Method for concentrating silica sol through ceramic membrane

Abstract

The invention discloses a method for concentrating silica sol through a ceramic membrane, and belongs to the technical field of ceramic membrane application. According to the method, a low-concentration silica sol raw material solution is pumped into a ceramic membrane assembly for cross-flow circulating filtration, water molecules and the like penetrate through a membrane to form a penetrating fluid, intercepted silicon dioxide particles form a concentrated solution, and the concentrated solution circularly flows back, so that the concentration of the raw material solution is continuously increased. In order to guarantee the quality of the final product, the system controls the temperature of the feed liquid in a circulating cooling water mode, and silica gel particles are prevented from growing up due to too high temperature in the operation process. The invention also designs a pulse-type gas-liquid back-flushing mechanism, and a mixed fluid formed by penetrating fluid and gas is periodically utilized to carry out reverse cleaning from the penetrating fluid side of the ceramic membrane, so that a pollution layer can be effectively stripped, and the filtration performance of the membrane is quickly recovered. The problems that a traditional concentration method is high in energy consumption and the stability of an organic film is poor are solved, high-magnification concentration of the silica sol can be achieved at the normal temperature or the low temperature with low energy consumption, and meanwhile the product quality is guaranteed.

Description

Technical Field

[0001] This invention relates to a method for concentrating silica sol using a ceramic membrane, belonging to the field of ceramic membrane application technology. Background Technology

[0002] Silica sol is a water / solvent-based stable colloidal system with nano-sized silica particles (5-150 nm in diameter) as the dispersed phase. Due to its colloidal stability and biocompatibility, silica sol shows significant application potential in precision coatings and composite materials. Industrial application data shows that concentrated silica sol has significant advantages in adhesion, strength, and coating performance, better meeting market demands. The main method for silica sol concentration is vacuum distillation dehydration. While this method achieves concentration, it suffers from high energy consumption, low thermal efficiency, long production cycles, and equipment corrosion. Furthermore, although organic ultrafiltration membranes reduce energy consumption to some extent, they face bottlenecks such as poor material thermal stability, insufficient chemical resistance, and rapid membrane flux decay. Summary of the Invention

[0003] The technical solution of this invention is used in the concentration process of silica sol. The ceramic membrane technology has the characteristics of high efficiency, stability and high durability. It uses an alumina-based gradient ceramic membrane in conjunction with a pulse backwashing process to achieve a breakthrough in concentration efficiency and control of product quality.

[0004] A method for concentrating silica sol using a ceramic membrane includes the following steps:

[0005] a) The silica sol raw material solution to be concentrated is pumped into a ceramic membrane module for cross-flow circulation filtration, so that water molecules and small molecule solvents permeate through the ceramic membrane to form a permeate, while silica particles are retained to form a concentrate, and the concentrate is circulated back to the raw material solution.

[0006] b) During the circulating filtration process, the temperature of the raw material liquid is controlled to prevent silica particles from growing or agglomerating due to excessive temperature.

[0007] c) Periodically pause the cross-flow circulation filtration and introduce a mixed flow of gas and permeate from the permeate side of the ceramic membrane to the feed liquid side to perform pulse backwashing on the ceramic membrane in order to restore its filtration performance.

[0008] In step b), the temperature of the raw material liquid is controlled within the range of 25-45°C.

[0009] In step a), the transmembrane pressure difference of the cross-flow circulation filtration is controlled at 0.1-0.5 MPa, and the tangential flow velocity of the feed liquid on the membrane surface is 2.0-6.0 m / s; the filtration adopts constant pressure filtration or constant flow filtration.

[0010] The ceramic membrane is an alumina-based ceramic membrane, and the pore size of its separation layer is 20-1000 nm; preferably 20-200 nm.

[0011] The periodic backwashing in step c) is based on a preset time interval or is automatically triggered, wherein the preset time interval is 3-50 minutes.

[0012] In step c), the duration of each backwash is 15-60 seconds, and the backwash pressure is 0.2-0.5 MPa.

[0013] In step c), when constant flow filtration is used, the time T for backwashing after each filtration is specified. backwash It is obtained by calculation using the following formula:

[0014]

[0015] k is the coefficient to be fitted, α is the parameter to be fitted, and d p It is the particle size of silica sol, d m C is the average pore size of the ceramic membrane. t It refers to the concentration of silica sol in the concentrate; R b It is the congestion rate;

[0016]

[0017] P 当前 This is the measurement of the transmembrane pressure difference, ΔP, under current conditions. 初始 The transmembrane pressure difference, ΔP, is measured at the initial stage of filtration. 极限 It refers to the transmembrane pressure difference when the flux reaches a stable level during the first concentration and filtration without backwashing.

[0018] C t It is obtained by converting the turbidity measured using a turbidity meter.

[0019] It also includes step d): monitoring the concentration of the raw material liquid in real time through an online detection unit, and stopping the concentration process when the concentration reaches a preset value.

[0020] The beneficial effects of this invention are:

[0021] 1. By using circulating water, the growth of silica particles caused by excessively high temperatures during operation can be avoided, thus affecting the quality of the final product; 2. Through a recycling architecture, a permeate storage tank and an online monitoring unit are set up, which automatically switches to the membrane module backwashing pipeline when a predicted value is detected.

[0022] 3. A pulse-type gas-liquid backwash mechanism is designed to clean the ceramic membrane using a mixed flow of permeate and gas. After cleaning, the filtration performance can be quickly restored, reducing downtime and maintenance costs. Attached Figure Description

[0023] Figure 1 The process flow diagram of this invention.

[0024] Figure 2 Graphs of silica sol and permeate at different time points in the experiment.

[0025] Figure 3 Particle size distribution diagram from the experiment. Detailed Implementation

[0026] like Figure 1 This invention provides a silica sol concentration system based on a ceramic membrane, the workflow of which is as follows: A low-concentration silica sol raw material solution to be processed is stored in a raw material tank. Upon startup, a circulation pump pumps the raw material solution from the raw material tank, which then enters a modular ceramic membrane module through pipelines. Driven by the pressure provided by the circulation pump, the raw material solution forms a high-speed cross-flow on the membrane surface of the ceramic membrane module. During this process, water molecules and other small molecule solvents can permeate through the micropores of the ceramic membrane, forming permeate, which is collected in a permeate storage tank through a permeate pipeline. Larger silica particles are retained by the ceramic membrane, forming a concentrate. This concentrate is returned to the raw material tank through a concentrate circulation pipeline, mixing with the liquid in the tank, thereby causing the concentration of silica sol in the raw material tank to continuously increase over time. The system is equipped with a temperature control system, typically a jacketed or external heat exchanger, which precisely controls the liquid temperature in the raw material tank and circulation pipelines through circulating cooling water. This prevents excessively high temperatures due to heat accumulation caused by circulation shear and pump generation, avoiding silica particle aggregation or growth, and ensuring the particle size distribution and stability of the final product. As the concentration process proceeds, concentration polarization and membrane fouling occur on the membrane surface due to the deposition of silica particles, leading to a decrease in membrane flux. By incorporating a pulsed gas-liquid backwash mechanism, the permeate in the permeate storage tank is used as backwash water. The system automatically triggers the backwash process according to a preset program (e.g., based on time or an increase in transmembrane pressure difference). During backwashing, the system briefly stops forward filtration. The backwash pump pressurizes the permeate in the permeate storage tank and mixes it with compressed gas (such as clean air or nitrogen) from a gas source, forming a gas-liquid mixture that flows back from the permeate side of the ceramic membrane towards the membrane surface. This pulsed impact effectively and quickly strips and washes away the fouling layer adhering to the membrane surface, allowing for rapid recovery of membrane performance. The backwash wastewater is discharged through a drain pipe. The entire concentration process continues until the silica sol concentration in the feed tank reaches the preset target value. An online detection unit, such as a concentration meter or refractometer, can be used for real-time concentration monitoring and automated control. Through the above-described "filtration-concentration-backflushing-recovery" cycle, the present invention can achieve high-rate concentration of silica sol at room temperature or low temperature with low energy consumption, while ensuring product quality.

[0027] Example 1

[0028] In this embodiment, 200L of industrial-grade alkaline silica sol was used as the feed solution, with an initial solid content of 1.96wt%, an initial average particle size of 22nm, and a pH of 9.5. The ceramic membrane module used in the concentration system was an Al2O3 / ZrO2 composite membrane, with a separation layer pore size of 50nm and a total effective membrane area of ​​2.5m². 2 .

[0029] After the experiment began, the system operated in a cross-flow full-circulation mode. The transmembrane pressure difference within the membrane module was maintained at 0.25 MPa by adjusting the circulation pump, and the tangential flow velocity of the feed liquid on the membrane surface was 4.0 m / s. To prevent localized overheating that could lead to silica particle growth, the feed liquid temperature was kept constant at 35 ± 2℃ throughout the concentration process using jacket cooling water. To effectively control membrane fouling and maintain stable separation efficiency, the system was programmed with a periodic online backwashing procedure. Specifically, a backwash was automatically initiated after every 20 minutes of continuous system operation, using system-generated permeate and clean compressed air as the backwashing media. Each backwash lasted 30 seconds, with the backwash pressure set at 0.3 MPa, utilizing the strong shear force of the gas-liquid pulse to clean the membrane surface.

[0030] After 6.5 hours of continuous concentration, the SiO2 solid content increased to 13.3 wt%, achieving a volumetric concentration of approximately 6.8 times. Throughout the experiment, the system exhibited excellent stability, with the average membrane flux remaining stable at 75 L / (m²). 2 The flux recovery rate was approximately 95% after each backwash (approximately h), and the final product had an average particle size of 23 nm. Figure 3 This is a particle size distribution diagram. As can be seen from the diagram, the final product did not experience particle enlargement during operation and maintained a narrow particle size distribution. Meanwhile, Figure 2 The images show the silica sol and permeate at different time points in the experiment. The permeate is becoming clearer, indicating that the concentration of the concentrate is increasing. The concentration of the target substance in the concentrate is high enough to make it more suitable for recycling. The SiO2 content is less than 0.01 wt%, indicating that the ceramic membrane has a silica rejection rate of over 99.8%.

[0031] Example 2

[0032] This embodiment illustrates the automatic backwashing control program design in this patent. The program design is based on the following considerations: the degree of matching between the particle size of the silica sol and the pore size of the ceramic membrane is closely related to the pore blockage. If the particle size is too small, it is easier to block the membrane pores; if the particle size is too large, it will mainly accumulate on the surface of the membrane and is easily washed away by cross-flow. Therefore, a value D representing the average pore size of the ceramic membrane is set. 平均孔径 and silica sol particle size D颗粒 The parameter is used to measure the relationship between the average pore size of the ceramic membrane and the size of the silica sol particles. Additionally, since the turbidity of the feed solution can be quantified using an online turbidity meter during the concentration process, and by establishing the relationship between turbidity NTU and silica sol concentration in the feed solution, the turbidity of the feed solution can be determined in real time using an online turbidity meter. This allows for the real-time acquisition of the feed solution concentration during the silica sol concentration process. The higher the feed solution concentration, the shorter the backwashing cycle Δt needs to be; therefore, the following relationship exists:

[0033]

[0034] Where k is the coefficient to be fitted, d p It is the particle size of silica sol, d m C is the average pore size of the ceramic membrane. t It refers to the concentration of silica sol in the concentrate;

[0035] For methods to determine the concentration of silica sol, please refer to the literature: Khlebtsov & Khanadeev, Determination of the Size, Concentration, and Refractive Index of Silica Nanoparticles from Turbidity Spectra (Langmuir, 2008). This method involves online real-time measurement and calculation using turbidity. The principle of this method is based on the single-scattering approximation theory, i.e., turbidity (τ) is related to the particle number concentration (N) and its scattering cross section (C). sca It is directly proportional to τ, and the relationship is τ=N·C sca Therefore, the concentration can be expressed by the formula N = τ / C. sca The calculations are as follows. In practice, the turbidity spectrum of the silica sol in a specific wavelength range (e.g., 400-600 nm) is first measured online using a spectrophotometer, and the "wavelength index" (w) related to the particle size is calculated from this. Based on this index, the average particle size can be determined using a theoretical calibration curve, and thus its scattering cross section (C) can be obtained. sca Finally, the concentration of silica sol can be calculated in real time by substituting the measured turbidity value into the above formula.

[0036] In addition, considering the blockage of the membrane surface by silica sol, this patent constructs a blockage rate R. b The correction factor is adjusted so that if the surface clogging rate is high, the backwashing cycle needs to be reduced. The clogging rate R b This can be calculated by measuring the transmembrane pressure difference. From this, the calculation process for the backwash interval can be derived, resulting in:

[0037]

[0038] Where α is the parameter to be fitted, R b It's the congestion rate.

[0039] R b In constant flow filtration mode, the clogging rate can be measured by the change in transmembrane pressure difference.

[0040]

[0041] ΔP 当前 This is the measurement of the transmembrane pressure difference, ΔP, under current conditions. 初始 The transmembrane pressure difference, ΔP, is measured at the initial stage of filtration. 极限 This refers to the transmembrane pressure difference when the flux reaches a stable level during the first concentration and filtration without backwashing (indicating that all membrane pores are blocked at this point).

[0042] Based on the above method, during the concentration process, ΔP is first measured. 极限 Then, the solution is concentrated for a certain period of time (15 minutes in the following examples), and the theoretical backwash interval is calculated. When this time is reached, a backwash is performed (excluding the time already spent on filtration). After the backwash, concentration is continued for a certain period of time (15 minutes in the examples as well). Under different conditions, the solution is run for 180 minutes, and the total number of backwashes is counted (a lower total number of backwashes represents a lower amount of permeate reuse).

[0043]

[0044] As can be seen from the table, when using the general fixed-interval backwashing strategy, a total of 8 backwashes are required. However, when using the adaptive backwashing strategy, the number of backwashes is reduced by automatically identifying parameters in the concentration process, thus saving permeate usage. In addition, by using membrane fouling rate correction, potential membrane fouling situations are further identified, and the backwashing cycle is adjusted in a timely manner, further reducing the number of backwashes.

Claims

1. A method for concentrating a silica sol by ceramic membrane, characterized by, The method comprises the following steps: a) pumping the raw liquid of the silica sol to be concentrated into a ceramic membrane module for cross-flow circulation filtration, so that water molecules and small molecular solvents permeate through the ceramic membrane to form a permeate, while silica particles are retained to form a concentrated liquid, and the concentrated liquid is circulated back to the raw liquid; b) during the circulation filtration, the temperature of the raw liquid is controlled to prevent the silica particles from growing or agglomerating due to excessive temperature; c) periodically pausing the cross-flow circulation filtration, and introducing a mixed flow of gas and permeate from the permeate side to the raw liquid side of the ceramic membrane to pulse backwash the ceramic membrane to restore its filtration performance.

2. The method of claim 1, wherein, In step b), the temperature of the raw liquid is controlled in the range of 25-45℃.

3. The method of claim 1, wherein, In step a), the transmembrane pressure difference of the cross-flow circulation filtration is controlled in the range of 0.1-0.5 MPa, and the tangential flow velocity of the raw liquid on the membrane surface is 2.0-6.0 m / s; the filtration adopts constant pressure filtration or constant flow filtration.

4. The method of claim 1, wherein, The ceramic membrane is an alumina-based ceramic membrane, and the pore size of the separation layer is 20-1000 nm; preferably 20-200 nm.

5. The method of claim 1, wherein, The periodic backwashing in step c) is based on a preset time interval or automatic triggering, wherein the preset time interval is 3-50 minutes.

6. The method of claim 1, wherein, In step c), the duration of each backwashing is 15-60 seconds, and the backwashing pressure is 0.2-0.5 MPa.

7. The method of claim 5, wherein, In step c), the time T of backwashing after each filtration is performed again under constant current filtration backwash is calculated by the following formula: k is a coefficient to be fitted, a is a parameter to be fitted, d p is the silica sol particle size, d m is the ceramic membrane average pore size, C t is the silica sol concentration in the concentrated solution; R b is the clogging rate; P 当前 is the measured transmembrane pressure difference, ΔP 初始 is the measured transmembrane pressure difference, ΔP 极限 refers to the transmembrane pressure difference at the time when the flux reaches a steady state for the first time without backflushing; the T backwash value is calculated based on the monitoring parameters after a set time of operation during concentration filtration; backflushing is initiated after the set time of operation has been deducted.

8. The method of claim 7, wherein, C t is obtained by converting the turbidity measured by a turbidimeter.

9. The method of claim 1, wherein, Further comprising step d): monitoring the concentration of the raw liquid in real time by an online detection unit, and stopping the concentration process when the concentration reaches a preset value.

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