A high-efficiency flocculation separation method for dissolved colloids in papermaking white water

Abstract

This invention discloses a highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water. The method includes adjusting the pH of the white water, adding a divalent cationic salt to compress the double electric layer, adding an amphoteric starch graft copolymer to form initial flocs, adding nano-silica sol for loading and densification, adding high molecular weight polyethylene oxide for secondary bridging, and finally separating the flocs using a stepped hydrocyclone. This invention significantly improves floc density and shear resistance through multi-step synergy of divalent salt destabilization, amphoteric starch bridging, nano-silica densification, PEO re-bridging, and stepped hydrocyclone separation, solving the problems of difficulty in achieving both destabilization efficiency and floc density, and incomplete solid-liquid separation in existing flocculation methods.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology in the papermaking industry, and in particular to a highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water. Background Technology

[0002] During the recycling of white water in papermaking, dissolved and colloidal substances, excluding pulp fibers and fillers, continuously accumulate. These substances mainly include hemicellulose, lignin derivatives, resin acids, fatty acids, starch, and various papermaking additive residues. Currently, common methods for removing dissolved colloidal substances from white water include chemical flocculation, flotation, and membrane separation. Chemical flocculation is widely used due to its simplicity and relatively low cost. Commonly used flocculants include inorganic coagulants such as polyaluminum chloride and polyferric sulfate, and organic flocculants such as polyacrylamide and polydimethyldiallyl ammonium chloride. In actual production, inorganic salts and polymeric flocculants are often combined to promote the destabilization of colloids and the formation of flocs through electrostatic neutralization and bridging, followed by solid-liquid separation through sedimentation or flotation.

[0003] First, while conventional inorganic coagulants such as aluminum and iron salts have good neutralizing ability for negatively charged colloids, papermaking white water often contains high concentrations of anionic waste and metal ion complexes. The dosage of inorganic coagulants needs to be significantly increased to achieve destabilization, which not only leads to increased chemical sludge production but also easily causes an increase in the system's conductivity, thus inhibiting subsequent flocculation. Second, when using cationic polyacrylamide and other polymeric flocculants alone, although the bridging effect is significant, the resulting flocs are soft, have high water content, and slow settling speed, making them prone to breakage under hydraulic shear, resulting in low separation efficiency. Increasing the dosage of inorganic particles or fillers to improve floc density increases reagent costs and sludge volume. Third, traditional solid-liquid separation is usually performed after flocculation using ordinary sedimentation tanks or vortex flotation. For organic colloidal flocs with densities close to water, the separation effect is not ideal, and the residual micro-flocs in the overflow still lead to high turbidity in the white water, limiting the high-proportion recycling of white water. Fourth, existing technologies do not consider the multi-scale structural regulation of floc formation. Most of them mix destabilization, coagulation, densification and bridging processes, causing mutual interference between agents and making it difficult to achieve orderly growth of flocs from the nanoscale to the microscale.

[0004] Therefore, in response to the problems mentioned above, this invention proposes a highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water. Summary of the Invention

[0005] In order to overcome the problems of difficulty in achieving both colloid destabilization efficiency and floc compactness, weak floc shear resistance, and incomplete solid-liquid separation in existing flocculation methods, this invention proposes a highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water.

[0006] The technical solution of this invention is: a highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water, comprising the following steps: S1. Adjust the pH of the papermaking white water to 6.0-7.0, then add divalent cation salts to a divalent cation concentration of 2-10 mmol / L, stir for 1-2 minutes, and compress the colloidal double layer; S2, add an amphoteric starch graft copolymer to liquid S1. The amphoteric starch graft copolymer has a cationic substitution degree of 0.2-0.4, an anionic substitution degree of 0.1-0.3, a molecular weight of 200,000-500,000, and an addition amount of 10-50 mg / L. Stir the reaction for 5-10 minutes to form initial flocs. S3, add nano-silica sol to liquid S2. The nano-silica has a particle size of 10-30nm and an addition amount of 5-20mg / L. Continue stirring for 2-3 minutes to load the nano-silica onto the surface of the initial flocs and increase the floc density. S4. Add high molecular weight polyethylene oxide (3-8 million) to the mixture obtained in S3 at a concentration of 0.1-1 mg / L. Stir slowly for 2-5 minutes to form large and dense flocs. S5, the white water containing flocs obtained in S4 is separated by a stepped hydrocyclone. The hydrocyclone has at least two column sections with different diameters to enhance the separation of flocs and water. The flocs are discharged from the underflow, and the purified white water is obtained from the overflow. S6, mechanically dewater the flocs discharged from the underflow in step S5 to obtain dewatered sludge and filtrate. The filtrate is returned to step S1 to be mixed with papermaking white water for treatment.

[0007] Preferably, the divalent cation salt is one or more of calcium chloride, magnesium chloride, calcium sulfate, or magnesium sulfate; in step S1, the pH is adjusted using sulfuric acid, hydrochloric acid, sodium hydroxide, or sodium carbonate, and the conductivity of the adjusted white water is controlled at 1000-3000 μS / cm; in step S2, the stirring speed is 200-400 rpm, in step S3, the stirring speed is 100-200 rpm, and in step S4, the stirring speed is 30-60 rpm. When calcium chloride or magnesium chloride is used as a divalent cation salt, calcium and magnesium ions have strong ability to compress the electric double layer and are inexpensive and readily available. They can undergo moderate cross-linking with the anionic groups in the subsequent amphoteric starch without affecting floc formation. Controlling the conductivity between 1000-3000 μS / cm can avoid excessive conductivity from inhibiting the polymer bridging effect, while ensuring sufficient destabilization of the colloid. Setting the stirring speed in stages, first quickly mixing and destabilizing, then medium-speed loading and densifying, and finally slow bridging and growth, is conducive to forming a dense and shear-resistant floc structure.

[0008] Preferably, the amphoteric starch graft copolymer is prepared by the following method: starch is grafted copolymerized with a cationic etherifying agent and an anionic graft monomer in the presence of an initiator, wherein the cationic etherifying agent is 3-chloro-2-hydroxypropyltrimethylammonium chloride or 2,3-epoxypropyltrimethylammonium chloride, and the anionic graft monomer is acrylic acid, methacrylic acid or 2-acrylamido-2-methylpropanesulfonic acid; The graft copolymerization method ensures that cationic and anionic groups are evenly distributed on the starch molecular chain, which not only guarantees a strong cationic charge density to neutralize the negative charge of the colloid, but also achieves a controllable loading effect through the electrostatic matching effect between the anionic groups and the subsequent nano-silica, avoiding excessive aggregation, and improving the stability of the flocs over a wide pH range.

[0009] Preferably, the nano-silica sol has a pH of 8.0-10.0 and its surface is negatively charged; the first column of the stepped hydrocyclone has a diameter of 50-150 mm, the second column has a diameter of 20-80 mm, and the cone angle is 10-30°. Among them, negatively charged nano-silica is stable under alkaline conditions and can be uniformly adsorbed onto the cationic sites of amphoteric starch through electrostatic action, which greatly improves the apparent density of flocs; the stepped hydrocyclone generates two-stage centrifugal force fields through column sections of different diameters, first coarse separation and then fine separation, which can achieve a collection efficiency of more than 90% for dense flocs, and the underflow concentration ratio is high, which is beneficial to subsequent dewatering.

[0010] Preferably, the high molecular weight polyethylene oxide has a molecular weight of 5-6 million and is added in an amount of 0.3-0.8 mg / L; the papermaking white water is the white water from the wire mesh or the white water from the press generated during the production of household paper, and its concentration of dissolved colloidal substances is 200-1000 mg / L in terms of COD, and its turbidity range is 50-300 NTU. The present invention uses ultra-high molecular weight polyethylene oxide, which can produce a significant bridging effect at extremely low dosage (<1mg / L), thereby avoiding the excessive polymer causing the floc surface to become oversaturated and thus re-stabilizing. Furthermore, the method can adapt to the white water characteristics of different paper types, and the turbidity of the treated white water can be reduced to below 10 NTU, with a COD removal rate of 70-85%.

[0011] The beneficial effects of this invention are: 1. This invention achieves highly efficient destabilization of colloids with extremely low metal salt dosage by synergistically combining the compression of the double electric layer by divalent cation salts with electrostatic neutralization by amphoteric starch graft copolymers. This avoids the problems of increased conductivity and sludge volume caused by the large addition of traditional coagulants such as aluminum salts, thus solving the problem of difficulty in achieving both destabilization efficiency and floc compaction.

[0012] 2. This invention introduces negatively charged nano-silica sol after the initial floc formation, and loads high-density inorganic particles onto the floc surface through electrostatic adsorption and hydrogen bonding, increasing the apparent density of the floc from 1.05 g / cm³. 3 The following values ​​were increased to 1.15-1.25 g / cm³. 3 Meanwhile, by utilizing the secondary bridging effect of ultra-high molecular weight polyethylene oxide, the size of the flocs is increased to 500-1000μm, which greatly enhances the shear resistance and settling performance of the flocs and overcomes the shortcomings of traditional flocs being soft and brittle.

[0013] 3. This invention uses a stepped hydrocyclone to replace ordinary sedimentation or flotation devices. It utilizes two-stage centrifugal force fields generated by column sections of different diameters to sequentially capture large-sized dense flocs and fine flocs. Combined with the high separation efficiency after floc densification, the turbidity of the purified white water is reduced to below 10 NTU and the COD removal rate reaches more than 80%. This solves the problems of incomplete solid-liquid separation and residual fine flocs in the overflow of existing technologies, providing a reliable guarantee for the high-proportion recycling of papermaking white water. Attached Figure Description

[0014] Figure 1 The diagram shown is a schematic representation of the process flow of this invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 some embodiments of the present invention, but 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.

[0016] This invention provides an embodiment: This example first prepares an amphoteric starch graft copolymer, specifically: 100g of industrial-grade corn starch was dispersed in 300mL of deionized water and gelatinized at 85℃ for 30 minutes, then cooled to 60℃. Cationic etherifying agent 3-chloro-2-hydroxypropyltrimethylammonium chloride (15% of starch mass) and anionic graft monomer acrylic acid (10% of starch mass) were added, along with ammonium persulfate initiator (0.5% of starch mass). The reaction was carried out at 60℃ for 4 hours under nitrogen protection. After the reaction, the product was precipitated with ethanol, washed, and vacuum dried to obtain a white powder. Colloidal titration showed that the cationic degree of substitution was 0.32, the anionic degree of substitution was 0.18, and the molecular weight was approximately 350 kDa, determined by viscosity analysis.

[0017] The graft copolymerization method in this example ensures that cationic and anionic groups are evenly distributed along the starch molecular chain. This not only guarantees a strong cationic charge density to neutralize the negative charge of the colloid, but also achieves a controllable loading effect through the electrostatic matching effect between the anionic groups and the subsequent nano-silica, avoiding excessive aggregation and improving the stability of the flocs over a wide pH range.

[0018] In this embodiment, bleached sulfate softwood pulp and hardwood pulp are used as raw materials, and the papermaking quantity is 18 g / m³. 2 The sample was taken from household paper (facial tissue). The white water sampling point was located in the white water tank behind the absorbent box in the mesh section. The water sample was collected directly and stored at 4℃, with the experiment completed within 24 hours. The dissolved colloid concentration (COD) of the white water was measured to be 320 mg / L, turbidity 85 NTU, pH 6.8, conductivity 1100 μS / cm, and total suspended solids (TSS) 120 mg / L. The COD contribution from dissolved colloids (the portion that can pass through a 0.45 μm filter membrane) was approximately 260 mg / L, indicating that colloids are the main source of COD in the white water. The calcium ion content in the white water was 45 mg / L, magnesium ion content was 8 mg / L, and anionic waste charge demand was 0.28 meq / L, showing moderate anionic colloidal stability. This white water exhibits the typical characteristics of household paper white water: low turbidity, low COD, and low conductivity. The colloids are mainly hemicellulose, fine fibers, and a small amount of resin acids.

[0019] Please see Figure 1 The present invention provides Embodiment 1: S1. Take 5L of the above-mentioned papermaking white water and place it in a 10L plexiglass reaction vessel. Start stirring at room temperature. First, adjust the pH of the white water from the original 6.8 to 6.5 using 30% dilute sulfuric acid. This adjustment process is completed within 1 minute, with a stirring speed of 300 rpm. After adjusting the pH, add a divalent cation salt to the white water. In this example, calcium chloride is used. Prepare a 10 g / L stock solution and add it slowly to achieve a final concentration of divalent calcium ions of 4 mmol / L in the white water. After the addition is complete, continue stirring at 300 rpm for 1.5 minutes to compress the colloidal double layer. In this step, divalent calcium ions enter the Stern layer of the colloidal particles through electrostatic interaction, reducing the absolute value of the zeta potential of the colloid and causing the colloid to change from a stable state to a destabilized state. For the negatively charged hemicellulose and resin salts in the white water of household paper, 4 mmol / L of calcium ions can increase the zeta potential from -28mV to about -12mV, and the electrostatic repulsion between colloidal particles is significantly weakened.

[0020] S2, prepare a 1 g / L solution of the amphoteric starch graft copolymer (it needs to be stirred and dissolved at 80℃ for 30 minutes and used after cooling), and add it to white water at an absolute dry weight of 10 mg / L, i.e., add 50 mL of this solution. After adding the copolymer, stir at 200 rpm for 5 minutes. This step is used to form initial flocs of amphoteric starch through electrostatic neutralization and bridging. The cationic groups (quaternary ammonium salts) of amphoteric starch undergo strong electrostatic adsorption with the destabilized colloidal particles. At the same time, the hydroxyl groups and a small number of anionic groups on the starch molecular chain form hydrogen bonds or coordination bonds with the polar groups on the surface of the colloidal particles, so that multiple colloidal particles are linked together by a polymer chain to form initial flocs with a size of about 30-100 μm.

[0021] S3 uses nano-silica sol with a silica content of 30 wt%, an average particle size of 20 nm, pH=9.2, and a surface zeta potential of -28 mV. It is diluted with deionized water to 10 g / L SiO2 and then slowly added dropwise to the reaction vessel at a rate of 5 mg / L SiO2. After addition, the stirring speed is reduced to 100 rpm and stirring continues for 2 minutes. This step is used to load nano-silica onto the surface of the initial flocs, increasing the floc density. Specifically, negatively charged nano-silica is stable at pH 9.2 and can be uniformly adsorbed onto unsaturated cation sites in amphoteric starch through electrostatic interactions; simultaneously, the silanol groups on the silica surface can form hydrogen bonds with the hydroxyl groups on the starch chains. The density of nano-silica is approximately 2.2 g / cm3, while the density of organic colloidal flocs is typically only 1.01-1.05 g / cm3. 3 The apparent density of the flocs after loading can be increased to 1.15-1.22 g / cm³. 3 This significantly improves the settling performance of the flocs. Furthermore, the nanoparticles embedded in the floc pores enhance the shear strength of the flocs.

[0022] S4. A non-ionic PEO (purity ≥99%) with a molecular weight of 5 million was prepared as a 0.1 g / L dilute solution. 15 mL of this solution was added at a concentration of 0.3 mg / L. After addition, the stirring speed was reduced to 30 rpm and stirred slowly for 3 minutes. This step utilizes the bridging mechanism of ultra-high molecular weight PEO to further promote floc growth. PEO is a linear polymer; the ether oxygen atoms on its molecular chain form strong hydrogen bonds with water molecules, and can also form hydrogen bonds with polar groups such as hydroxyl and carboxyl groups on the floc surface. Under slow stirring, a single PEO molecular chain can simultaneously adsorb multiple initial flocs or micro-flocs loaded with nano-silica, bridging them into large flocs with a size of 400-800 μm. Due to the high molecular weight of PEO (5 million), even a very low addition (0.3 mg / L) can produce a significant bridging effect, preventing excessive polymer from causing surface oversaturation and subsequent re-stabilization of the flocs.

[0023] S5 uses a stepped hydrocyclone (first section diameter 100mm, second section diameter 40mm, cone angle 20°, overflow pipe diameter 25mm, underflow outlet diameter 12mm) for separation, with a feed pressure of 0.15MPa (the pressure is appropriately reduced due to the low viscosity of the white water from household paper), and a feed flow rate of 2.5m³ / h. 3 / h. White water enters the first column section tangentially, forming a high-speed rotating vortex field. Dense flocs migrate towards the wall under centrifugal force and spiral downwards, accelerating further in the second column section before being discharged from the underflow outlet; while purified white water flows upwards from the center through the overflow pipe. This invention's stepped design generates two levels of centrifugal force fields through column sections of different diameters. The first column section (large diameter) generates a medium centrifugal force (approximately 80g) for initial separation of larger flocs, while the second column section (small diameter) generates a high centrifugal force (approximately 250g) to capture fine flocs, thereby significantly improving separation efficiency. The volume of flocs discharged from the underflow is approximately 4-6% of the feed volume, while purified white water is obtained from the overflow.

[0024] In step S6, a filter press is used to filter the flocs. Before filtration, a small amount of cationic PAM (molecular weight 8 million, dosage 1 mg / L) is added as a filter aid. After filtration, dewatered sludge is obtained with a moisture content reduced to 52%. The filtrate turbidity is 18 NTU and COD is 150 mg / L. The filtrate is returned to step S1 and mixed with fresh papermaking white water at a volume ratio of 1:10 for recycling. The filtrate recirculation recovers residual chemicals and fine particles, reduces the amount of fresh water used, and achieves closed-loop white water recycling.

[0025] After the above treatment, the purified white water was collected from the overflow outlet, sampled, and tested. The results showed that: The purified white water had a turbidity of 2.8 NTU and a COD of 58 mg / L, with a COD removal rate of 81.9%. Suspended solids were not detected. The zeta potential was -5.8 mV, indicating that the colloids had been fully destabilized. The floc settling velocity was measured using the static settling column method, with an average settling velocity of 5.6 m / h. The floc moisture content (after filtering the underflow flocs through a 0.45 μm filter membrane and drying at 105 °C) was 89.5%, demonstrating that efficient separation could still be achieved with cyclone separation. The solids content of the underflow flocs after pressure filtration reached 48%.

[0026] This invention provides Embodiment 2: To further verify the applicability of different divalent cation salts, magnesium chloride was used instead of calcium chloride in this example, and the remaining parameters were basically the same as in Example 1. Specifically: In step S1, magnesium chloride was used as the divalent cation salt, and its addition brought the magnesium ion concentration to 4 mmol / L. Since the hydration radius of magnesium ions is slightly larger than that of calcium ions, the stirring time was extended to 2 minutes to ensure thorough mixing. The parameters for other steps (S2 to S5) remained unchanged. The white water used for treatment was from the same batch as in Example 1, with a raw water COD of 325 mg / L and a turbidity of 87 NTU.

[0027] The test results of the treated overflow purified white water were as follows: turbidity 3.5 NTU, COD 68 mg / L, COD removal rate 79.1%; floc settling velocity 5.2 m / h; and water content of the underflow flocs after mechanical dewatering 54%. Compared with Example 1, the treatment effect of magnesium chloride was slightly lower but still within the excellent range. This is mainly because magnesium ions have a slightly weaker ability to compress the electric double layer than calcium ions. This is because magnesium ions have a small ionic radius and high charge density, but a thicker hydration layer, resulting in a slightly lower actual effective charge density. However, magnesium chloride has the advantage of not easily forming scale in certain specific water qualities (such as high-alkalinity white water) and can be selected according to actual needs.

[0028] This invention provides embodiment 3: To further verify the treatment effect of this invention at higher drug dosages, this embodiment uses high molecular weight polyethylene oxide with a molecular weight of 6 million, added at a concentration of 0.5 mg / L; simultaneously, the amount of amphoteric starch added is adjusted to 15 mg / L, and the amount of nano-silica is adjusted to 10 mg / L. Other parameters are the same as in Example 1, specifically: S1: Calcium chloride concentration 4 mmol / L, pH 6.5; S2: Stirring 200 rpm for 5 minutes; S3: Stirring 100 rpm for 2 minutes; S4: Slow stirring 30 rpm for 3 minutes; S5: Step hydrocyclone parameters remain unchanged. Raw water parameters: COD 318 mg / L, turbidity 84 NTU.

[0029] The test results of the treated overflow purified white water were as follows: the turbidity of the purified white water decreased to 1.9 NTU, the COD decreased to 51 mg / L, and the COD removal rate reached 84.0%; the floc settling velocity reached 6.3 m / h; and the moisture content of the underflow flocs after pressure filtration was 49%. This example illustrates that appropriately increasing the dosage of amphoteric starch and nano-silica, and combining it with higher molecular weight PEO, can further improve floc density and settling performance, making it particularly suitable for white water reuse systems for household paper products (such as facial tissues and toilet paper) with extremely high requirements for effluent turbidity. However, it should be noted that when the amount of PEO added exceeds 1 mg / L, steric stabilization may occur on the floc surface, which may lead to a slight decrease in separation efficiency.

[0030] Comparative Example 1 provided by the present invention: To further verify the synergistic effect of steps S3 and S4 in this invention, this comparative example only performs steps S1 and S2 and then directly proceeds to step S5 using a stepped hydrocyclone separator, without adding nano-silica or polyethylene oxide. The remaining operating conditions are the same as in Example 1. The raw water COD is 322 mg / L, and the turbidity is 86 NTU.

[0031] Treatment results: The turbidity of the purified white water was 42 NTU, and the COD was 185 mg / L, with a COD removal rate of only 42.5%. The floc settling velocity was 1.8 m / h. From the appearance of the flocs, the size was small, approximately 50-120 μm, and the structure was loose, resulting in poor separation in the hydrocyclone. The solids content of the underflow flocs was only 15%. This is because without the densifying effect of nano-silica, the floc density was close to that of water, making effective settling difficult during centrifugation. Simultaneously, without the secondary bridging effect of PEO, the floc size was insufficient, and a large number of fine flocs entered the overflow, leading to high turbidity of the purified white water. This comparative example demonstrates that relying solely on divalent salts and amphoteric starch is insufficient to achieve efficient solid-liquid separation.

[0032] Comparative Example 2 is provided in this invention: Compared with existing technologies, this comparative example uses the most commonly used combination in papermaking white water treatment, specifically polyaluminum chloride (PAC) and cationic polyacrylamide (CPAM). The specific operating parameters are as follows: First, adjust the pH of the white water to 6.5, add PAC (calculated as Al2O3, 100 mg / L), stir at 200 rpm for 2 minutes, then add CPAM (molecular weight 8 million, cation content 20%, 1.5 mg / L), stir at 40 rpm for 4 minutes, and then pass it into a stepped hydrocyclone (with the same parameters as in Example 1) for separation. The raw water was from the same batch, with a COD of 323 mg / L and a turbidity of 85 NTU.

[0033] Treatment results: The turbidity of the purified white water was 16 NTU, COD was 112 mg / L, and the COD removal rate was 65.3%; the floc settling velocity was 3.5 m / h; the underflow floc moisture content was high (65% moisture content after pressure filtration). Although this effect is better than Comparative Example 1, it is still significantly lower than Example 1 of this invention. The main reason is that the PAC hydrolysis products are strongly positively charged, and the flocs formed after rapid neutralization with colloids are relatively dense, but the large addition of aluminum salts leads to a high production of chemical sludge; at the same time, the CPAM bridging effect is interfered with by the PAC hydrolysis products, resulting in uneven floc size distribution. In addition, the residual aluminum ion concentration in the white water after PAC treatment is high (about 3-5 mg / L), which is not conducive to the reuse of white water in the production of high-grade paper. This invention uses divalent cation salts to replace aluminum salts, avoiding the problem of metal ion accumulation, and both amphoteric starch and nano-silica are environmentally friendly materials with good biodegradability.

[0034] Comparative Example 3 is provided in this invention: To further verify the effect of the stepped hydrocyclone, the operation steps of this comparative example are exactly the same as those in Example 1, except that in step S5, the stepped hydrocyclone is replaced with a common single-stage hydrocyclone (column diameter 100mm, cone angle 20°, overflow pipe diameter 25mm, underflow port diameter 12mm, operating pressure 0.2MPa). The raw water COD is 321mg / L and turbidity is 86NTU.

[0035] Treatment results: The turbidity of the purified white water was 12.8 NTU, COD was 92 mg / L, and the COD removal rate was 71.3%. Due to the low centrifugal force of ordinary hydrocyclones (maximum of approximately 120 g per section), some fine flocs were not captured and entered the overflow, leading to increased effluent turbidity. Analysis of the overflow floc particle size distribution showed that 18% of the volume fraction of particles were smaller than 10 μm. However, in the stepped hydrocyclone, the high centrifugal force generated by the second section (small diameter) effectively captured these fine particles, reducing the proportion of particles smaller than 10 μm to below 2%.

[0036] The table below shows the data for the above embodiments and comparative examples, in detail: Table 1 Comparison of processing effects between each embodiment and the comparative example

[0037] Note: *: The relative cost of the reagents is calculated based on the total cost of the reagents in Example 1, which is 1.00. It only includes flocculants, divalent salts, nano-SiO2 and PEO, and does not include equipment investment and energy consumption.

[0038] **: Comparative Example 1: The floc settling performance is poor, and the solids content of the bottom flow floc is only 15%, which cannot effectively carry out mechanical dewatering. The moisture content of the dewatered sludge is not listed.

[0039] ***: The settling velocity in Comparative Example 3 is separated from the settling velocity of the flocs themselves due to the different types of hydrocyclones, and is not directly compared in the table.

[0040] To further verify the present invention, morphological observation and shear strength tests were performed on the flocs of Example 1 and Comparative Example 2.

[0041] 1) Take the floc sample from Example 1 after step S4 and observe it under an optical microscope at 200x magnification. The flocs are spherical or near-spherical with clear outlines, dense surfaces, and internal voids filled with nano-silica particles. The floc diameter ranges from 350-750 μm, with an average of 5500 μm. In contrast, the flocs from Comparative Example 2 are irregularly shaped with a loose surface, showing protruding polymer chains with diameters ranging from 80-250 μm, averaging 150 μm, and are easily broken under slight pressure from a glass slide.

[0042] 2) The suspensions of Example 1 and Comparative Example 2 after flocculation were placed in a shearing device and sheared at 200 rpm for 5 minutes, then allowed to stand for 1 minute before the turbidity of the upper layer was measured. The turbidity of Example 1 increased from 2.8 NTU to 4.1 NTU after shearing, an increase of 46%; the turbidity of Comparative Example 2 increased from 16 NTU to 58 NTU after shearing, an increase of 263%. These results demonstrate that the flocs formed by this invention have excellent shear resistance. This is because the skeletal support of nano-silica and the elastic bridging effect of the PEO flexible chains make the flocs less prone to breakage in the shear field, making them very suitable for the shear environment generated by the circulating pumping of white water in a household paper system.

[0043] To further verify the reflux adaptation effect of the method of the present invention, the same batch of household paper white water was continuously circulated for 10 batches using the process described in Example 1 (5L of fresh white water per batch, but the filtrate from step S6 was refluxed at a ratio of 1:10). The stability of the treatment effect was examined, and samples were taken from each batch to test the turbidity and COD removal rate of the purified water. The results are shown in Table 2: Table 2. Stability data for 10 consecutive batches.

[0044] As shown in Table 2, after 10 consecutive batches of treatment, the turbidity of the purified water remained stable within the range of 2.5-3.1 NTU, and the COD removal rate remained stable within the range of 80.9-82.5%, with extremely small standard deviations. This indicates that the method of the present invention has good reproducibility and anti-interference ability. Filtrate reflux did not lead to pollutant accumulation because the small amount of colloidal matter not completely removed in each cycle was effectively removed after passing through the destabilization, flocculation, and separation processes again.

[0045] To verify the universality of the method of the present invention under different water quality fluctuations in white water for tissue paper production, tests were also conducted on the same tissue paper production line at different basis weights (14 g / m³). 2 18g / m 2 22g / m 2 White water samples were collected at different vehicle speeds (800 m / min, 1200 m / min, 1500 m / min), with COD ranging from 280 to 380 mg / L, turbidity from 70 to 110 NTU, and conductivity from 950 to 1300 μS / cm. The samples were processed using the same parameters as in Example 1 (only adjusted to 6.3-6.7 in step S1 based on the initial pH). The results are shown in Table 3. Table 3. Treatment effect of white water on three types of paper

[0046] As shown in Table 3, the method of this invention has good treatment effect on white water under different operating conditions of tissue paper production, with COD removal rate maintained above 81% and turbidity of purified water below 4 NTU. This fully meets the requirements for high-proportion recycling of tissue paper white water (typically requiring turbidity < 10 NTU and COD removal rate > 70%). Therefore, it can be concluded that the method of this invention has broad raw material adaptability and can be stably applied to the treatment of white water from the wire mesh or press generated during tissue paper production.

[0047] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A highly efficient flocculation and separation method for dissolved colloidal substances in papermaking white water, characterized in that, Includes the following steps: S1. Adjust the pH of the papermaking white water to 6.0-7.0, then add divalent cation salts to a divalent cation concentration of 2-10 mmol / L, stir for 1-2 minutes, and compress the colloidal double layer; S2, add an amphoteric starch graft copolymer to liquid S1. The amphoteric starch graft copolymer has a cationic substitution degree of 0.2-0.4, an anionic substitution degree of 0.1-0.3, a molecular weight of 200,000-500,000, and an addition amount of 10-50 mg / L. Stir the reaction for 5-10 minutes to form initial flocs. S3, add nano-silica sol to liquid S2. The nano-silica has a particle size of 10-30nm and an addition amount of 5-20mg / L. Continue stirring for 2-3 minutes to load the nano-silica onto the surface of the initial flocs and increase the floc density. S4. Add high molecular weight polyethylene oxide (34-8 million) to the mixture obtained in S3 at a concentration of 0.1-1 mg / L. Stir slowly for 2-5 minutes to form large and dense flocs. S5, the white water containing flocs obtained in S4 is separated by a stepped hydrocyclone. The hydrocyclone has at least two column sections with different diameters to enhance the separation of flocs and water. The flocs are discharged from the underflow, and the purified white water is obtained from the overflow.

2. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: The divalent cation salt is one or more of calcium chloride, magnesium chloride, calcium sulfate, or magnesium sulfate.

3. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that, The amphoteric starch graft copolymer is prepared by the following method: starch is grafted copolymerized with a cationic etherifying agent and an anionic graft monomer in the presence of an initiator.

4. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 3, characterized in that: The cationic etherifying agent is 3-chloro-2-hydroxypropyltrimethylammonium chloride or 2,3-epoxypropyltrimethylammonium chloride, and the anionic grafting monomer is acrylic acid, methacrylic acid or 2-acrylamido-2-methylpropanesulfonic acid.

5. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: The nano-silica sol has a pH of 8.0-10.0 and its surface carries a negative charge.

6. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: The high molecular weight polyethylene oxide has a molecular weight of 5-6 million and is added at a rate of 0.3-0.8 mg / L.

7. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: The first column of the stepped hydrocyclone has a diameter of 50-150mm, the second column has a diameter of 20-80mm, and the cone angle is 10-30°.

8. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: In step S1, the pH is adjusted using sulfuric acid, hydrochloric acid, sodium hydroxide, or sodium carbonate, and the conductivity of the adjusted white water is controlled between 1000-3000 μS / cm.

9. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: The stirring speed in step S2 is 200-400 rpm, the stirring speed in step S3 is 100-200 rpm, and the stirring speed in step S4 is 30-60 rpm.

10. The efficient flocculation and separation method for dissolved colloidal substances in papermaking white water according to claim 1, characterized in that: In step S5, the flocs discharged from the underflow are mechanically dewatered to obtain dewatered sludge and filtrate. The filtrate is returned to step S1 to be mixed with papermaking white water for further treatment.

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