A low-rank coal dewatering method based on grafting modification of polyacrylamide to regulate pore structure of filter cake

Abstract

This invention belongs to the field of low-rank coal dewatering technology, and particularly relates to a method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake. Specifically, it includes: dissolving anionic polyacrylamide in a deionized water-ethanol mixed solvent to obtain a polyacrylamide solution; pre-emulsifying an acrylate hydrophobic monomer with a carbon chain length of [value missing] with an emulsifier to obtain an emulsion; adding an initiator stepwise to the polyacrylamide solution under nitrogen protection and stirring conditions, then uniformly adding the emulsion to carry out a graft polymerization reaction, and obtaining graft-modified polyacrylamide after post-treatment; adding it to low-rank coal slurry water, stirring to form flocs, and dewatering by pressure filtration to obtain a filter cake. This invention utilizes the synergistic effect of charge neutralization, hydrophobic association, and adsorption bridging of graft-modified polyacrylamide to form a high-porosity floc skeleton; combined with a gradient pressure filtration method, a hydrophobic interface is formed on the pore wall of the filter cake, reducing capillary retention resistance, resulting in a filter cake with a moisture content as low as 22.5%~24.3%.

Description

Technical Field

[0001] This invention belongs to the field of low-rank coal dewatering technology, and particularly relates to a method for dewatering low-rank coal based on grafted modified polyacrylamide to regulate the pore structure of the filter cake. Background Technology

[0002] Coal washing and processing is a crucial step in achieving clean and efficient coal utilization. In my country, wet separation is the primary method, requiring approximately 3 tons of water to process every ton of raw coal. The washed product carries a significant amount of moisture and necessitates dewatering to meet performance requirements. With increasing mechanization in coal mining, the content of fine particles in the raw coal being processed has been rising continuously. Currently, particles smaller than 0.5 mm account for over 20%. Due to the influence of slurry properties, particle size distribution, and residual reagents, dewatering fine coal has become a technical bottleneck in the coal preparation field.

[0003] Low-rank coal is a type of coal with a relatively low degree of coalification, mainly including lignite, long-flame coal, non-caking coal, and weakly caking coal. Its most prominent feature is its high intrinsic moisture content, which stems from its highly developed hierarchical pore system and the abundance of oxygen-containing functional groups such as carboxyl and hydroxyl groups on its surface. These polar groups form strong hydrogen bonds with water molecules to form a stable hydration film. At the same time, clay minerals such as kaolinite and montmorillonite, which are commonly found in coal, have strong water absorption and swelling properties. When they disperse into colloidal particles upon contact with water, they further clog the pores and increase the resistance to dehydration.

[0004] Dewatering and upgrading of low-rank coal has significant industrial value in terms of transportation economy, combustion efficiency, and compatibility with deep processing, and is a key link in achieving its efficient and clean utilization. Mechanical dewatering has become the main method for dewatering low-rank coal due to its mature technology, low energy consumption, and relatively small investment. However, even with high-pressure diaphragm filtration and extended pressing time, the moisture content of low-rank coal filter cake is still as high as 20-35%, which cannot meet the requirements of modern coal chemical processes for raw material moisture (such as fixed-bed gasification requiring moisture <15%).

[0005] Polyacrylamide (PAM) is currently the most widely used polymeric flocculant in the field of low-rank coal dewatering. Its molecular weight is usually 3 million to 20 million. Depending on the charged groups, it can be divided into anionic (APAM), cationic (CPAM), and nonionic (NPAM). Its mechanism of action mainly includes charge neutralization and adsorption bridging: the former neutralizes the negative charge on the surface of coal particles by ionic groups, compresses the double layer to reduce the zeta potential, weakens electrostatic repulsion and promotes particle proximity; the latter uses long molecular chains to simultaneously adsorb multiple particles to form a bridging structure, so that fine particles aggregate into large flocs. However, PAM has inherent limitations in the deep dewatering of low-rank coal. The PAM molecular chain is rich in hydrophilic groups such as amide and carboxyl groups, which not only fail to destroy the hydration film on the surface of coal particles but may even enhance its stability through hydrogen bonding. The flocs formed by PAM have well-developed micropores (<2 nm) and mesopores (2~50 nm), with a large amount of water physically encapsulated. Capillary water and surface adsorbed water account for 40~60% of the total water content, and the pore walls remain hydrophilic, making it difficult to release under conventional mechanical pressure. This results in a filter cake moisture content reduction of only 1~3 percentage points, ultimately still reaching 25~35%. Furthermore, PAM can only indirectly affect the macroscopic structure of the filter cake by changing the floc size; it cannot precisely control the pore size distribution, pore connectivity, and pore wall wettability, making it difficult to form an ideal filter cake structure with gradient pore size distribution, good connectivity, and hydrophobic pore walls.

[0006] To improve the dehydration effect of polyacrylamide (PAM), researchers have explored various technical routes, including ultrasonic modification, ionic and chemical group modification, and surfactant formulation. However, none of these methods have achieved deep dehydration of the filter cake. Ultrasonic modification utilizes cavitation to shear molecular chains, which can increase flocculation activity by 10-20%, but the effect is unstable, energy consumption is high, and it is difficult to industrialize. Ionic and chemical group modification (such as sulfonyl methylation and hydroxymethylation) can improve charge characteristics, water solubility, and salt tolerance, but because it does not introduce hydrophobic groups, its effect on reducing filter cake moisture is limited. Surfactant formulation (such as the combination of PAM and CTAB) can take into account both flocculation and hydrophobic effects, but it has problems such as competitive adsorption of reagents, complex processes, and environmental risks.

[0007] In summary, existing low-rank coal dewatering technologies suffer from poor filter cake pore structure and high capillary retention resistance, resulting in low dewatering efficiency and high filter cake moisture content. Therefore, developing a graft-modified polyacrylamide and its dewatering method that can actively regulate the filter cake pore structure and reduce capillary retention resistance is of great significance for overcoming the bottlenecks in low-rank coal dewatering technology. Summary of the Invention

[0008] The purpose of this invention is to provide a method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of the filter cake, so as to solve the defects of existing low-rank coal dewatering technology, such as poor filter cake pore structure, large capillary retention resistance, low dewatering efficiency, and high filter cake moisture content.

[0009] To achieve the above objectives, the technical solution adopted by this invention is: a method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake, comprising the following steps:

[0010] Step S1: Dissolve anionic polyacrylamide in a deionized water-ethanol mixed solvent to obtain a polyacrylamide solution with a mass fraction of 8-12%.

[0011] Step S2: The carbon chain length is... The acrylate hydrophobic monomers and emulsifiers were pre-emulsified in a high-speed shear emulsifier to obtain an emulsion with a particle size of 100~300 nm.

[0012] Step S3: Under nitrogen protection and stirring conditions, potassium persulfate-sodium bisulfite redox initiator is added stepwise to the polyacrylamide solution, and then the emulsion is added dropwise at a rate of 2~5 mL / min. Under the action of the initiator, the graft polymerization reaction is carried out to obtain the reaction product.

[0013] Step S4: Quench, cool, filter, purify and dry the reaction product to obtain grafted modified polyacrylamide;

[0014] Step S5: Add the grafted modified polyacrylamide to the low-rank coal slurry water, stir and mix evenly to form flocs, and then perform pressure filtration and dewatering to obtain filter cake.

[0015] Preferably, in step S1, the anionic polyacrylamide has a viscosity-average molecular weight of 8 million to 12 million, a degree of hydrolysis of 20% to 30%, and an anionicity of 1.5 to 2.5 mmol / g.

[0016] Preferably, in step S1, the amount of ethanol used is 5 to 10% of the total mass of the deionized water-ethanol mixed solvent.

[0017] Preferably, in step S2, the amount of the acrylate hydrophobic monomer is 5-15% of the mass of the anionic polyacrylamide.

[0018] Preferably, in step S2, the emulsifier is nonylphenol polyoxyethylene ether, and the amount of emulsifier used is 2 to 5% of the mass of the acrylate hydrophobic monomer.

[0019] Preferably, in step S3, the amount of potassium persulfate-sodium bisulfite redox initiator is 0.3-0.8% of the mass of the acrylate hydrophobic monomer, and the molar ratio of potassium persulfate to sodium bisulfite is 1:1-1.2.

[0020] Preferably, step S3 specifically includes: under nitrogen protection and stirring conditions, adding 60-70% of the total amount of potassium persulfate-sodium bisulfite redox initiator to the polyacrylamide solution, then uniformly adding the emulsion at a rate of 2-5 mL / min, controlling the temperature at 40-45℃ during the addition process, and reacting for 30 min; then adding the remaining amount of potassium persulfate-sodium bisulfite redox initiator, raising the temperature to 50-55℃ and reacting for 2-3 h, and finally raising the temperature to 55-60℃ and reacting for 1-2 h, obtaining the reaction product through graft polymerization.

[0021] Preferably, step S4 specifically includes: after the graft polymerization reaction is completed, the temperature is lowered to 45~50℃, and 0.01~0.05% by weight of hydroquinone, an acrylate hydrophobic monomer, is added to quench residual free radicals. The temperature is maintained and stirred for 30 min to ensure the system is homogeneous and stable. Then, the product is cooled to 25~30℃ at a rate of 1~2℃ / min and discharged. The product is filtered through an 80~120 mesh stainless steel screen to remove gel particles, and then unreacted monomers, oligomers, and small molecule impurities are removed by ethanol precipitation or dialysis. The purified material is then vacuum dried or spray dried to obtain graft-modified polyacrylamide.

[0022] Preferably, in step S5, the amount of grafted modified polyacrylamide added is 0.002~0.01% of the total weight of low-rank coal slurry.

[0023] Preferably, in step S5, the pressure filtration dewatering adopts a gradient pressurization method, specifically including: applying a pressure of 0.1~0.2 MPa during the initial sedimentation stage and pressing for 1~3 min to retain the porous structure of the flocs; increasing the pressure to 0.4~0.6 MPa during the main growth stage and pressing for 5~15 min to promote the migration of fine particles to the depth of the pores and prevent surface clogging; increasing the pressure to 1.0~2.0 MPa during the compaction dewatering stage and pressing for 5~10 min, and coordinating with air pressure flow to promote the removal of residual water.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1. This invention utilizes graft polymerization to achieve a carbon chain length of... Acrylic ester hydrophobic monomers were chemically bonded into the backbone of anionic polyacrylamide (APAM) to prepare graft-modified polyacrylamide HM-PAM. When HM-PAM is used for dewatering low-rank coal, it forms a high-porosity flocculent skeleton through the synergistic effects of charge neutralization, hydrophobic association, and adsorption bridging. This is combined with a gradient pressurization method for filter cake dewatering: lower pressure is applied during the cake formation stage to retain the porous structure of the flocculents; increased operating pressure during the main pressurization stage promotes the migration of fine particles to deeper pores and prevents surface clogging; and the highest operating pressure is applied during the deep dewatering stage, along with air pressure flow to promote the removal of residual moisture. Simultaneously, the hydrophobic interfaces formed by the long hydrophobic chains of HM-PAM on the pore walls of the filter cake reduce capillary resistance to water retention, accelerate the discharge of residual moisture, and improve dewatering efficiency. Through these synergistic effects, a filter cake with a loose structure, abundant pores, and low moisture content (22.5–24.3%) is ultimately obtained.

[0026] 2. In the preparation of HM-PAM, this invention uses emulsifier OP-10 to pre-emulsify the acrylate hydrophobic monomers, forming a fine emulsion with a particle size of 100-300 nm, increasing the contact area between the hydrophobic monomers and the active sites of the anionic polyacrylamide (APAM) backbone. Simultaneously, a constant-rate dropping method is used to maintain a low concentration of hydrophobic monomers in the system, effectively suppressing homopolymerization side reactions and improving grafting selectivity and efficiency. Furthermore, the initiator is added in steps: 60-70% of the total amount is added at the start of the reaction to maintain a stable free radical concentration, avoiding excessively high free radical concentrations in the early stages that could lead to chain transfer and chain termination; the remaining amount is added to ensure continuous polymerization in the later stages, improving monomer conversion. Through the above synergistic effect, the hydrophobic long chains are uniformly grafted onto the anionic polyacrylamide (APAM) backbone, with a grafting rate of over 75% and the residual monomer content reduced to below 0.4%. This gives HM-PAM better hydrophobic association ability, which is conducive to forming a uniform hydrophobic interface on the filter cake pore wall and further reducing capillary retention resistance. Detailed Implementation

[0027] The present invention will now be described in more detail through specific embodiments.

[0028] Example 1

[0029] A method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake includes the following steps:

[0030] Step S1: Deionized water and ethanol are mixed at a mass ratio of 95:5 to obtain a deionized water-ethanol mixed solvent. The mixed solvent is added to a reaction vessel. Under the conditions of stirring speed of 60 r / min and temperature of 35℃, anionic polyacrylamide (APAM) powder with a viscosity-average molecular weight of 8 million, a degree of hydrolysis of 20%, and an anionic degree of 1.5 mmol / g is added to dissolve and obtain a polyacrylamide solution with a mass fraction of 8%.

[0031] Step S2: Select a carbon chain length of... The hydrophobic monomer of acrylate (isooctyl acrylate) was added at 5% of the mass of APAM. The hydrophobic monomer was mixed with emulsifier OP-10 (nonylphenol polyoxyethylene ether) at a mass ratio of 100:2 and pre-emulsified in a high-speed shear emulsifier to obtain an emulsion with a particle size of 100 nm.

[0032] Step S3: High-purity nitrogen gas was bubbled into the reactor for 30 min to remove oxygen. Under nitrogen positive pressure of 0.03 MPa and stirring at 110 r / min, potassium persulfate-sodium bisulfite (KPS-SHS) redox initiator was added stepwise to the polyacrylamide solution. The total amount of initiator was 0.3% of the mass of the hydrophobic acrylate monomer, and the molar ratio of potassium persulfate (KPS) to sodium bisulfite (SHS) was 1:1. The stepwise addition was as follows: 60% of the total initiator was added first, followed by the emulsion being added dropwise at a rate of 2 mL / min, with the temperature controlled at 40℃ during the dropwise addition, and the reaction was allowed to proceed for 30 min; then the remaining 40% of the initiator was added, the temperature was raised to 50℃, the stirring speed was reduced to 90 r / min, and the reaction was allowed to proceed for 3 h; finally, the temperature was raised to 55℃, the stirring speed was reduced to 70 r / min, and the reaction was allowed to proceed for 2 h. The entire process was carried out under a nitrogen protective atmosphere. The reaction product was obtained through graft polymerization.

[0033] Step S4: After the graft polymerization reaction is completed, the temperature is lowered to 45℃, and 0.01% hydroquinone (by mass) of the acrylate hydrophobic monomer is added to quench residual free radicals. The temperature is maintained and stirred for 30 min to ensure the system is homogeneous and stable. Then, the product is cooled to 25℃ at a rate of 1℃ / min and discharged. The product is filtered through a 120-mesh stainless steel screen to remove gel particles, and then unreacted monomers, oligomers, and small molecule impurities are removed by ethanol precipitation. The purified material is vacuum dried at 45℃ and a vacuum degree of -0.09 MPa to obtain graft-modified polyacrylamide HM-PAM.

[0034] Step S5: Take coal slurry from a long-flame coal mine with a solids content of 12%, add 0.002% (by weight) of grafted modified polyacrylamide HM-PAM to the slurry, stir at 300 r / min for 1 min, then stir at 60 r / min for 5 min to form flocs. Then, use a gradient pressurization method for dewatering: apply 0.1 MPa pressure for 3 min during the initial sedimentation stage to retain the porous structure of the flocs; increase the pressure to 0.4 MPa for 15 min during the main growth stage to encourage fine particles to migrate to deeper pores and prevent surface clogging; increase the pressure to 1.0 MPa for 10 min during the compaction and dewatering stage, and use air pressure flow to promote the removal of residual moisture. After pressurization, a filter cake is obtained.

[0035] The grafting rate of the grafted modified polyacrylamide HM-PAM obtained in this embodiment was 75%, the intrinsic viscosity was 8.7 dL / g, the solid content was 90%, and the residual monomer content was 0.39%. The moisture content of the filter cake obtained after pressure filtration was 24.3%.

[0036] Example 2

[0037] A method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake includes the following steps:

[0038] Step S1: Deionized water and ethanol are mixed at a mass ratio of 92:8 to obtain a deionized water-ethanol mixed solvent. The mixed solvent is added to a reaction vessel. Under the conditions of stirring speed of 70 r / min and temperature of 38℃, anionic polyacrylamide (APAM) powder with a viscosity-average molecular weight of 10 million, a degree of hydrolysis of 25%, and an anionic degree of 2.0 mmol / g is added to dissolve and obtain a polyacrylamide solution with a mass fraction of 10%.

[0039] Step S2: Select a carbon chain length of C 12 The hydrophobic monomer of acrylate (dodecyl acrylate) was added at 10% of the mass of APAM. The hydrophobic monomer was mixed with emulsifier OP-10 (nonylphenol polyoxyethylene ether) at a mass ratio of 100:3 and pre-emulsified in a high-speed shear emulsifier to obtain an emulsion with a particle size of 200 nm.

[0040] Step S3: High-purity nitrogen gas was bubbled into the reactor for 30 min to remove oxygen. Under nitrogen positive pressure of 0.03 MPa and stirring at 110 r / min, potassium persulfate-sodium bisulfite (KPS-SHS) redox initiator was added stepwise to the polyacrylamide solution. The total amount of initiator was 0.6% of the mass of the hydrophobic acrylate monomer, and the molar ratio of potassium persulfate (KPS) to sodium bisulfite (SHS) was 1:1.15. The stepwise addition was as follows: first, 65% of the total initiator was added, then the emulsion was added dropwise at a rate of 3 mL / min, with the temperature controlled at 42℃ during the dropwise addition, and the reaction was allowed to proceed for 30 min; then, the remaining 35% of the initiator was added, the temperature was raised to 52℃, the stirring speed was reduced to 90 r / min, and the reaction was allowed to proceed for 2.5 h; finally, the temperature was raised to 58℃, the stirring speed was reduced to 70 r / min, and the reaction was allowed to proceed for 1.5 h. The entire process was carried out under a nitrogen protective atmosphere. The reaction product was obtained through graft polymerization.

[0041] Step S4: After the graft polymerization reaction is completed, the temperature is lowered to 48℃, and 0.03% hydroquinone (by mass) of the acrylate hydrophobic monomer is added to quench residual free radicals. The temperature is maintained and stirred for 30 min to ensure the system is homogeneous and stable. Then, the product is cooled to 28℃ at a rate of 1.5℃ / min and discharged. The product is filtered through a 100-mesh stainless steel screen to remove gel particles, and then unreacted monomers, oligomers, and small molecule impurities are removed by ethanol precipitation. The purified material is vacuum dried at 45℃ and a vacuum degree of -0.09 MPa to obtain graft-modified polyacrylamide HM-PAM.

[0042] Step S5: Take lignite slurry from a certain mine, with a solids content of 15%, and add 0.005% (by weight) of grafted modified polyacrylamide HM-PAM to the slurry. Stir at 300 r / min for 1 min, then stir at 60 r / min for 5 min to form flocs. Then, use a gradient pressurization method for dewatering: apply 0.15 MPa pressure for 2 min during the initial sedimentation stage to retain the porous structure of the flocs; increase the pressure to 0.5 MPa for 10 min during the main growth stage to encourage fine particles to migrate to deeper pores and prevent surface clogging; increase the pressure to 1.5 MPa for 8 min during the compaction and dewatering stage, and use air pressure flow to promote the removal of residual moisture. After pressurization, a filter cake is obtained.

[0043] The grafting rate of the grafted modified polyacrylamide HM-PAM obtained in this embodiment was 78%, the intrinsic viscosity was 9.5 dL / g, the solid content was 92%, and the residual monomer content was 0.31%. The moisture content of the filter cake obtained after pressure filtration was 22.5%.

[0044] Example 3

[0045] A method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake includes the following steps:

[0046] Step S1: Deionized water and ethanol are mixed at a mass ratio of 90:10 to obtain a deionized water-ethanol mixed solvent. The mixed solvent is added to a reaction vessel. Under the conditions of stirring speed of 80 r / min and temperature of 40℃, anionic polyacrylamide (APAM) powder with a viscosity-average molecular weight of 12 million, a degree of hydrolysis of 30%, and an anionic degree of 2.5 mmol / g is added to dissolve and obtain a polyacrylamide solution with a mass fraction of 12%.

[0047] Step S2: Select a carbon chain length of C 16 The hydrophobic monomer of acrylate (hexadecyl acrylate) was added at 15% of the mass of APAM. After the hydrophobic monomer was heated and melted, it was mixed with emulsifier OP-10 (nonylphenol polyoxyethylene ether) at a mass ratio of 100:5. The mixture was pre-emulsified in a high-speed shear emulsifier to obtain an emulsion with a particle size of 300 nm.

[0048] Step S3: High-purity nitrogen gas was bubbled into the reactor for 30 min to remove oxygen. Under nitrogen positive pressure of 0.03 MPa and stirring at 110 r / min, potassium persulfate-sodium bisulfite (KPS-SHS) redox initiator was added stepwise to the polyacrylamide solution. The total amount of initiator was 0.8% of the mass of the hydrophobic acrylate monomer, and the molar ratio of potassium persulfate (KPS) to sodium bisulfite (SHS) was 1:1.2. The stepwise addition was as follows: 70% of the total initiator was added first, followed by the emulsion being added dropwise at a rate of 5 mL / min, with the temperature controlled at 45℃ during the dropwise addition, and the reaction was allowed to proceed for 30 min; then the remaining 30% of the initiator was added, the temperature was raised to 55℃, the stirring speed was reduced to 90 r / min, and the reaction was allowed to proceed for 2 h; finally, the temperature was raised to 60℃, the stirring speed was reduced to 70 r / min, and the reaction was allowed to proceed for 1 h. The entire process was carried out under a nitrogen protective atmosphere. The reaction product was obtained through graft polymerization.

[0049] Step S4: After the graft polymerization reaction is completed, the temperature is lowered to 50℃, and 0.05% hydroquinone (by mass) of the acrylate hydrophobic monomer is added to quench residual free radicals. The temperature is maintained and stirred for 30 min to ensure the system is homogeneous and stable. Then, the product is cooled to 30℃ at a rate of 2℃ / min and discharged. The product is filtered through an 80-mesh stainless steel screen to remove gel particles, and then unreacted monomers, oligomers, and small molecule impurities are removed by ethanol precipitation. The purified material is vacuum dried at 45℃ and a vacuum degree of -0.09 MPa to obtain graft-modified polyacrylamide HM-PAM.

[0050] Step S5: Take non-sticky coal slurry from a mine, with a solids content of 18%, and add 0.01% (by weight) of grafted modified polyacrylamide HM-PAM to the slurry. Stir at 300 r / min for 1 min, then at 60 r / min for 5 min to form flocs. Then, use a gradient pressurization method for dewatering: apply 0.2 MPa pressure for 1 min during the initial sedimentation stage to retain the porous structure of the flocs; increase the pressure to 0.6 MPa for 5 min during the main growth stage to encourage fine particles to migrate to deeper pores and prevent surface clogging; increase the pressure to 2.0 MPa for 5 min during the compaction and dewatering stage, and use air pressure flow to promote the removal of residual moisture. After pressurization, a filter cake is obtained.

[0051] The grafting rate of the grafted modified polyacrylamide HM-PAM obtained in this embodiment was 76%, the intrinsic viscosity was 9.0 dL / g, the solid content was 91%, and the residual monomer content was 0.35%. The moisture content of the filter cake obtained after pressure filtration was 23.2%.

[0052] Comparative Example 1

[0053] The difference between Comparative Example 1 and Example 2 is that no reagents were added, and a purely mechanical pressure filtration method was used.

[0054] Specifically, Comparative Example 1 provides a method for dewatering low-rank coal, which includes the following steps:

[0055] Lignite slurry from a certain mine, with a solids content of 15%, was dewatered by pressure filtration using a gradient pressurization method: first, a pressure of 0.15 MPa was applied for 2 minutes; then the pressure was increased to 0.5 MPa for 10 minutes; finally, the pressure was increased to 1.5 MPa for 8 minutes, with air pressure flow used to promote the removal of residual moisture. After pressure filtration, a filter cake was obtained.

[0056] The moisture content of the filter cake obtained after pressure filtration in Comparative Example 1 was found to be 35.8%.

[0057] Comparative Example 2

[0058] The difference between Comparative Example 2 and Example 2 is that the same anionic polyacrylamide (APAM) as in Example 2 was used, but no grafting modification was performed.

[0059] Specifically, Comparative Example 2 provides a method for dewatering low-rank coal, which includes the following steps:

[0060] Take lignite slurry from a certain mine, with a solids content of 15%, and add 0.005% anionic polyacrylamide (APAM) by weight of the slurry. Stir at 300 r / min for 1 min, then stir at 60 r / min for 5 min. Then, dewater the mixture by pressure filtration using a gradient pressurization method, with the same filtration conditions as in Example 2. After pressure filtration, a filter cake is obtained.

[0061] The moisture content of the filter cake obtained after pressure filtration in Comparative Example 2 was found to be 30.2%.

[0062] Comparative Example 3

[0063] The difference between Comparative Example 3 and Example 2 is that it does not include step S2. In step S3: high-purity nitrogen gas is bubbled into the reaction vessel for 30 min to remove oxygen. Under nitrogen positive pressure of 0.03 MPa and stirring at 110 r / min, potassium persulfate-sodium bisulfite (KPS-SHS) redox initiator is added stepwise to the polyacrylamide solution. The total amount of initiator is 0.6% of the mass of the hydrophobic acrylate monomer, and the molar ratio of potassium persulfate (KPS) to sodium bisulfite (SHS) is 1:1.15. Specifically, the stepwise addition is as follows: first, 65% of the total initiator is added, and then a carbon chain with a length of C is added dropwise at a rate of 3 mL / min. 12 The hydrophobic monomer of acrylate (dodecyl acrylate) was added dropwise at a total amount of 10% of the mass of APAM, with the temperature controlled at 42°C during the dropwise addition, and the reaction was carried out for 30 min. Then, the remaining 35% of the initiator was added, the temperature was raised to 52°C, the stirring speed was reduced to 90 r / min, and the reaction was carried out for 2.5 h. Finally, the temperature was raised to 58°C, the stirring speed was reduced to 70 r / min, and the reaction was carried out for 1.5 h. The entire process was carried out under a nitrogen protective atmosphere, and the reaction product was obtained through graft polymerization. The remaining steps were the same as in Example 2.

[0064] Testing revealed that the grafting rate of the grafted modified polyacrylamide HM-PAM obtained in Comparative Example 3 was 70%, the intrinsic viscosity was 8.1 dL / g, the solid content was 88%, and the residual monomer content was 1.1%. The moisture content of the filter cake obtained after pressure filtration was 25.6%.

[0065] Comparative Example 4

[0066] The difference between Comparative Example 4 and Example 2 is that in step S3: high-purity nitrogen gas is bubbled into the reactor for 30 min to remove oxygen. Under the protection of nitrogen positive pressure of 0.03 MPa and stirring at 110 r / min, potassium persulfate-sodium bisulfite (KPS-SHS) redox initiator is added to the polyacrylamide solution in one step. The amount of initiator is 0.6% of the mass of the hydrophobic acrylate monomer, and the molar ratio of potassium persulfate (KPS) to sodium bisulfite (SHS) is 1:1.15. Then, the emulsion is added dropwise at a rate of 3 mL / min, and the temperature is controlled at 42°C during the dropwise addition. The reaction is carried out for 30 min. The temperature is then raised to 52°C, the stirring speed is reduced to 90 r / min, and the reaction is carried out for 2.5 h. Finally, the temperature is raised to 58°C, the stirring speed is reduced to 70 r / min, and the reaction is carried out for 1.5 h. The entire process is carried out under a nitrogen protective atmosphere. The reaction product is obtained through graft polymerization. The remaining steps are the same as in Example 2.

[0067] Testing revealed that the grafting rate of the grafted modified polyacrylamide HM-PAM obtained in Comparative Example 4 was 66%, the intrinsic viscosity was 7.8 dL / g, the solid content was 83%, and the residual monomer content was 1.7%. The moisture content of the filter cake obtained after pressure filtration was 26.0%.

[0068] Comparative Example 5

[0069] The difference between Comparative Example 5 and Example 2 is that in step S5: take lignite slurry from a certain mine with a solid content of 15%, add 0.005% of grafted modified polyacrylamide HM-PAM by weight of the slurry, stir at 300 r / min for 1 min, and then stir at 60 r / min for 5 min to form flocs; then perform pressure filtration dewatering: apply 0.5 MPa, filter for 20 min, and after the filtration is completed, obtain filter cake; the remaining steps are the same as in Example 2.

[0070] The moisture content of the filter cake obtained after pressure filtration in Comparative Example 5 was found to be 27.3%.

[0071] The test results from Examples 1-3 above show that the present invention uses emulsifier OP-10 to affect carbon chain lengths of... Graft-modified polyacrylamide (HM-PAM) was prepared by pre-emulsifying acrylate hydrophobic monomers and adding initiators stepwise to graft the hydrophobic monomers onto anionic polyacrylamide (APAM). The grafting rate reached 75-78%, the intrinsic viscosity reached 8.7-9.5 dL / g, the solid content reached 90-92%, and the residual monomer content was as low as 0.31-0.39%. Using the prepared graft-modified polyacrylamide HM-PAM, low-rank coal slurry was dewatered by pressure filtration using a gradient pressurization method, resulting in a filter cake with a moisture content as low as 22.5-24.3%.

[0072] In contrast, Comparative Example 1, which used only mechanical pressure filtration to dewater low-rank coal slurry, produced a filter cake with a moisture content as high as 35.8%, which was higher than the 22.5% in Example 2. This demonstrates that the present invention, through the synergistic effect of flocculation and hydrophobicity of grafted modified polyacrylamide HM-PAM, combined with a graded gradient pressure filtration method, can effectively destroy the hydration film on the surface of low-rank coal, construct a high-porosity filter cake skeleton, and reduce capillary water retention resistance, thereby significantly improving the dewatering effect.

[0073] Comparative Example 2, which used ungrafted modified anionic polyacrylamide (APAM) to dewater low-rank coal slurry water by pressure filtration, yielded a filter cake with a moisture content as high as 30.2%, higher than the 22.5% of Example 2. This demonstrates that the present invention introduces hydrophobic long chains onto the APAM main chain through chemical grafting, enabling the modified product to possess both flocculation and hydrophobic functions. Compared with ordinary APAM (Comparative Example 2), the grafted modified polyacrylamide HM-PAM of the present invention can not only form large-sized flocs through charge neutralization and adsorption bridging, but its hydrophobic side chains can also form a hydrophobic interface on the pore wall of the filter cake, effectively reducing the capillary water retention resistance, thereby improving the dewatering effect of low-rank coal.

[0074] Comparative Example 3, which did not pre-emulsify the acrylate hydrophobic monomers, yielded a grafting rate of only 70% for the grafted modified polyacrylamide HM-PAM, with an intrinsic viscosity of 8.1 dL / g, both lower than that of Example 2. When used for pressure filtration dewatering of low-rank coal slurry, the resulting filter cake had a moisture content as high as 25.6%, higher than the 22.5% of Example 2. This indicates that the pre-emulsification-constant-rate dripping method of this invention can pre-disperse the acrylate hydrophobic monomers into nanoscale fine emulsions, increasing the contact area and reaction probability between the hydrophobic monomers and the active sites of the APAM backbone, effectively inhibiting the homopolymerization reaction of the hydrophobic monomers themselves, thereby obtaining a higher grafting rate. The increased grafting rate allows more hydrophobic long chains to be successfully introduced into the APAM backbone. These hydrophobic long chains can form a dense hydrophobic interface on the pore wall of the filter cake during pressure filtration, significantly reducing the capillary water retention resistance, thereby reducing the moisture content of the filter cake.

[0075] Comparative Example 4, which uses a one-time addition of initiator, shows that the grafting rate of the grafted modified polyacrylamide HM-PAM is only 66%, and the intrinsic viscosity is reduced to 7.8 dL / g, both lower than that of Example 2. When used to dewater low-rank coal slurry, the moisture content of the resulting filter cake is as high as 26.0%, higher than 22.5% in Example 2. This indicates that the stepwise addition of initiator in this invention can effectively maintain a stable free radical concentration during polymerization, avoiding the aggravation of chain transfer and chain termination reactions due to excessively high free radical concentration in the early stage, thereby obtaining a higher grafting rate. Furthermore, the subsequent addition of initiator ensures monomer conversion rate, reduces the plasticizing effect of residual monomer on polymer, and ultimately endows the product with better flocculation and filter cake pore control capabilities, thereby reducing the moisture content of the filter cake.

[0076] Comparative Example 5, which dewatered low-rank coal slurry water by constant pressure filtration, yielded a filter cake with a moisture content as high as 27.3%, higher than the 22.5% of Example 2. This demonstrates that the present invention can improve the dewatering effect of low-rank coal slurry water through a gradient pressurization method. Specifically, applying a smaller pressure during the filter cake formation stage can preserve the porous structure of the flocs; increasing the operating pressure during the main pressurization stage can promote the migration of fine particles to the depth of the pores and prevent surface clogging; applying the highest operating pressure during the deep dewatering stage, combined with air pressure flow, promotes the removal of residual moisture; the hydrophobic interface formed by the HM-PAM hydrophobic long chains on the pore wall of the filter cake can reduce the capillary retention resistance of water and accelerate the discharge of residual moisture. Through this gradient pressurization method, a filter cake with a loose structure, rich pores, and lower moisture content can ultimately be obtained.

[0077] The above embodiments are merely illustrative of the present invention and are not intended to limit the invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake, characterized in that, Includes the following steps: Step S1: Dissolve anionic polyacrylamide in a deionized water-ethanol mixed solvent to obtain a polyacrylamide solution with a mass fraction of 8-12%. Step S2: pre-emulsify the acrylate hydrophobic monomer with carbon chain length of 100-300 nm in a high-speed shearing emulsifier; Step S3: Under nitrogen protection and stirring conditions, potassium persulfate-sodium bisulfite redox initiator is added stepwise to the polyacrylamide solution, and then the emulsion is added dropwise at a rate of 2~5 mL / min. Under the action of the initiator, the graft polymerization reaction is carried out to obtain the reaction product. Step S4: Quench, cool, filter, purify and dry the reaction product to obtain grafted modified polyacrylamide; Step S5: Add the grafted modified polyacrylamide to the low-rank coal slurry water, stir and mix evenly to form flocs, and then perform pressure filtration and dewatering to obtain filter cake.

2. The low-rank coal dewatering method based on graft-modified polyacrylamide for regulating filter cake pore structure according to claim 1, characterized in that, In step S1, the anionic polyacrylamide has a viscosity-average molecular weight of 8 million to 12 million, a degree of hydrolysis of 20% to 30%, and an anionicity of 1.5 to 2.5 mmol / g.

3. The low-rank coal dewatering method based on graft-modified polyacrylamide for regulating filter cake pore structure according to claim 1, characterized in that, In step S1, the amount of ethanol used is 5 to 10% of the total mass of the deionized water-ethanol mixed solvent.

4. The low-rank coal dewatering method based on graft-modified polyacrylamide for regulating filter cake pore structure according to claim 1, characterized in that, In step S2, the amount of the acrylate hydrophobic monomer used is 5-15% of the mass of the anionic polyacrylamide.

5. The low-rank coal dewatering method based on graft-modified polyacrylamide for regulating filter cake pore structure according to claim 4, characterized in that, In step S2, the emulsifier is nonylphenol polyoxyethylene ether, and the amount of emulsifier used is 2 to 5% of the mass of the acrylate hydrophobic monomer.

6. The method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of the filter cake as described in claim 4, characterized in that, In step S3, the amount of potassium persulfate-sodium bisulfite redox initiator is 0.3-0.8% of the mass of the acrylate hydrophobic monomer, and the molar ratio of potassium persulfate to sodium bisulfite is 1:1-1.

2.

7. The method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake as described in any one of claims 1 to 6, characterized in that, Step S3 specifically includes: under nitrogen protection and stirring conditions, adding 60-70% of the total amount of potassium persulfate-sodium bisulfite redox initiator to the polyacrylamide solution, then uniformly adding the emulsion at a rate of 2-5 mL / min, controlling the temperature at 40-45℃ during the addition process, and reacting for 30 min; then adding the remaining amount of potassium persulfate-sodium bisulfite redox initiator, raising the temperature to 50-55℃ and reacting for 2-3 h, and finally raising the temperature to 55-60℃ and reacting for 1-2 h, obtaining the reaction product through graft polymerization.

8. The method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake as described in any one of claims 1 to 6, characterized in that, Step S4 specifically includes: after the graft polymerization reaction is completed, the temperature is lowered to 45~50℃, and 0.01~0.05% of hydroquinone (by mass of acrylate hydrophobic monomer) is added to quench residual free radicals. The temperature is maintained and stirred for 30 min to ensure the system is homogeneous and stable. Then, the product is cooled to 25~30℃ at a rate of 1~2℃ / min and discharged. The product is filtered through an 80~120 mesh stainless steel screen to remove gel particles, and then unreacted monomers, oligomers and small molecule impurities are removed by ethanol precipitation or dialysis. The purified material is vacuum dried or spray dried to obtain graft-modified polyacrylamide.

9. The method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of filter cake as described in any one of claims 1 to 6, characterized in that, In step S5, the amount of grafted modified polyacrylamide added is 0.002~0.01% of the total weight of low-rank coal slurry water.

10. The method for dewatering low-rank coal based on graft-modified polyacrylamide to regulate the pore structure of the filter cake as described in claim 9, characterized in that, In step S5, the pressure filtration dewatering adopts a gradient pressurization method, specifically including: applying a pressure of 0.1~0.2 MPa during the initial sedimentation stage and pressing for 1~3 min to retain the porous structure of the flocs; increasing the pressure to 0.4~0.6 MPa during the main growth stage and pressing for 5~15 min to promote the migration of fine particles to the depth of the pores and prevent surface clogging; increasing the pressure to 1.0~2.0 MPa during the compaction dewatering stage and pressing for 5~10 min, and coordinating with air pressure flow to promote the removal of residual water.

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