization reaction, directly affecting the formation quality and particle size uniformity of silica sol crystal nuclei. A metering accuracy deviation exceeding ±0.5% will cause significant fluctuations in the pH value of the reaction system, resulting in uncontrolled crystal nucleus growth.

[0039] Tubular micromixing rapid nucleation reaction

[0040] The two metered solutions are simultaneously fed into a tubular micro-mixing reactor. The reactor is equipped with three layers of staggered sinusoidal mixing channels with an inner diameter of 2.5 mm and a polytetrafluoroethylene coating on the surface.

[0041] The reaction temperature was controlled at 50℃, the reaction pressure at 0.4MPa, and the material residence time at 12 seconds to generate silica sol crystal nuclei with an average particle size of 4nm. Pressure and temperature sensors were installed at the reactor inlet and outlet, and the metering pump flow rate was automatically adjusted when the pressure fluctuation exceeded ±0.05MPa.

[0042] Technical Principle: The sinusoidal mixing channel continuously changes the fluid flow direction, generating high-intensity micro-vortices to achieve millisecond-level uniform mixing and avoid uneven crystal nucleus size caused by localized over-acidity or over-alkaliness. The polytetrafluoroethylene coating prevents silica sol from depositing and scaling on the channel's inner wall.

[0043] Segmented gradient heating for crystal nucleus growth regulation

[0044] The reaction solution containing crystal nuclei is continuously fed into a three-stage tubular growth reactor: the first temperature control section is 65°C with a residence time of 25 minutes; the second temperature control section is 80°C with a residence time of 35 minutes; and the third temperature control section is 92°C with a residence time of 18 minutes.

[0045] Each temperature control section is equipped with an online dynamic light scattering particle size analyzer to detect the silica sol particle size in real time. The temperature and residence time are automatically adjusted according to the detection results to obtain primary silica sol with an average particle size of 20nm.

[0046] Technical principle: Gradient heating allows for precise control of the crystal nucleus growth rate. The low-temperature stage promotes uniform crystal nucleus growth, while the high-temperature stage accelerates particle maturation, preventing abnormal particle growth and agglomeration. Online particle size monitoring enables closed-loop control of the growth process, ensuring consistent product particle size.

[0047] Continuous countercurrent ion exchange purification

[0048] Primary silica sol was continuously fed into a series of cation exchange resin columns and anion exchange resin columns at a flow rate of 2.0 L / min, and ion exchange was carried out in a countercurrent contact manner.

[0049] The resin column is equipped with a regeneration bypass, automatically switching to the standby column when the adsorption capacity reaches saturation, while simultaneously regenerating the saturated column online. The total metal ion content of the purified silica sol is below 50 ppm.

[0050] Technical principle: Countercurrent contact improves ion exchange efficiency and removes sodium sulfate and metal ions such as potassium, calcium, and magnesium generated during the reaction. These impurities affect the charge balance in the wet end of papermaking, reducing the retention and filtration effects.

[0051] Tangential flow ultrafiltration concentration and particle size fractionation

[0052] The purified silica sol was passed into a tangential flow ultrafiltration system, with the transmembrane pressure controlled at 0.3 MPa and the feed flow rate at 4 m / s. Agglomerated particles with a diameter greater than 30 nm were retained, and the silica sol was concentrated to a solid content of 20 wt%.

[0053] The concentrate reflux pipeline returns the intercepted large-diameter particles to the first temperature control section as seed crystals, with the reflux ratio controlled at 1:9 to improve the utilization rate of raw materials.

[0054] Technical principle: Tangential flow ultrafiltration can simultaneously achieve concentration and particle size classification, removing small molecule impurities and agglomerated particles to obtain narrow-distribution nano-silica sol. The concentrate reflux process increases the raw material utilization rate by more than 10%.

[0055] Online addition of composite stabilizers and precise pH control

[0056] The concentrated silica sol was fed into a static mixer, and a composite stabilizer (PVP to HEC mass ratio 2:1) was added at a rate of 0.45% of the silica sol mass using an independent metering pump.

[0057] A 10% sodium hydroxide solution was added using an automatic potentiometric titration system, and the pH value was precisely adjusted to 9.0, with titration accuracy controlled within ±0.05 pH.

[0058] Technical principle: The composite stabilizer prevents silica sol particle agglomeration through a combination of electrostatic repulsion and steric hindrance. A pH of 9.0 is the most stable range for silica sol, and it also offers the best compatibility with the wet end environment of papermaking.

[0059] High-shear homogeneous dispersion and online direct delivery

[0060] The pH-adjusted silica sol was fed into a high-shear homogenizer with a stator-rotor gap of 0.15 mm, and continuously homogenized at 16,500 rpm for 2.5 minutes.

[0061] The temperature is controlled to not exceed 40℃ by cooling water in the jacket. After homogenization, the material is directly transported to the inlet of the wire mesh box of the paper production line through a closed pipeline. The conveying temperature is 28℃ and the pressure is 0.25MPa.

[0062] Technical principle: High-shear homogenization further disperses any potential micro-agglomerates, ensuring uniform dispersion of silica sol particles. Direct online delivery avoids quality changes during storage and transportation, guaranteeing that the silica sol enters the papermaking wet end with optimal activity.

[0063] III. Product Characterization and Performance Testing

[0064] Characterization of silica sol properties

[0065] Average particle size: 20.2 nm, Particle size distribution index (PDI) = 0.12

[0066] Solid content: 20.1 wt%

[0067] pH value: 9.02

[0068] Total metal ion content: 32 ppm

[0069] Storage stability: No stratification or gelling observed after 6 months of sealed storage at room temperature.

[0070] Paper retention and filtration performance test

[0071] Pulp mix composition: 70% bleached sulfate hardwood pulp, 30% light calcium carbonate, pulp concentration 0.8%.

[0072] Additive addition sequence and dosage: Add 0.04% CPAM (octane-dry slurry) to the inlet of the slurry pump, add 0.15% modified bentonite (octane-dry slurry) to the inlet of the pressure screen, and add 0.3% nano-silica sol (octane-dry slurry) to the inlet of the screen box.

[0073] Test results: Fine fiber retention rate 87.2%, filler retention rate 73.5%, filtration rate 125 mL / 10 s, paper tensile strength 38.6 kN / m, tear strength 12.4 mN・m² / g

[0074] Application Example 1 (Verification of Segmented Gradient Heating Process)

[0075] I. Raw Materials and Methods

[0076] The raw materials are exactly the same as those in the basic embodiment.

[0077] Preparation method: Except for the crystal nucleus growth step, the remaining steps are completely consistent with the basic embodiment. The crystal nucleus growth adopts the segmented gradient heating process of this invention: the first temperature control segment is 65℃ / 25min, the second temperature control segment is 80℃ / 35min, and the third temperature control segment is 92℃ / 18min.

[0078] The testing conditions for papermaking applications are exactly the same as those for the basic implementation example.

[0079] II. Product Characterization and Performance Testing

[0080] Average particle size of silica sol: 20.5 nm, PDI = 0.13

[0081] Total metal ion content: 35 ppm

[0082] Fine fiber retention rate: 86.8%

[0083] Packing material retention rate: 72.9%

[0084] Filtration rate: 122mL / 10s

[0085] Paper tensile strength: 38.2 kN / m

[0086] Comparative Example 1 (Feature Missing: Isothermal Growth Instead of Gradient Heating)

[0087] I. Raw Materials and Methods

[0088] The raw materials are exactly the same as those in Application Example 1.

[0089] Preparation method: Except for the crystal nucleus growth step, the remaining steps are completely consistent with those in Application Example 1. The crystal nucleus growth adopts an isothermal process: 92℃ throughout, with a total residence time of 78 minutes (the same as the total residence time of gradient heating).

[0090] The testing conditions for papermaking applications are exactly the same as those in Application Example 1.

[0091] II. Product Characterization and Performance Testing

[0092] Average particle size of silica sol: 32.7 nm, PDI = 0.38

[0093] Total metal ion content: 34 ppm

[0094] Fine fiber retention rate: 72.1%

[0095] Filler retention rate: 58.3%

[0096] Filtration rate: 89mL / 10s

[0097] Paper tensile strength: 32.5 kN / m

[0098] III. Comparative Analysis

[0099] Under isothermal growth processes, silica sol particles grow rapidly at high temperatures, exhibiting significant abnormal growth, widening of particle size distribution, and a marked decrease in colloidal activity. This is because the nucleus growth rate at high temperatures far exceeds the nucleation rate, with large particles continuously engulfing smaller particles, resulting in uneven particle size. In contrast, the segmented gradient heating process of this invention, by gradually increasing the temperature, allows the nuclei to grow uniformly at low temperatures and mature slowly at high temperatures, effectively controlling the particle growth rate and yielding narrowly distributed nano-silica sol, thus exhibiting superior retention and filtration effects.

[0100] Application Example 2 (Verification of Crystal Nucleus Growth Temperature Parameters)

[0101] I. Raw Materials and Methods

[0102] The raw materials are exactly the same as those in the basic embodiment.

[0103] Preparation method: Except for the third temperature control stage of crystal nucleation growth, which is 93℃, the other steps are completely consistent with the basic embodiment. Other parameters: first temperature control stage 65℃ / 25min, second temperature control stage 80℃ / 35min, third temperature control stage 93℃ / 18min.

[0104] The testing conditions for papermaking applications are exactly the same as those for the basic implementation example.

[0105] II. Product Characterization and Performance Testing

[0106] Average particle size of silica sol: 21.8 nm, PDI = 0.14

[0107] Total metal ion content: 33 ppm

[0108] Fine fiber retention rate: 85.7%

[0109] Packing material retention rate: 71.6%

[0110] Filtration rate: 119 mL / 10 s

[0111] Paper tensile strength: 37.8 kN / m

[0112] Comparative Example 2 (Parameters out of range: third temperature control section temperature 100℃)

[0113] I. Raw Materials and Methods

[0114] The raw materials are exactly the same as those in Application Example 2.

[0115] Preparation method: Except that the temperature of the third temperature control section of crystal nucleation growth is 100℃ (exceeding the 90-95℃ range of this invention), the other steps are completely consistent with application example 2.

[0116] The testing conditions for papermaking applications are exactly the same as those in Application Example 2.

[0117] II. Product Characterization and Performance Testing

[0118] Average particle size of silica sol: 28.6 nm, PDI = 0.27

[0119] Total metal ion content: 36 ppm

[0120] Fine fiber retention rate: 76.4%

[0121] Packing material retention rate: 62.8%

[0122] Filtration rate: 97mL / 10s

[0123] Paper tensile strength: 34.1 kN / m

[0124] III. Comparative Analysis

[0125] When the temperature in the third temperature control section exceeds 95℃, the growth rate of silica sol particles accelerates dramatically, leading to some particle agglomeration, a wider particle size distribution, and reduced colloidal surface activity. This is because excessively high temperatures damage the hydration film on the surface of the silica sol particles, resulting in increased interparticle interaction forces and a tendency for agglomeration. The 90-95℃ temperature range defined in this invention is the optimal range for crystal nucleus growth, ensuring a sufficient growth rate while effectively preventing particle agglomeration and ensuring stable product quality.

[0126] Application Example 3 (Online Continuous Process Validation)

[0127] I. Raw Materials and Methods

[0128] The raw materials are exactly the same as those in the basic embodiment.

[0129] Preparation method: The entire process of online continuous reaction of the present invention is adopted, with a production cycle of 2.5 hours. The product is directly transported to the paper production line online.

[0130] The papermaking application test conditions are exactly the same as those in the basic embodiment: continuous production for 72 hours, with sampling and testing every 2 hours.

[0131] II. Product Characterization and Performance Testing

[0132] The average particle size of the silica sol fluctuates between 19.6 and 20.8 nm.

[0133] PDI fluctuation range: 0.11-0.14

[0134] Fine fiber retention rate fluctuation range: 85.9-87.5%

[0135] Packing material retention rate fluctuation range: 71.8-73.9%

[0136] Filtration rate fluctuation range: 118-126mL / 10s

[0137] Comparative Example 3 (Prior Technology: Batch Reactor Process)

[0138] I. Raw Materials and Methods

[0139] The raw materials are exactly the same as those in Application Example 3.

[0140] Preparation method: A traditional batch reactor process is used, and the specific steps are as follows:

[0141] Dilute the sodium silicate solution to 10% SiO2 by mass, add it to the reaction vessel, stir and heat to 50℃.

[0142] Slowly add dilute sulfuric acid solution, controlling the pH value between 9 and 10, over a period of 2 hours.

[0143] Heat to 90℃ and age for 6 hours.

[0144] After cooling to room temperature, the solution was purified by ion exchange and then concentrated by ultrafiltration to a solid content of 20 wt%.

[0145] Add a compound stabilizer, adjust the pH to 9.0, and store for later use.

[0146] The production cycle is 18 hours, and the product is stored for 7 days before use. The paper application test conditions are exactly the same as those in Application Example 3. Three batches are produced continuously, and samples are taken and tested once for each batch.

[0147] II. Product Characterization and Performance Testing

[0148] The average particle size of the silica sol fluctuates between 25.3 and 36.7 nm.

[0149] PDI fluctuation range: 0.32-0.45

[0150] Fine fiber retention rate fluctuation range: 68.5-79.2%

[0151] Filler retention rate fluctuation range: 54.7-65.8%

[0152] Filtration rate fluctuation range: 82-103 mL / 10 s

[0153] III. Comparative Analysis

[0154] Traditional batch reactor processes exhibit significant batch-to-batch quality variations, with an average particle size deviation reaching 11.4 nm and a PDI generally greater than 0.3. This is due to uneven material mixing and large fluctuations in temperature and concentration fields during batch reactions, leading to unstable product quality across different batches. Furthermore, the colloidal activity decreases after storage, further impacting retention and filtration efficiency. In contrast, the fully online continuous process of this invention achieves precise control of the production process, ensuring stable product quality with an average particle size deviation of less than ±1.2 nm. Online direct delivery avoids quality changes during storage, making it perfectly compatible with continuous papermaking production.

[0155] R&D Data Comparison Table

[0156] Average particle size of silica sol (nm) 20.2 20.5 32.7 21.8 28.6 19.6-20.8 25.3-36.7 Particle Size Distribution Index (PDI) 0.12 0.13 0.38 0.14 0.27 0.11-0.14 0.32-0.45 Total metal ion content (ppm) 32 35 34 33 36 31-36 45-62 Fine fiber retention rate (%) 87.2 86.8 72.1 85.7 76.4 85.9-87.5 68.5-79.2 Filler retention rate (%) 73.5 72.9 58.3 71.6 62.8 71.8-73.9 54.7-65.8 Filtration rate (mL / 10s) 125 122 89 119 97 118-126 82-103 Paper tensile strength (kN / m) 38.6 38.2 32.5 37.8 34.1 37.5-38.9 31.2-35.7 Paper tear strength (mN・m² / g) 12.4 12.1 9.8 11.9 10.5 11.8-12.5 9.2-11.3 Production cycle (h) 2.5 2.5 2.5 2.5 2.5 2.5 18 Energy consumption (kWh / ton of product) 320 325 322 328 330 315-330 480

[0157] Data Validity Statement

[0158] The particle size and distribution of silica sol were measured using a Malvern Zetasizer Nano ZS90 dynamic light scattering instrument. Each sample was tested three times, and the average value was taken.

[0159] The metal ion content was measured using inductively coupled plasma mass spectrometry (ICP-MS), with a detection limit of 0.1 ppb.

[0160] Retention rate was tested using a dynamic water filter (DDJ) at 750 rpm for 30 seconds.

[0161] The filtration rate was tested using a Schubert-Riegeler filterability meter, in accordance with the GB / T 3332-2019 standard.

[0162] The physical strength of the paper was tested according to GB / T 12914-2008 and GB / T 455-2002 standards. Ten samples were tested for each index, and the average value was taken.

[0163] Energy consumption data comes from the electricity meter readings of the actual production unit, representing the average value after 72 hours of continuous operation.

[0164] The online continuous reaction method for nano-silica sol provided by this invention, through several core technological innovations, completely solves the problems of unstable product quality, wide particle size distribution, low production efficiency, and incompatibility with continuous papermaking production inherent in traditional intermittent processes. Its core innovations are mainly reflected in the following aspects:

[0165] First, the tubular micro-mixing rapid nucleation technology achieves precise control of silica sol crystal nuclei. Through three layers of staggered sinusoidal mixing channels, high-intensity micro-vortices are generated, enabling sodium silicate and sulfuric acid to be uniformly mixed within milliseconds, producing crystal nuclei with uniform particle size of 3-5 nm. This technology ensures the uniformity of silica sol particle size from the source, avoiding the differences in crystal nuclei size caused by uneven mixing in traditional reactors.

[0166] Secondly, the segmented gradient heating nucleus growth control technology is key to achieving narrow-distribution nano-silica sol. By gradually increasing the reaction temperature through three series-connected temperature control sections, the nuclei grow uniformly in the low-temperature section and mature slowly in the high-temperature section, effectively controlling the particle growth rate and avoiding abnormal particle growth and agglomeration. The results of comparing Examples 1 and 2 show that omitting the gradient heating or exceeding the temperature range leads to a wider particle size distribution and a decrease in colloidal activity, fully demonstrating the necessity of this technical feature and the rationality of the parameter range.

[0167] Third, the continuous production process enables stable control of product quality and a significant increase in production efficiency. The combination of online ion exchange and tangential flow ultrafiltration not only effectively removes impurity ions but also achieves the concentration and particle size classification of silica sol, with the total metal ion content below 50 ppm. The concentrate reflux process reuses the retained large-diameter particles as seed crystals, improving raw material utilization. The production cycle is shortened from 12-24 hours in the traditional process to 2-3 hours, production efficiency is increased by 4-6 times, and energy consumption is reduced by more than 30%.

[0168] Fourth, the online direct delivery technology completely avoids quality changes during storage and transportation. Online addition of composite stabilizers and precise pH control significantly improve the storage stability of silica sol. High-shear homogeneous dispersion further ensures uniform particle dispersion. After product preparation, it is directly transported to the wet end of papermaking through a closed pipeline, maintaining optimal surface activity at all times.

[0169] In papermaking applications, the ternary retention and filtration aid system composed of the nano-silica sol prepared in this invention, cationic polyacrylamide, and modified bentonite exhibits excellent synergistic effects. The retention rate of fine fibers is increased to over 85%, the filler retention rate to over 70%, and the filtration speed is increased by 20-30%, while effectively improving the physical strength of the paper. Compared with traditional batch processes, product quality fluctuations are significantly reduced, and it can perfectly adapt to high-speed paper machines with speeds of 800-1500 m / min, significantly improving the operating efficiency of the paper machine and the consistency of paper quality.

[0170] In summary, this invention achieves continuous and intelligent production of nano-silica sol through technological innovation, resulting in stable product quality, excellent performance, and significant economic and social benefits. It is suitable for large-scale application in the modern papermaking industry.

[0171] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0172] Finally, the following points should be noted: First, in the description of this application, it should be noted that, unless otherwise specified and limited, the terms "installation", "connection", and "linkage" should be interpreted broadly, and can refer to mechanical or electrical connections, or internal connections between two components, or direct connections. "Up", "down", "left", "right", etc., are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may change.

[0173] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for online continuous reaction of nano-silica sol, characterized in that: The method is as follows: Step 1: Raw material pretreatment and precise metering and conveying Industrial-grade sodium silicate solution is filtered through a plate and frame filter press to remove insoluble impurities, resulting in a clarified sodium silicate solution with a silica mass fraction of 8-12% and a modulus of 3.2-3.

4. 98% industrial concentrated sulfuric acid is diluted with deionized water at a volume ratio of 1:5 to obtain a dilute sulfuric acid solution. The clarified sodium silicate solution and the dilute sulfuric acid solution are then separately fed into high-precision diaphragm metering pumps, delivered at a constant flow rate according to a silica to sulfuric acid molar ratio of 1:0.85-0.95, with metering accuracy controlled within ±0.2%. Step 2: Rapid nucleation reaction using tubular micromixing The metered sodium silicate solution and dilute sulfuric acid solution are simultaneously introduced into a tubular micro-mixing reactor. The reactor is equipped with three layers of staggered sinusoidal mixing channels with an inner diameter of 2-3 mm. The reaction temperature is controlled at 45-55℃, the reaction pressure at 0.3-0.5 MPa, and the residence time of the materials in the reactor is 10-15 seconds. Through high-intensity micro-mixing, an instantaneous and uniform neutralization reaction is achieved, generating silica sol crystal nuclei with uniform particle size. The average particle size of the crystal nuclei is controlled at 3-5 nm. Step 3: Segmented gradient temperature control for crystal nucleus growth The reaction solution containing crystal nuclei is continuously fed into a segmented tubular growth reactor, which is divided into three temperature-controlled sections connected in series: the first temperature-controlled section has a temperature of 60-70℃ and a residence time of 20-30 minutes; the second temperature-controlled section has a temperature of 75-85℃ and a residence time of 30-40 minutes; and the third temperature-controlled section has a temperature of 90-95℃ and a residence time of 15-20 minutes. By controlling the growth rate of crystal nuclei through gradient heating, abnormal particle growth is avoided, and primary silica sol with an average particle size of 15-25 nm is obtained. Step 4: Continuous countercurrent ion exchange purification The primary silica sol was continuously fed into a series of cation exchange resin columns and anion exchange resin columns at a flow rate of 1.5-2.5 L / min, and ion exchange was carried out in a countercurrent contact manner. The cation exchange resin was a strongly acidic styrene-based cation exchange resin, and the anion exchange resin was a weakly basic acrylic-based anion exchange resin. The sodium sulfate and metal ion impurities such as potassium, calcium, and magnesium generated in the reaction were removed, and the total metal ion content in the purified silica sol was controlled to be less than 50 ppm. Step 5: Tangential flow ultrafiltration concentration and particle size fractionation The purified silica sol was passed into a tangential flow ultrafiltration system. A polyethersulfone ultrafiltration membrane with a molecular weight cutoff of 50 kDa was selected, and the transmembrane pressure was controlled at 0.2-0.4 MPa and the feed flow rate was 3-5 m / s. Small molecule impurities and excess water in the system were removed by ultrafiltration, while agglomerated particles with a particle size greater than 30 nm were retained. The silica sol was concentrated to a solid content of 18-22 wt%, resulting in narrow-distribution nano-silica sol with a particle size distribution index (PDI) < 0.

15. Step 6: Online addition of composite stabilizer and precise pH control The concentrated silica sol is passed into a static mixer, and a composite stabilizer is continuously added to the mixer using a metering pump. The amount of composite stabilizer added is 0.3-0.6% of the silica sol mass. After mixing evenly, a 10% sodium hydroxide solution is added dropwise to the silica sol using an automatic potentiometric titration system to precisely adjust the pH value of the silica sol to 8.8-9.2, with the titration accuracy controlled within ±0.05 pH. Step 7: High-shear homogeneous dispersion and online direct delivery The pH-adjusted silica sol is fed into a high-shear homogenizer and continuously homogenized at a speed of 15,000-18,000 rpm for 2-3 minutes. After homogenization, the nano-silica sol is directly transported to the wet end addition point of the paper production line through a closed pipeline. During the transport process, the temperature is maintained at 25-30℃ and the pressure is maintained at 0.2-0.3MPa.

2. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: The mixing channel surface of the tubular micro-mixing reactor described in step 2 is coated with polytetrafluoroethylene. Pressure sensors and temperature sensors are installed at the reactor inlet and outlet to monitor the pressure and temperature changes during the reaction process in real time. When the pressure fluctuation exceeds ±0.05MPa, the delivery flow rate of the metering pump is automatically adjusted.

3. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: In step 3, each temperature control section of the segmented tubular growth reactor is equipped with an online dynamic light scattering particle size analyzer to detect the average particle size and particle size distribution of the silica sol in real time, and automatically adjust the temperature and material residence time of the corresponding temperature control section based on the detection results.

4. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: In step 4, both the cation exchange resin column and the anion exchange resin column are equipped with regeneration bypasses. When the adsorption capacity of the ion exchange resin reaches saturation, the system automatically switches to the standby resin column for operation, while simultaneously regenerating the saturated resin column online, thus achieving continuous and uninterrupted operation of the ion exchange process.

5. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: The tangential flow ultrafiltration system described in step 5 is equipped with a concentrate reflux pipeline, which returns the intercepted large-diameter particles to the first temperature control section in step 3, so that they can participate as seed crystals in the subsequent crystal nucleus growth process, thereby improving the utilization rate of raw materials. The reflux ratio is controlled at 1:8-1:

10.

6. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: The composite stabilizer mentioned in step 6 consists of polyvinylpyrrolidone and hydroxyethyl cellulose in a mass ratio of 2:

1. The two stabilizers are added separately through independent metering pumps, and the addition ratio can be dynamically adjusted according to the pulp properties of the papermaking production line.

7. The method for online continuous reaction of nano-silica sol according to claim 1, characterized in that: In step 7, the stator and rotor gap of the high-shear homogenizer is 0.1-0.2 mm. During the homogenization process, the temperature of the silica sol is controlled to not exceed 40°C by cooling water in the jacket to prevent the silica sol particles from agglomerating due to high temperature.

8. The application of the online continuous reaction method for nano-silica sol according to claims 1-7 in the multi-element retention and filtration aid in papermaking, characterized in that: The application is as follows: A ternary retention and filtration aid system is formed by combining online-prepared nano-silica sol, cationic polyacrylamide, and modified bentonite, and applied to the production process of cultural paper. First, cationic polyacrylamide is added to the inlet of the pulp pump of the paper machine at a dosage of 0.03-0.05% of the oven-dry pulp mass. After thorough mixing, modified bentonite is added to the inlet of the pressure screen at a dosage of 0.1-0.2% of the oven-dry pulp mass. Finally, online-prepared nano-silica sol is added to the inlet of the wire mesh box at a dosage of 0.2-0.4% of the oven-dry pulp mass. The nano-silica sol forms flocs with the fine fibers and fillers adsorbed by cationic polyacrylamide through electrostatic adsorption. Simultaneously, bentonite is interspersed between the flocs, forming a dense and relatively strong network structure, improving the retention rate of fine fibers and fillers, and accelerating the filtration speed of the pulp. This application is suitable for machine speeds of 800-1500 m / min. The high-speed cultural paper machine can effectively reduce white water concentration, reduce fiber loss, and improve the operating efficiency of the paper machine.

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