Composite modified coal-based solid waste filler as well as preparation method and application thereof

Abstract

The invention discloses a composite modified coal-based solid waste filler as well as a preparation method and application thereof, and belongs to the technical field of inorganic solid waste recycling and green packaging materials. According to the process, the composite modified filler is obtained through multi-step synergistic modification of acid treatment, alkali treatment, silane coupling agent treatment and cationic polymer treatment in sequence; and mixing the filler with defibered plant paper pulp fibers, and carrying out wet molding and drying to obtain the high-performance packaging material. According to the synergistic modification process, high-content (35-55 wt%) filling of the coal-based solid waste is realized, the prepared material has excellent mechanical properties and environmental compatibility, the wet strength retention rate is greater than or equal to 70%, and the water absorption rate is less than or equal to 5%.

Description

Technical Field

[0001] This application belongs to the technical field of inorganic solid waste resource utilization and green composite packaging materials, specifically involving a composite modified coal-based solid waste filler, its preparation method and application. Background Technology

[0002] With the strengthening of global environmental regulations and the deepening of the concept of sustainable development, the development of fully biodegradable and renewable green packaging materials has become an urgent need. Paper-based packaging materials, primarily made from plant fibers (such as molded pulp products and cardboard made from waste paper fibers), are considered an important alternative to traditional plastics due to their renewability and biodegradability. However, current disposal methods (stockpiling or utilization in low-value-added building materials) pose environmental risks and low economic benefits. Therefore, seeking new ways to achieve high-value and large-scale utilization of coal-based solid waste is of great significance.

[0003] Using coal-based solid waste as filler in plant fiber-based composites can reduce costs. However, due to the extremely poor interfacial compatibility between coal-based solid waste (hydrophilic inorganic matter) and plant fibers, direct mixing not only fails to strengthen the composite but also disrupts the hydrogen bond network between fibers, leading to a severe decline in the mechanical properties of the composite and causing filler detachment (powdering). Especially when using wet molding to prepare composites with high filler content, the small size of the modified filler particles makes them easily lost with water during the molding and dehydration process, resulting in an actual filler content far lower than the design value, severely affecting performance consistency and material utilization. Existing technologies often address this problem by adding excessive retention aids or using extremely expensive specialized molding equipment, leading to high costs and environmental pollution. To improve compatibility, existing technologies typically employ a single physical or chemical modification method, or a simple combination of steps. However, traditional methods have the following deep-seated limitations: First, they usually rely on only a single mechanism of action, making it difficult to cope with complex interfacial stresses under high filler content, especially prone to failure in humid environments. Secondly, existing technologies do not consider the synergistic effects between various modification steps for special fillers such as coal-based solid waste, which have complex surface compositions and uneven activity. Finally, existing methods lack systematic design and cannot construct a multi-layered, composite interface structure between the filler and fibers that integrates physical anchoring, chemical bonding, and strong electrostatic adsorption. Therefore, existing technologies struggle to achieve high-proportion filling of coal-based solid waste (e.g., >30%, especially reaching 35-55%) while simultaneously ensuring excellent overall performance of the composite material, particularly high wet strength retention (≥70%) and low water absorption (≤5%), and preventing powder shedding.

[0004] In summary, there is an urgent need for a process and composite material preparation method that can solve the above problems, so as to achieve high-value utilization of coal-based solid waste and produce high-performance, cost-controllable biodegradable and environmentally friendly packaging materials. Summary of the Invention

[0005] To achieve the above objectives, this application provides a method for preparing a composite modified coal-based solid waste filler, wherein the coal-based solid waste is subjected to acid treatment, alkali treatment, silane coupling agent treatment and cationic polymer treatment in sequence to obtain the composite modified coal-based solid waste filler. The acid treatment uses an acid concentration of 0.5-5 wt% and a treatment temperature of 20-80℃. The alkaline treatment uses an alkaline solution concentration of 1-10 wt% and a treatment temperature of 40-90℃. The silane coupling agent treatment involves adding alkali-treated coal-based solid waste into a pre-hydrolyzed silane coupling agent for reaction. The cationic polymer is a polymer containing quaternary ammonium salt groups.

[0006] Furthermore, the coal-based solid waste is selected from at least one of fly ash, gasification slag, or coal gangue, and its particle size D90 ≤ 100 μm.

[0007] Furthermore, the plant fiber-based packaging material comprises the following raw materials in parts by weight: 45-90 parts plant fiber and 35-55 parts composite modified coal-based solid waste filler; The composite modified coal-based solid waste filler is obtained by the preparation method according to any one of claims 1-2.

[0008] Furthermore, the raw materials include 45-65 parts of plant fiber and 35-55 parts of composite modified coal-based solid waste filler.

[0009] Furthermore, the raw material also includes one or more of the following additives: 0.5-2.0 parts of wet strength agent, 0.1-0.5 parts of retention and filtration aid, and 0.5-3.0 parts of softener.

[0010] On the other hand, this application also provides a method for preparing the plant fiber-based packaging material as described above, comprising the following steps: S1, the composite modified coal-based solid waste filler obtained by the above preparation method is mixed with the sludged plant fiber to form a composite slurry; S2, the composite slurry in S1 is wet-formed and then dried to obtain plant fiber-based packaging material.

[0011] Furthermore, the composite modified coal-based solid waste filler is present in the plant fiber-based packaging material at a mass percentage of 10%-55%.

[0012] Furthermore, the mass percentage is 35%-55%.

[0013] Furthermore, the wet molding process in S2 includes the following steps: performing initial solid-liquid separation on the composite slurry, collecting the filtrate generated from the initial solid-liquid separation, and reusing the filtrate for the re-molding of the same batch of slurry.

[0014] On the other hand, this application also provides the application of the composite modified coal-based solid waste filler obtained according to the above preparation method in the preparation of plant fiber-based packaging materials.

[0015] Beneficial effects

[0016] (1) Overcoming the bottleneck of interfacial compatibility under high filling conditions. The four-step sequential synergistic modification strategy of "acid-base-silane-polymer" provided in this application is not a simple superposition of single methods. This sequential design has strict logical dependence and functional synergy: acid treatment cleans and micro-etches the surface, base treatment further activates it by hydroxylation, laying the foundation for subsequent reactions; silane coupling agent constructs strong chemical bridge bonds; and the final cationic polymer coating achieves strong electrostatic anchoring and covers residual active sites. This systematic process constructs a multi-layered, composite interfacial structure integrating "physical anchoring, chemical bonding and electrostatic adsorption" between coal-based solid waste and plant fibers, fundamentally solving the core problem of weak interfacial bonding under high filling content.

[0017] (2) High-value utilization of solid waste and high-performance materials have been achieved. Thanks to the strong interfacial bonding mentioned above, this application has achieved a high proportion of coal-based solid waste filling in plant fiber matrix (up to 35-55 wt%), opening up a new high-value-added path for the resource utilization of bulk solid waste. The prepared composite material exhibits excellent comprehensive performance, with a wet strength retention rate of not less than 70% and a water absorption rate of not more than 5%, fully meeting the requirements for use in humid environments, and its performance is significantly better than that of materials prepared by unmodified or single modified fillers.

[0018] (3) This application uses solid waste (coal-based solid waste and waste paper fiber) as the main raw materials. The product is biodegradable and environmentally friendly. The process is simple, and while maintaining excellent performance, it significantly reduces the raw material cost of traditional plant fiber packaging materials and reduces solid waste disposal costs, thus possessing outstanding industrialization advantages.

[0019] (4) Wide range of applications. The packaging materials made by this method have excellent performance and can be used to manufacture various packaging and structural materials that require strength and water resistance, including but not limited to cardboard, pulp molded products, corrugated cardboard, honeycomb boards, cushioning and shockproof linings, egg trays, fruit trays and seedling containers.

[0020] (5) It ensures the reliability and economy of the high-filling process. The innovative multi-step molding process effectively solves the problem of loss of high-content fine fillers in conventional wet molding. Without relying on expensive special equipment, it achieves efficient retention of fillers (>95%), ensuring the precise control of filler content and stable performance of the product, and greatly improving the utilization rate of raw materials and the universality of the process. Attached Figure Description

[0021] The above and other objects, features and advantages of this application will become more apparent from the more detailed description of exemplary embodiments thereof in conjunction with the accompanying drawings.

[0022] Figure 1 The images show the surface morphology of fly ash after synergistic modification at various stages.

[0023] Figure 2 It is an infrared spectrum of the changes in surface functional groups of fly ash after synergistic modification at various stages.

[0024] Figure 3 The specific surface area and pore structure of fly ash after synergistic modification at various stages are shown (left: nitrogen adsorption-desorption curve, right: pore size distribution curve).

[0025] Figure 4 This is a Zeta potential diagram of fly ash after synergistic modification at various stages.

[0026] Figure 5 These are microscopic morphology images of fly ash added to pulp fibers before and after treatment; A is the sample with unmodified fly ash, i.e., No. 1 fly ash, and B is the sample with fly ash modified by the four systems described in this application, i.e., No. 5 fly ash. Detailed Implementation

[0027] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. While embodiments of this application are shown in the drawings, it should be understood that this application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0028] Unless otherwise specified, all materials and reagents used in the following examples are purchased from the market and are available to the public.

[0029] The purpose of this application is to provide a synergistic surface modification method for improving the interfacial compatibility between coal-based solid waste and plant fibers, a modified filler prepared by this method, and the application of this filler in the preparation of high-performance biodegradable packaging materials. This technical solution aims to systematically solve the interfacial bonding problem when coal-based solid waste is used as a high-proportion filler through a specific multi-step synergistic process, thereby achieving high-value utilization of bulk solid waste while obtaining plant fiber-based packaging materials with excellent performance and water resistance.

[0030] To fully disclose and verify the technical effects of this application, and to demonstrate its inventiveness and progress compared to the prior art, specific comparative examples and embodiments are provided below. These experimental data will clearly demonstrate the necessity, sequential dependence, and unexpected technical effects of the "four-step synergistic modification" strategy of this application. The following are the analytical characterization results of fly ash undergoing each treatment process: the original fly ash sample is recorded as 1#; the ash sample after acid treatment is recorded as 2#; the ash sample after acid treatment + alkali treatment is recorded as 3#; the ash sample after acid treatment + alkali treatment + silane treatment is recorded as 4#; and the ash sample after acid treatment + alkali treatment + silane treatment + ammonium chloride treatment is recorded as 5#.

[0031] Coal-based solid waste (selected from at least one of fly ash, gasification slag or coal gangue, with a particle size D90≤100μm) is subjected to surface functionalization through the following four steps to obtain composite modified coal-based solid waste filler.

[0032] (1) Acid treatment: The coal-based solid waste is placed in an acid solution with a concentration of 0.5-5 wt% and treated at 20-80℃. The acid is an inorganic acid (such as hydrochloric acid or sulfuric acid) or an organic acid (such as citric acid or acetic acid).

[0033] (2) Alkali treatment: The acid-treated coal-based solid waste is placed in an inorganic alkaline solution (such as sodium hydroxide solution) with a concentration of 1-10 wt% and treated at 40-90℃.

[0034] (3) Silane coupling agent treatment: The coal-based solid waste after alkali treatment is reacted with a silane coupling agent. The silane coupling agent is of the type that can combine with the surface of the filler through hydrolysis and condensation, and its organic functional groups (such as amine groups and epoxy groups) can form chemical bonds or electrostatic interactions with the fiber.

[0035] (4) Cationic polymer treatment: The coal-based solid waste treated with silane coupling agent is mixed with a cationic polymer solution for treatment. The cationic polymer is a water-soluble polymer containing quaternary ammonium salt groups or secondary / tertiary amino groups (such as polydimethyldiallylammonium chloride, polyethyleneamine).

[0036] The present application will be further described in detail below through embodiments, but the scope of protection of the present application is not limited to the content described herein. Example 1: Preparation of Synergistically Modified Coal-Based Solid Waste Packing Material 1. Experimental Procedure a. Acid treatment: Take 200g of dry fly ash (particle size D50=45μm), add 2000ml of 2wt% hydrochloric acid solution, and mechanically stir at 50℃ for 60min. After the reaction, filter, wash with deionized water until the pH of the filtrate is ≈7, dry at 105℃ to constant weight, and set aside the filtrate for later use.

[0037] b. Alkali treatment: Add 2000 ml of a 3 wt% sodium hydroxide solution to the acid-treated fly ash and stir at 70°C for 90 min. Filter, wash with deionized water until the pH of the filtrate is approximately 9, dry, and set aside the filtrate for later use.

[0038] c. Silane Coupling Agent Treatment: Disperse 100g of alkali-treated fly ash in 300ml of ethanol / water (9:1 volume ratio) mixed solvent. Add 1.5g of KH550 silane coupling agent (pre-hydrolyzed for 30min), and reflux at 80℃ for 120min. Filter, wash with ethanol, dry, and collect the ethanol-water treatment solution. Determine the residual concentration of KH550 in the solution by gas chromatography, and replenish with fresh KH550 to the initial concentration, then use directly for the next batch treatment.

[0039] d. Cationic polymer treatment: Silane-treated fly ash was dispersed in 500 ml of deionized water, and 5 g of polydimethyldiallylammonium chloride (PDADMAC, 20% solids content, cationic polymer) solution was added. The mixture was stirred and coated at 60 °C for 60 min. After filtration, drying, and grinding, composite modified fly ash filler was obtained, and the aqueous phase treatment solution was collected. The residual concentration of PDADMAC was determined by colloidal titration, and fresh PDADMAC solution was added to the initial concentration for direct use in the next batch treatment.

[0040] Comparative Example 1: Unmodified coal-based solid waste filling (1) Take 100g of dry fly ash (particle size D50=45μm), without any surface treatment, and use it directly as filler.

[0041] (2) 500g of oven-dried waste paper pulp board was broken down into a fiber suspension with a concentration of about 3.5%.

[0042] (3) Add the above unmodified fly ash to the fiber suspension and stir to mix evenly so that the target filling amount of the filler is 40% of the total dry material.

[0043] (4) Add 1.5% of the total oven-dry weight of polyamide epichlorohydrin resin, 0.3% of cationic polyacrylamide, and 1.0% of polyvinyl alcohol, and continue stirring until homogeneous. Form the paper using a standard paper forming machine, and hot-press and dry it at 95℃ and 0.5MPa for 5 minutes to obtain a yield of approximately 300 g / m³.2 Paper-based materials.

[0044] Comparative Example 2: Acid-treated modification only (1) Take 100g of dry fly ash, add 1000ml of 2wt% hydrochloric acid solution, and stir at 50℃ for 60min. Filter, wash with water until neutral, and dry.

[0045] (2) The subsequent steps are exactly the same as those in Comparative Example 1 (steps 2-5), using only acid-treated fly ash as filler, with a filling amount of 40%.

[0046] Comparative Example 3: Silane Coupling Agent Treatment Only (1) Take 100g of dry fly ash and mix it directly with 3-aminopropyltriethoxysilane (KH550, the amount of which is 1.5% of the mass of fly ash) that has been pre-hydrolyzed with ethanol-water solution, and react at 80℃ for 90min. Filter and dry.

[0047] (2) The subsequent steps are exactly the same as those in Comparative Example 1 (steps 2-5), except that fly ash treated with only silane is used as filler.

[0048] Comparative Example 4: Acid treatment + silane coupling agent treatment (two-step method) (1) Take 100g of dry fly ash and first treat it with acid according to the method of Comparative Example 2.

[0049] (2) The acid-treated fly ash is then treated with silane coupling agent according to the method of Comparative Example 3.

[0050] Comparative Example 5: Treatment with scrambled order (base-acid-silane-polymer) (1) Take 100g of dry fly ash, place it in a 5wt% sodium hydroxide solution, and treat it at 70℃ for 90min. Filter and wash with water.

[0051] (2) Then place it in a 2wt% hydrochloric acid solution and treat it at 50°C for 60 min. Filter, wash with water until neutral, and dry.

[0052] (3) The same silane coupling agent treatment as in Comparative Example 3 and the same cationic polymer treatment as in Example 1 were subsequently performed.

[0053] Example 2 Detection Results Figure 1 These are SEM images of the original fly ash sample and the fly ash sample after each stage of co-processing. Figure 1It can be seen that the original fly ash sample (1#) has a dense surface with a small number of irregular particles firmly adhered to the particle surface, exhibiting a typical spherical shape. After acid treatment (2#), obvious micro-etching phenomena appeared on the sample surface, with some glassy material being dissolved, forming a worm-like rough texture. This indicates that the acid treatment effectively cleaned and activated the filler surface, providing a larger specific surface area and physical anchoring points for subsequent reactions. After further alkaline treatment (3#), the morphology changed significantly on the rough substrate formed by acid etching, with the worm-like structure on the surface transforming into a more uniform nanostructure (exhibiting needle / rod-like characteristics). Combined with the systematic weakening of the Si-O peak in FTIR and the significant increase in the volume of BET mesopores, this morphological change can be attributed to the deep attack of the alkaline solution on the silicon-oxygen network, accompanied by a dissolution-redeposition process, forming a micro-nano composite structure rich in active sites. The sample (4#) treated with silane coupling agent had its surface rough details covered or modified by a layer of material, and its morphology tended to be homogenized. The final composite modified filler (5#) treated with cationic polymer exhibits a complete coating state on its surface, forming a continuous thin film structure.

[0054] Figure 2 This shows the changes in the surface chemical structure of fly ash during the stepwise modification process. The original fly ash sample (1#) at 1086 cm⁻¹... -4 A strong Si-O-Si asymmetric stretching vibration peak is observed at 3431 cm⁻¹. -1 The presence of an OH stretching vibration peak indicates that its main structure is a hydrophilic inorganic silicate. After acid treatment (2#), all Si-O related characteristic peaks (1086, 791, 464 cm⁻¹) were observed. -1 The intensity of all peaks decreased, indicating that acid treatment effectively dissolved some of the amorphous components on the surface, achieving surface cleaning and preliminary etching, providing a more open surface for subsequent reactions. Further alkaline treatment (3#) led to a systematic and significant decrease in the intensity of the Si-O characteristic peaks. This key phenomenon indicates that alkaline treatment does not simply increase surface hydroxyl groups, but rather deeply attacks and depolymerizes the surface silicon-oxygen network, forming a chemically activated layer with reshaped microstructure, rich in defects, and highly reactive. This activated layer provides a crucial reaction substrate for the subsequent hydrolytic condensation of the silane coupling agent. After treatment with the silane coupling agent (4#), two decisive changes occurred in the spectrum: one was at 2975 cm⁻¹. -1 A significant CH stretching vibration peak appeared nearby, which is a characteristic signal of the introduced silane organic segment; secondly, a peak appeared at 1000-1100 cm⁻¹. -1 In the Si-O-Si region, the spectral shape changes significantly, at 1086 cm⁻¹. -1A shoulder peak appeared next to the main peak, forming a "double shoulder peak" characteristic. This confirms that the silane coupling agent has successfully grafted onto the filler surface through a condensation reaction between the Si-OH formed by its hydrolysis and the Si-OH of the activated layer on the fly ash surface, forming a new Si-O-Si bond. Finally, after treatment with a cationic polymer (5#), the spectrum further evolved: 1086 cm⁻¹ -1 The intensity of the Si-O-Si main peak at 2975 cm⁻¹ is significantly enhanced. -1 The intensity of the CH peak at the point changed. This indicates that the cationic polymer is not simply physically adsorbed; rather, the polar / ionic groups in the polymer chain or during the treatment process interact strongly with the grafted silane layer or fly ash surface, forming an effective coating and potentially altering the vibrational modes of the surface structure. This interaction is key to achieving positive surface modification and thus strong bonding with negatively charged plant fibers.

[0055] Figure 3 The data show the specific surface area and pore size structure of the original fly ash sample and the samples after each stage of synergistic treatment. According to the IUPAC classification of nitrogen adsorption-desorption isotherms, the isotherms of all five fly ash samples can be classified as Type IV isotherms with an H3-type hysteresis loop. Adsorption capacity increases slowly in the low-pressure region, while a significant increase and hysteresis phenomenon occur in the high-pressure region (P / P0→1), indicating that the samples are predominantly composed of mesoporous-macroporous structures formed by the accumulation of plate-like particles, with relatively limited contribution from micropores. Relevant data for each sample are shown in Table 1.

[0056] Table 1 Physical structural properties of each sample

[0057] The nitrogen adsorption isotherm of the original fly ash sample (1#) showed a small hysteresis loop and low adsorption capacity in the high-pressure zone, with a corresponding BET specific surface area of ​​only 1.0596 m². 2 / g, total pore volume 0.002693 cm³ 3 / g, average pore size 10.1666 nm, BJH cumulative pore volume only 0.002501 cm³ 3 / g indicates that the original fly ash is relatively dense overall, with limited usable pore structure. After acid treatment (2#), the adsorption capacity of the isotherm increased across the entire pressure range, the hysteresis loop slightly enlarged, and the adsorption slope in the low-pressure region increased, indicating that acid washing, while removing surface impurities, also created a certain number of micropores and small mesopores. Compared with 1#, the BET specific surface area of ​​2# significantly increased to 4.3162 m². 2 / g, the total pore volume increased to 0.005112 cm³. 3 / g, the average pore size decreased to 4.7375 nm; the t-Plot micropore area increased from 0.4651 m² / g. 2 / g increased to 3.2438 m 2 / g, micropore volume increased from 0.000186 cm³ 3 / g increased to 0.001354 cm 3 / g, the pore volume of BJH also increased to 0.003825 cm³. 3 / g. In the pore size distribution curve, #2 showed a more obvious peak in the approximately 2-5nm range than #1, quantitatively confirming the surface micro-etching phenomenon visible in SEM. This indicates that acid treatment not only cleaned the particle surface but also effectively etched and opened the original closed channels, exposing more internal surfaces and micro / small mesoporous structures. Further alkaline treatment (#3) on this basis resulted in a significantly higher adsorption capacity in the medium-high pressure region compared to #2, with a wider hysteresis loop, indicating that mesopores and macropores were extensively activated. The BET specific surface area of ​​#3 continued to increase to 5.2622 m². 2 / g, the total pore volume jumps to 0.015341 cm³. 3 / g, BJH cumulative pore volume reaches 0.014954 cm³. 3 / g, significantly higher than #2; the average pore size has increased again to 11.6614 nm. Notably, #3's t-Plot micropore area and micropore volume (1.0565 m²) are significantly higher. 2 / g, 0.000432 cm 3 The ratio of / g) is slightly lower than that of #2, while the external surface area is 1.0724 m². 2 / g increased to 4.2057 m 2 / g. Combining the pore size distribution, it can be seen that alkali treatment further dissolves and widens some micropores and small mesopores into mesopores within the 3-50 nm range, forming more interconnected channels and larger open pores. This is closely related to the selective dissolution of the glass phase and active silica-alumina components during the alkali dissolution-precipitation process, thus significantly increasing the external surface area and mesopore volume that can participate in reactions and adsorption. Subsequent treatment with a silane coupling agent yielded sample #4, whose overall isotherm morphology remained type IV, but the hysteresis loop shape became slightly smoother, and the adsorption capacity in the high-pressure zone further increased, indicating that while maintaining the integrity of the mesoporous framework, the pore structure underwent a certain rearrangement. The BET specific surface area of ​​#4 increased slightly to 5.7136 m². 2 / g, the total pore volume further increased to 0.019598 cm³. 3 / g, with an average pore size of 13.7206 nm and a pore volume of 0.019589 cm³. 3 / g. However, the t-Plot micropore area and micropore volume decreased to 0.5128 m. 2 / g and 0.000115 cm 3 / g, and the external surface area increases to 5.2008 m². 2The / g indicates that silane molecules preferentially undergo condensation and grafting on the inner walls of micropores and smaller mesopores, partially filling or shielding the finest pores while preserving and slightly expanding the original medium-to-large mesopore network. In the pore size distribution curve, the peak value of <5 nm micropores / small mesopores weakens, while the pore volume contribution in the 10-30 nm range increases, indicating that silane treatment tends to form an organic-inorganic composite layer on the coarse pore walls formed after alkali activation, thus providing active anchoring points for subsequent interface modification. Further ammonium chloride treatment (5#) on the basis of acid-alkali-silane treatment resulted in a slight decrease in adsorption capacity in the low-pressure region, and the adsorption curve and hysteresis loop in the medium-to-high-pressure region also slightly contracted compared to 4#, reflecting that some pores were further occupied or the structure underwent slight collapse. The BET specific surface area of ​​5# decreased to 4.4447 m². 2 / g, the total pore volume decreased to 0.014765 cm³. 3 / g, BJH pore volume also decreased to 0.014595 cm³. 3 The micropore area increased by approximately 13.2873 nm / g, but the average pore size remained within the mesoporous range. t-Plot results showed that the micropore area slightly increased to 0.5468 m² / g. 2 / g, micropore volume is 0.000173cm³ 3 / g, while the external surface area decreased to 3.8980 m². 2 / g. Based on the pore size distribution, the intensity of fly ash #5 decreases in the medium pore size region (approximately 5-30 nm), and the overall pore volume is slightly reduced. This can be attributed to the adsorption, ion exchange, and possible crystal salt deposition of ammonium chloride in the pores, causing some mesopores to be "occupied" or narrowed. Overall, although the specific surface area and pore volume of #5 are lower than #3 and #4, they are still significantly higher than the original fly ash #1, indicating that it has formed a functionalized surface containing ammonium chloride species while maintaining the basic mesoporous framework, making it more suitable as an active filler in subsequent adsorption or reaction systems. In summary, after being treated stepwise through "acid → acid + alkali → acid + alkali + silane → acid + alkali + silane + ammonium chloride," the pore structure of fly ash has undergone an evolution from dense low specific surface area → open micropores / small mesopores → significantly developed mesopores → micropores partially filled with organosilicon layers and salts, with moderate pore size regulation. The BET and pore size distribution data, along with the characterization results from SEM, corroborate each other, fully demonstrating that the initial acid-base treatment effectively etched and activated the fly ash skeleton. The subsequent silane and ammonium chloride treatments, through a "site-coating-pore-adjusting" mechanism, regulated the pore size and surface chemical environment, providing a designable structural basis for the application of fly ash in composite materials and adsorption / catalysis.

[0058] Figure 4The fundamental transformation of the surface electrical properties of fly ash during the stepwise modification process was clearly revealed. Unmodified raw fly ash (1#) became negatively charged due to the dissociation of surface silanol groups, with a Zeta potential of -8.49 mV. After acid treatment (2#) and alkali treatment (3#), the potentials changed to -23.3 mV and -19.8 mV, respectively. The increase in the absolute value of the negative potential confirms the deep etching and activation of the fly ash surface by acid and alkali treatment, exposing and creating more abundant surface active sites. The key turning point occurred after silane coupling agent treatment (4#). After grafting with aminosilane (KH550), the surface charge of the sample reversed from negative to a positive value of +9.88 mV. This indicates that the grafted aminosilane successfully transferred its organic functional group (protonated -NH3) to the surface. + The introduction of this material onto the surface achieved the first fundamental transformation of the filler from "hydrophilic and negatively charged" to "organic-phase-friendly and positively charged." Finally, after coating with the cationic polymer (PDADMAC) (5#), the surface positive charge was further enhanced, with the Zeta potential significantly increased to +15.4 mV. This marks the successful construction of a composite surface with high strength and stable positive charge through a four-step synergistic modification process involving "acid-base-silane-polymer." This systematic evolution of electrical properties is of great significance: a significant potential difference of over 25 mV exists between the final modified filler (+15.4 mV) and the plant fiber (approximately -11.5 mV). According to the principles of colloid chemistry, this potential difference with opposite signs and considerable numerical value will generate a strong electrostatic attraction. This force is one of the core mechanisms ensuring the efficient adsorption of the filler onto the fiber surface during wet molding and the formation of a strong interfacial bond in the final composite material, thereby completely solving the "powdering" problem.

[0059] Example 3: Fly ash / waste paper fiber composite material with high filling ratio (50%) a. Soak 300g of oven-dry waste paper pulp board in the solid-liquid separation alkaline solution of Example 1(b) above for 12h, then dissolve it in a standard disintegrator until there are no pulp clumps, then mix it with the solid-liquid separation acid solution of Example 1(a) above to adjust the pH value to neutral, and prepare a fiber suspension with a concentration of about 3.5%.

[0060] b. Take 150 g of the above-mentioned composite modified fly ash filler (the target filling amount is 50% of the oven-dry mass of the pulp) and slowly add it to the fiber suspension of this embodiment (a). Stir at high speed for 30 min to make it evenly dispersed, and separate the solid and liquid. The resulting filtrate is used as process recycled water for subsequent preparation of fresh acid and alkali solutions of the required concentration.

[0061] c. Add 1.5% of the total dry weight of wet strength agent (polyamide epichlorohydrin resin), 0.3% of the retention aid (cationic polyacrylamide), and 1.0% of the softener (polyvinyl alcohol) to the slurry, and continue stirring for 15 min.

[0062] d. Using a pulp molding machine, form the pulp under a vacuum of 0.03 MPa. Collect the turbid filtrate obtained from the first filtration and pour it all back into the remaining unformed pulp, stirring thoroughly again. Repeat this "filtration-collection of turbid filtrate-reuse" process 1-2 times until the filtrate in the filtration flask becomes clear and transparent. Then, heat-press the resulting wet paper sheet in a hot press dryer at 95°C and 0.07 MPa for 5 minutes to obtain the final product.

[0063] Example 4: Gasification slag / bamboo pulp fiber composite material with medium filling ratio (25%) a. Take the dried gasification slag (particle size D50=75μm) and sequentially perform the same acid treatment (using 1wt% sulfuric acid), alkali treatment, silane coupling agent treatment (using γ-(2,3-epoxypropoxy)propyltrimethoxysilane KH560) and cationic polymer treatment (using polyethyleneamine) as in Example 1.

[0064] b. Dissolve 450g of oven-dried bamboo pulp fiber into a suspension.

[0065] c. Add 150g of the above-mentioned composite modified gasification slag packing (target filling amount is 25%).

[0066] d. Add 1.0% wet strength agent, 0.2% retention aid and 0.8% softener.

[0067] e. The paper is formed on a four-wire paper machine using a papermaking process, and then dried in a drying cylinder to obtain a basis weight of 250 g / m³. 2 Packaging cardboard.

[0068] Example 5: Performance Testing and Comparative Analysis The materials obtained from the above comparative examples and embodiments were subjected to performance tests, and the results are summarized in Table 2 below.

[0069] Dry tensile strength: determined according to GB / T 12914-2018 standard.

[0070] Wet tensile strength: The tensile strength of the sample is measured after immersing it in water for 30 minutes.

[0071] Wet strength retention rate: (wet tensile strength / dry tensile strength) × 100%.

[0072] Water absorption rate: Measured according to GB / T 1540-2002 standard, and the water absorption rate was recorded after 2 hours.

[0073] Table 2. Relationship between modified coal-based solid waste additive amount and various properties of packaging materials

[0074] Table 2 clearly demonstrates the technical effectiveness of the four-step sequential synergistic modification strategy of "acid-base-silane coupling agent-cationic polymer" described in this application. Specific analysis is as follows: (1) There is a strict sequential dependency and functional synergy among the treatment steps, rather than a simple additive process. The results of Comparative Example 4 (acid + silane two-step method) and Comparative Example 5 (base-acid-silane-polymer method with disordered order) show that neither missing steps nor disordered order can achieve the same effect as this application. Although the performance of both is improved compared to single modification, their wet strength retention rate (maximum only 45%) and water absorption rate (minimum 38%) are significantly different from those of Example 3 of this application (wet strength retention rate 75%, water absorption rate 4.5%). Specifically, the wet strength retention rate of Example 3 is 1.67 times that of Comparative Example 5, while the water absorption rate is only about 11.8% of that of Comparative Example 5. This confirms that the "base treatment" step must be performed after the "acid treatment" to form a structure rich in active hydroxyl groups on a clean surface, which is a key prerequisite for the subsequent efficient chemical grafting of silane; while the "cationic polymer treatment" must be performed last to achieve the final electrostatic coating and stabilization of the filler surface. This sequence is a necessary condition for constructing a multi-interface structure of "physical anchoring, chemical bonding and electrostatic adsorption".

[0075] (2) This application achieves excellent comprehensive performance under high filling amount, breaking through the traditional technical bottleneck.

[0076] ① When the filler content is as high as 50 wt% (Example 3), the dry tensile strength (5.2 kN / m) and wet tensile strength (3.9 kN / m) of the obtained material exceed those of all comparative materials with lower filler contents (40-45 wt%). This indicates that the filler modified in this application can effectively enhance the overall mechanical properties of the material when filled in a high proportion.

[0077] ② Example 3 simultaneously achieved a wet strength retention rate as high as 75% and a water absorption rate as low as 4.5%. In contrast, Comparative Examples 1-5, with similar filler amounts, achieved an optimal wet strength retention rate of only 45%, and a corresponding water absorption rate as high as 38%, demonstrating the inherent contradiction between strength and water resistance in traditional methods. This application successfully synergistically optimized these two key performance indicators.

[0078] ③ Furthermore, at a filler content of 25% (Example 4), the material prepared by the method of this application still showed superior dry / wet tensile strength (6.8 kN / m, 5.2 kN / m) and water resistance (water absorption rate 3.8%) compared to all comparative examples, demonstrating the good applicability of this modification strategy under different filler systems.

[0079] In summary, the data in Table 2 show that the four-step sequential synergistic modification method specified in this application can transform hydrophilic coal-based solid waste into a functional filler that is tightly bound to plant fibers, thereby achieving high filler content (35-55 wt%) while endowing the composite material with excellent mechanical properties and water resistance.

[0080] Analysis of paper samples made from waste paper fiber pulp with raw fly ash (1#) and quaternary ammonium salt ash (5#) after four-step synergistic treatment at a solid content of 50%. The paper sample with raw fly ash (1#) added is designated as A, and the paper sample with quaternary ammonium salt ash (5#) after four-step synergistic treatment is designated as B.

[0081] The surface morphology of paper samples filled with two different types of fly ash was analyzed using scanning electron microscopy (SEM), such as... Figure 5 As shown, this visually reveals the differences in the distribution and bonding state of the filler within the fiber network. For example... Figure 5 As shown in Figure A (filled with unmodified fly ash, #1), the paper fiber network is relatively loose. Fly ash particles are distributed independently on the fiber surface or in the pores in distinct spherical or near-spherical shapes. The particle-fiber interface is clear, and physical gaps are visible, exhibiting typical "physical blending" characteristics. This indicates that due to the repulsive effect caused by the negatively charged surfaces, the filler retention rate is low and the bonding is loose. In stark contrast, Figure 5 B (Fly Ash with Four-Step Synergistic Modification, #5) exhibits a denser fiber network, with a significantly increased number and more uniform distribution of modified fly ash particles. Crucially, the boundaries between most particles and fibers become blurred, with particles tightly wrapped or embedded in the fibers, exhibiting no obvious exposure or gaps. This demonstrates that the filler and fibers have formed a tightly bonded composite structure. This morphological transformation directly confirms that the stable positively charged surface constructed through the synergistic modification of "acid-base-silane-cationic polymer" can effectively promote the efficient retention and firm bonding of fillers on negatively charged fibers through strong electrostatic adsorption, fundamentally improving interfacial interactions and providing a reliable microstructural basis for enhancing paper physical properties and eliminating "dust shedding."

[0082] Table 1 shows a comparison of the physical properties of sample A (filled with unmodified fly ash) and sample B (filled with composite modified filler). Using the paper sample B prepared in this application, at a high filling content of 50%, the wet strength retention rate significantly increased to 75%, the water absorption rate drastically decreased to 4.5%, and there was no dust shedding. Its overall performance is far superior to that of comparative sample A. Furthermore, density and structural analysis were performed on the two materials. Using the helium displacement method (AccuPycⅡ 1345), the true density of paper sample A filled with unmodified fly ash was determined to be 1.7838 g / cm³. 3 The true density of paper sample B filled with composite modified filler is 1.7758 g / cm³. 3The slight decrease in true density demonstrates that the low-density silane coupling agent and cationic polymer have successfully bonded and coated the fly ash surface, forming a stable organic-inorganic composite structure. Apparent density determination using a geometric method yielded an apparent density of 1.25 g / cm³ for paper sample B. 3 Significantly higher than paper sample A (0.95 g / cm³) 3 This indicates that a tight interfacial bond is formed between the composite modified filler and plant fibers through strong electrostatic interaction, chemical anchoring, and polymer entanglement. This bond effectively reduces porosity in the fiber network, resulting in a denser material structure during drying. In summary, the change in true density confirms the success of the surface chemical modification; while the increase in apparent density, along with the synergistic improvement in mechanical properties and water resistance, jointly confirms the superior interfacial reinforcement effect achieved by this modification. This series of data, corroborated by characterization results such as SEM, FTIR, and Zeta potential, constitutes a complete chain of evidence, fully demonstrating the unexpected technical effects achieved by the synergistic modification strategy of this application in overcoming the bottleneck of high filler interfacial compatibility.

[0083] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a composite modified coal-based solid waste filler, characterized in that, Coal-based solid waste was subjected to acid treatment, alkali treatment, silane coupling agent treatment and cationic polymer treatment in sequence to obtain composite modified coal-based solid waste filler; The acid treatment uses an acid concentration of 0.5-5 wt% and a treatment temperature of 20-80℃. The alkaline treatment uses an alkaline solution concentration of 1-10 wt% and a treatment temperature of 40-90℃. The silane coupling agent treatment involves adding alkali-treated coal-based solid waste into a pre-hydrolyzed silane coupling agent for reaction. The cationic polymer is a polymer containing quaternary ammonium salt groups.

2. The preparation method according to claim 1, characterized in that, The coal-based solid waste is selected from at least one of fly ash, gasification slag or coal gangue, and its particle size D90≤100μm.

3. A plant fiber-based packaging material, characterized in that, The plant fiber-based packaging material comprises the following raw materials in parts by weight: 45-90 parts plant fiber and 35-55 parts composite modified coal-based solid waste filler. The composite modified coal-based solid waste filler is obtained by the preparation method according to any one of claims 1-2.

4. The plant fiber-based packaging material according to claim 3, characterized in that, The raw materials contain 45-65 parts of plant fiber and 35-55 parts of composite modified coal-based solid waste filler.

5. The plant fiber-based packaging material according to claim 3, characterized in that, The raw materials also include one or more of the following additives: 0.5-2.0 parts of wet strength agent, 0.1-0.5 parts of retention and filtration aid, and 0.5-3.0 parts of softener.

6. A method for preparing the plant fiber-based packaging material as described in any one of claims 3-5, characterized in that, Includes the following steps: S1, the composite modified coal-based solid waste filler obtained by the preparation method according to any one of claims 1 or 2 is mixed with the sludged plant fiber to form a composite slurry; S2, the composite slurry in S1 is wet-formed and then dried to obtain plant fiber-based packaging material.

7. The method according to claim 6, characterized in that, The composite modified coal-based solid waste filler has a mass percentage of 10%-55% in the plant fiber-based packaging material.

8. The method according to claim 7, characterized in that, The mass percentage is 35%-55%.

9. The method according to claim 6, characterized in that, The wet molding process in S2 includes the following steps: performing initial solid-liquid separation on the composite slurry, collecting the filtrate generated from the initial solid-liquid separation, and reusing the filtrate for the re-molding of the same batch of slurry.

10. The application of a composite modified coal-based solid waste filler obtained by the preparation method according to any one of claims 1 or 2 in the preparation of plant fiber-based packaging materials.

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