Preparation method of high-performance coal water slurry additive

Abstract

The application discloses a preparation method of a high-performance coal water slurry additive. Tannic acid, crotonic acid and allyloxy polyoxyethylene ether are used as raw materials, a tannic acid-based ternary polymer is obtained through oxidation reaction and free radical polymerization reaction, and then the tannic acid-based ternary polymer is compounded with methylene dianaphthalene sulfonic acid sodium to obtain the high-performance coal water slurry additive. The high-performance additive prepared by the application has a smaller adding amount in a slurry preparation process, and the prepared slurry has better performance. In addition, the additive overcomes the poor matching problem of traditional additives, has good universality, and has good slurry preparation effect in a traditional slurry preparation system, a high-concentration coal water slurry preparation system and a slurry preparation system mixed with organic waste, and especially solves the slurry preparation problems in the high-concentration slurry preparation system and the slurry preparation system mixed with organic waste.

Description

Technical Field

[0001] This invention relates to the field of coal-water slurry additives, and more specifically to a method for preparing a high-performance coal-water slurry additive. Background Technology

[0002] In the preparation of coal-water slurry, additives are needed to improve the zeta potential and steric hindrance of the surface of coal powder, a hydrophobic nonpolar hydrocarbon, thereby ensuring its uniform and stable dispersion in water. High-performance additives can result in coal-water slurry with high concentration, low viscosity, and strong stability. Therefore, additives play a crucial role in the preparation of coal-water slurry.

[0003] Currently, widely used additives include lignin-based, naphthalene-based, polycarboxylic acid-based, and humic acid-based additives. However, since the coal used in coal chemical industry is mainly low-rank bituminous coal, its characteristics result in poor slurry-forming performance, and the coal quality varies greatly, placing higher demands on the performance and adaptability of additives. In recent years, to significantly improve the slurry-forming performance of coal, scholars have proposed improving the slurry properties by changing the coal-water slurry preparation process. Two main enrichment processes developed—graded grinding slurry preparation and discontinuous grading slurry preparation—have significantly improved the slurry concentration of coal samples. Among them, the graded grinding slurry preparation process can increase the coal slurry concentration by 2%–3%, and the discontinuous grading slurry preparation process can increase the coal slurry concentration by 4%–6%. Li Xingyi's paper, titled "Research on the Technology of Graded Grinding for Preparing High-Concentration Coal-Water Slurry," studied the technology of graded grinding for preparing high-concentration coal-water slurry and found that, considering slurry performance and additive costs, the optimal additive dosage is 0.3% (dry basis / dry powder) when the concentration is increased by about 3 percentage points. In their paper titled "Experimental Study on the Preparation of Gasified Coal-Water Slurry by Intermittent Grading Process," Du Liwei et al. used lignite and coke as raw materials and employed an intermittent grading slurry preparation process to conduct slurry-forming experiments. The results showed that when lignite and coke were uniformly mixed at a mass ratio of 6:4 and the coarse to fine powder ratio was 7:3, the highest concentration of the prepared coal-water slurry was 61.24%, requiring an additive dosage of 0.3% (dry basis / dry powder). This study demonstrates that while the slurry preparation process can significantly increase the concentration of coal-water slurry, it requires a large amount of additives, resulting in higher costs and increased expenses for enterprises.

[0004] In addition, in recent years, with the scarcity of coal resources and the increase in organic waste emissions, the country has begun to advocate the coordinated development of coal-water slurry technology coupled with the comprehensive utilization of organic waste. Organic waste is mixed into the coal-water slurry preparation system to achieve the dual goals of coal saving and waste resource utilization. However, organic waste is diverse, including biochemical sludge, biomass, organic waste liquid, coal-oil co-refining residue, etc., with complex composition and great differences, resulting in a complex pulping environment and high pulping difficulty. It is difficult to prepare mixed slurry with high blending ratio, high concentration and good performance using conventional additives.

[0005] Developing high-performance, low-cost, and widely adaptable novel additives is key to advancing coal-water slurry preparation technology. Currently, the main methods for developing novel additives include compounding, novel molecular synthesis, and synthesis-compounding coupling. Compounding is a common way to improve the effectiveness of additives; however, most current compounding methods use conventional additives in proportions, resulting in problems such as high addition ratios, high costs, and limited applications. For example, Chinese invention patent CN111154520A discloses a lignin-free coal-water slurry additive and the coal-water slurry prepared using it. This invention uses a synergistic compounding of naphthalene-based dispersants, polystyrene sulfonates, polyoxyethylene series nonionic synthetic dispersants, and sulfonated melamine-formaldehyde condensates to prepare a higher concentration of coal-water slurry. However, the amount of dispersant added is still too high, and the proportion of nonionic dispersants is high, leading to a high price for the additive, which is not conducive to its industrial application. Chinese invention patent CN117660065A discloses a humic acid-based coal-water slurry additive and its preparation method. It is prepared by compounding sodium humate, naphthalene sulfonate and coal-water slurry additives (one or more of polyacrylamide, sodium tripolyphosphate, and sodium carboxymethyl cellulose). Although the additive can improve the slurry-forming performance of Shenhua coal and Ningtaota coal compared with naphthalene-based additives alone, the slurry performance is relatively poor for Shenhua coal. The additive still has the problem of poor compatibility with coal types during use.

[0006] Novel molecular synthesis is a relatively efficient method to improve the performance of coal-water slurry. In their paper titled "Preparation and Performance Study of a Tannic Acid-Based Ternary Polymer Coal-Water Slurry Additive," Yu Yunfeng et al. obtained a novel ternary polymer coal-water slurry additive through a graft copolymerization reaction of tannic acid (TA), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), and maleic anhydride (MA). They then investigated its effect on the performance of the coal-water slurry. The results showed that using 1.5‰ of the dry coal mass of the additive could produce a 62% concentration coal-water slurry with good fluidity and stability, and low viscosity. Using a concentration-enhancing process, a maximum concentration of 65% coal-water slurry could be obtained. However, the amount of additive added during the entire slurry preparation process was still relatively high, and the viscosity of the slurry was relatively high at a concentration of 65%. Furthermore, this additive was not applied to a diversified slurry preparation system for organic waste. From the perspective of the additive's working principle, single-type additives often lack good universality in complex slurry-forming environments.

[0007] Based on this, in view of the prominent problems existing in the high-concentration slurry preparation system and organic waste slurry preparation system in the current coal-water slurry preparation technology, and in combination with the current research status of additives, it is very necessary to develop new additives coupled by synthesis and compounding, which is of great significance to promoting the development of coal-water slurry preparation technology. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a high-performance additive with good pulping effect, strong compatibility, wide applicability, low dosage, and low cost, especially showing good pulping effect in the fields of organic waste pulping and high-concentration coal-water slurry preparation. This invention achieves multifunctionality of the coal-water slurry additive groups through a clever epitaxial star-shaped molecular design coupling compounding method. Multiple hydrophilic and hydrophobic groups provide the additive with multiple active sites, allowing for more complete interaction with coal and water molecules during pulping. Their synergistic effect ensures stable and uniform dispersion of coal molecules in the aqueous medium. The resulting slurry not only has high concentration, low viscosity, good fluidity and stability, but also requires a small dosage and has good compatibility during use, thus overcoming the shortcomings of traditional additives in the pulping process.

[0009] A method for preparing a high-performance coal-water slurry additive involves using tannic acid, crotonic acid, and allyloxy polyoxyethylene ether as raw materials, and obtaining a tannic acid-based ternary polymer through oxidation and free radical polymerization. This polymer is then compounded with sodium methylene dinaphthalene sulfonate to obtain the high-performance coal-water slurry additive.

[0010] Preferably, the preparation method of the tannic acid-based ternary polymer is as follows: crotonic acid, allyloxy polyoxyethylene ether, and deionized water are placed in a reaction vessel, ultrasonically mixed to ensure uniformity, heated to 70℃~95℃, and simultaneously added dropwise with tannic acid solution and potassium persulfate solution. The mixture is stirred for 3h~5h, cooled to room temperature, pH adjusted to 9, filtered, concentrated, washed with ethanol, and dried to obtain the tannic acid-based ternary polymer.

[0011] Preferably, the mass ratio of the tannic acid-based terpolymer to sodium methylene dinaphthalene sulfonate is (1-2.5):(1-3).

[0012] Preferably, the conditions for the ultrasound are: ultrasound temperature 40℃~50℃, ultrasound time 15min~25min.

[0013] Preferably, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:(13-20):(8-15).

[0014] Preferably, the mass of the potassium persulfate is 6% to 9% of the mass of tannic acid.

[0015] Preferably, the time for adding the tannic acid solution and potassium persulfate solution is 15 min to 20 min.

[0016] The beneficial effects of this invention are: (1) This invention introduces crotonic acid monomer and allyloxy polyoxyethylene ether monomer into tannic acid matrix through oxidation reaction and free radical polymerization reaction by ingenious molecular design, and synthesizes a molecular polymer with a star structure and multiple functional groups. This molecular polymer has multiple active sites and can act on the coal-water system / coal-water-waste system in a multi-node and multi-plane manner. By forming a three-dimensional hydration film structure on the surface of coal particles / waste, the coal-water slurry exhibits a stable dispersion state. (2) This invention constructs a tannic acid-based ternary polymer-methylene dinaphthalene sulfonate binary additive system through synthesis-compounding. In the additive system, the benzene ring on the tannic acid molecule, the alkyl groups with different carbon chain lengths, the naphthyl group and methylene group on the sodium methylene dinaphthalene sulfonate molecule and other hydrophobic groups, together with the unreacted hydroxyl group on the tannic acid molecule, the introduced carboxyl group, ether group, hydroxyl group and sulfonic acid group and other hydrophilic groups, can significantly improve the hydrophilicity of the coal-water system or the coal-water-waste system, effectively reduce the interfacial tension between solid and liquid, reduce the viscosity of the system, and improve the stability of the system. The two additives have a synergistic effect, which enhances the pulping effect and has significant pulping advantages. (3) Compared with traditional additives or existing additives, the high-performance additive prepared in this invention requires less addition during slurry preparation and produces slurry with better performance. In addition, this additive overcomes the problem of poor compatibility of traditional additives and has good versatility. It has good slurry preparation effects in traditional slurry systems, high-concentration coal-water slurry preparation systems and organic waste blending slurry systems. In particular, it solves the slurry preparation problems in high-concentration slurry systems and organic waste material slurry systems. Detailed Implementation

[0017] Example 1 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 40°C for 15 min. The mixture was then heated to 70°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 15 min. The mixture was stirred for 3 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:3 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:13:8, and the mass of potassium persulfate is 6% of the mass of tannic acid. The container described above is a three-necked round-bottom flask.

[0018] Example 2 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 50°C for 25 min. The mixture was then heated to 85°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 17 min. The mixture was stirred for 4 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:1 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:17:12, and the mass of potassium persulfate is 7.5% of the mass of tannic acid.

[0019] Example 3 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 95°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 20 min. The mixture was stirred for 5 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:1 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:20:15, and the mass of potassium persulfate is 9% of the mass of tannic acid.

[0020] Example 4 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 85°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 20 min. The mixture was stirred for 4 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:1 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:13:15, and the mass of potassium persulfate is 7.5% of the mass of tannic acid.

[0021] Example 5 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 85°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 20 min. The mixture was stirred for 4 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:1 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:20:8, and the mass of potassium persulfate is 7.5% of the mass of tannic acid.

[0022] Example 6 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 70°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 18 min. The mixture was stirred for 5 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 1:3 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:17:12, and the mass of potassium persulfate is 6.0% of the mass of tannic acid.

[0023] Example 7 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 95°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 17 min. The mixture was stirred for 4 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 2.5:1 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:17:12, and the mass of potassium persulfate is 9.0% of the mass of tannic acid.

[0024] Example 8 A method for preparing a high-performance coal-water slurry additive includes the following steps: Crotonic acid, allyloxy polyoxyethylene ether and deionized water were added to a container and ultrasonically mixed at 45°C for 20 min. The mixture was then heated to 85°C, and tannic acid solution and potassium persulfate solution were added dropwise at a time of 17 min. The mixture was stirred for 3 h, cooled to room temperature, and the pH was adjusted to 9. The mixture was then filtered, concentrated, washed with ethanol, and dried to obtain a tannic acid-based terpolymer. Then, the tannic acid-based ternary polymer and sodium methylene dinaphthalene sulfonate were mixed evenly at a mass ratio of 2.5:3 to obtain a high-performance coal-water slurry additive. In the entire reaction system, the molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:17:12, and the mass of potassium persulfate is 7.5% of the mass of tannic acid.

[0025] Comparative Example 1: The inventors used a commercially available sodium naphthalene sulfonate formaldehyde condensate water-coal slurry additive as a comparative example to compare its performance with that of the water-coal slurry additives prepared in Examples 1 to 8 of this invention.

[0026] Preparation method of coal-water slurry: In the coal-water slurry preparation experiment, this invention selected Shenfu coal from northern Shaanxi and prepared coal-water slurry using both the conventional one-time dry grinding and wet blending preparation process (hereinafter referred to as "conventional slurry preparation process") and the high-concentration slurry preparation technology independently developed by Shaanxi Chemical Research Institute Co., Ltd. (hereinafter referred to as "high-concentration slurry preparation process").

[0027] In the organic waste pulping experiment, this invention used Shaanxi Shenfu coal as the main raw material, and selected biomass coke and biochemical sludge as the blending objects to investigate the pulping performance of various organic wastes.

[0028] Performance testing methods for coal-water slurry: Concentration determination: determined according to GB / T18856.2-2008 standard; Apparent viscosity determination: The apparent viscosity was determined according to GB / T18856.4-2008 standard using a coal-water slurry viscometer. The apparent viscosity was taken as the shear rate of 100 s⁻¹. -1 The average viscosity at that time; Flowability determination: The flow characteristics of coal-water slurry were evaluated using an observational method. Based on their flow characteristics, they were classified into four levels: A, B, C, and D. A: Continuous flow, smooth and uninterrupted; B: Relatively continuous flow, fluid surface is not smooth; C: Flows only with external force; D: Does not form a slurry, cannot flow. To indicate minor differences in flowability within a certain level range, "+" and "-" signs were used for differentiation. "+" indicates better flowability within a certain level; "-" indicates poorer flowability within a certain level. Water separation rate determination: Place the slurry in a 25 ml graduated cylinder, let it stand for 24 hours, measure the volume of the clear liquid in the upper layer of the cylinder, calculate the ratio of the clear liquid to the total volume, and record it as the water separation rate of the slurry. The larger the water separation rate value, the worse the stability; the smaller the value, the better the stability.

[0029] Test results: Tables 1 and 2 show the comparative experimental results of the high-performance coal-water slurry additives in Examples 1 to 8 and the commercially available additives in the comparative examples under different organic waste blending slurry systems. It should be noted that in Tables 1 and 2, the amount of biomass coke added and the amount of biochemical sludge added refer to the mass percentage of the total solids content of the system under their respective slurry concentration conditions; the amount of additive added refers to the dry basis mass percentage of the total solids content of the system under their respective slurry concentration conditions.

[0030] Table 1 Comparative experimental results of water-coal slurry additives in biomass-coke blending and pulping systems Table 2 Comparative experimental results of coal-water slurry additives in biochemical sludge blending and pulping systems Table 1 shows that in the biomass-coke blending pulping system, when the biomass-coke addition amount is 50%, the high-performance coal-water slurry additives prepared in Examples 1 to 8 all have better pulping effects than the commercially available coal-water slurry additives in the comparative examples, with a significant decrease in apparent viscosity, an increase in coal slurry concentration of 1.5%, and a higher blending ratio of biomass-coke. Table 2 shows that in the biochemical sludge blending pulping system, when the biochemical sludge addition amount is 10%, the high-performance coal-water slurry additives prepared in Examples 1 to 8 show the same results as the biomass-coke blending pulping experiments in Table 1, also exhibiting a positive improvement effect. Furthermore, regardless of whether it is biomass-coke pulping or biochemical sludge pulping experiments, the dosage of the high-performance coal-water slurry additive is reduced by at least 50% compared to the dosage of conventional commercially available coal-water slurry additives.

[0031] Tables 3 and 4 show the comparative experimental results of the high-performance coal-water slurry additives in Examples 1 to 8 and the commercially available additives in the comparative examples under different process conditions of the coal-water slurry preparation system. The additive dosage refers to the percentage of the dry basis mass of the raw coal under the respective slurry concentration conditions.

[0032] Table 3 Comparative experimental results of coal-water slurry additives under conventional coal-water slurry preparation process conditions Table 4 Comparative experimental results of coal-water slurry additives under high-concentration slurry preparation process conditions As shown in Tables 3 and 4, under the same pulping process conditions in the coal-water slurry preparation system, the high-performance coal-water slurry additives prepared in Examples 1 to 8 show a significantly reduced dosage compared to the commercially available coal-water slurry additives in the comparative examples. Furthermore, the resulting slurry has a higher concentration, lower apparent viscosity, and better flowability and stability. This is particularly evident under the high-concentration pulping process conditions in Table 4, where the apparent viscosity of the coal-water slurry decreases significantly. In addition, comparing Examples 1 to 8 comprehensively, the coal-water slurry prepared under the conditions of Example 2 exhibits the lowest viscosity, best flowability, best stability, and better overall performance.

[0033] As can be seen from the results of the above embodiments, the high-performance coal-water slurry additive prepared by the present invention exhibits good slurry preparation effects in traditional coal-water slurry preparation systems, high-concentration coal-water slurry preparation systems, and organic waste blending slurry preparation systems. It has good versatility, and compared with traditional commercially available coal-water slurry additives, the amount added is greatly reduced, and the slurry forming performance is significantly improved. It overcomes the technical difficulties existing in the preparation process of traditional coal-water slurry, especially organic waste slurry and high-concentration coal-water slurry, and has good application prospects.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a high-performance coal-water slurry additive, characterized in that, Tannic acid, crotonic acid, and allyloxy polyoxyethylene ether are used as raw materials. Tannic acid-based ternary polymers are obtained through oxidation and free radical polymerization. Then, they are compounded with sodium methylene dinaphthalene sulfonate to obtain a high-performance coal-water slurry additive. The preparation method of the tannic acid-based ternary polymer is as follows: crotonic acid, allyloxy polyoxyethylene ether, and deionized water are placed in a container, ultrasonically mixed to ensure uniformity, heated to 70℃~95℃, and simultaneously added dropwise with tannic acid solution and potassium persulfate solution. The mixture is stirred for 3h~5h, cooled to room temperature, pH adjusted to 9, filtered, concentrated, washed with ethanol, and dried to obtain the tannic acid-based ternary polymer. The mass ratio of the tannic acid-based ternary polymer to sodium methylene dinaphthalene sulfonate is (1-2.5):(1-3). The molar ratio of tannic acid, crotonic acid, and allyloxy polyoxyethylene ether is 1:(13-20):(8-15).

2. The method for preparing the high-performance coal-water slurry additive according to claim 1, characterized in that, The conditions for the ultrasound are: ultrasound temperature 40℃~50℃, ultrasound time 15min~25min.

3. The method for preparing the high-performance coal-water slurry additive according to claim 1, characterized in that, The mass of potassium persulfate is 6% to 9% of the mass of tannic acid.

4. The method for preparing the high-performance coal-water slurry additive according to claim 1, characterized in that, The time for adding the tannic acid solution and potassium persulfate solution is 15 min to 20 min.

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