A method for preparing geopolymer by using low-temperature pyrolysis-water washed waste incineration fly ash as raw material

Abstract

This application relates to the field of waste treatment, and in particular to a method for preparing geopolymers using low-temperature pyrolysis-water washing of waste incineration fly ash as raw material. This application provides a method for activating waste fly ash after treatment, which adopts a system of low-temperature water washing desalination + nano-aluminum-based materials + citric acid composite activation. This system not only breaks down the agglomeration and clay coating of water-washed fly ash, but also achieves deep fixation of heavy metals, solving the dual problems of "decreased activity after desalination" and "dissolution of heavy metals".

Description

Technical Field

[0001] This application relates to the field of waste treatment, and in particular to a method for preparing geopolymers using low-temperature pyrolysis-water washing of waste incineration fly ash as raw material. Background Technology

[0002] With the acceleration of urbanization, the amount of urban domestic waste generated has increased dramatically. Waste incineration technology, due to its advantages of volume reduction, harmlessness, and high resource recovery, has become one of the mainstream waste treatment methods. However, the incineration process produces a large amount of fly ash, which contains heavy metals (such as Pb, Cd, Cr, Hg, etc.), dioxins, and other toxic and harmful substances. It is classified as hazardous waste, and improper handling can cause serious environmental pollution to soil, water bodies, and the atmosphere, threatening human health.

[0003] Currently, the main methods for treating fly ash from waste incineration include solidification / stabilization, high-temperature melting, and chemical stabilization. While solidification / stabilization technology is simple to operate and low-cost, the increased volume of the solidified material increases the difficulty and cost of subsequent disposal, and its poor long-term stability poses a risk of heavy metal leaching. High-temperature melting technology can achieve the harmlessness and volume reduction of fly ash, but it consumes a lot of energy, requires large equipment investments, and is prone to secondary pollution. Chemical stabilization technology involves adding chemical agents to react with heavy metals in fly ash to form stable compounds, but chemical agents are expensive, have poor stabilization effects on certain heavy metals, and are difficult to treat dioxins in fly ash.

[0004] Geopolymers are a novel type of inorganic cementitious material formed by the dissolution, hydrolysis, and condensation reaction of aluminosilicate raw materials under the action of an alkaline activator. They possess advantages such as high strength, high durability, high temperature resistance, and low carbon footprint, making them promising for applications in construction, road construction, and solid waste disposal. Existing technologies include the preparation of geopolymers from industrial solid wastes such as fly ash, slag, and steel slag. However, when using waste incineration fly ash to prepare geopolymers, the presence of large amounts of heavy metals and dioxins in the fly ash leads to excessive leaching of heavy metals in the geopolymer product, and the presence of dioxins poses a potential threat to the environment and human health, thus limiting the application of waste incineration fly ash in geopolymer preparation. Low-temperature pyrolysis is a common method for treating dioxins, effectively decomposing over 99% of them in the system to reduce the harm to human health and the environment from fly ash during subsequent geopolymer preparation. However, fly ash after pyrolysis and washing typically exhibits higher silica-alumina purity and lower reactivity, containing large amounts of inert crystals such as quartz and mullite. Therefore, when used in geopolymer preparation, it can easily lead to problems such as difficulty in controlling the system's setting time and excessive use of alkali activators. Currently, thermal activation is often used to reactivate dioxins. However, this process has several drawbacks. First, activation may lead to the recombination of dioxins, resulting in the volatilization of dioxins and heavy metals, causing environmental pollution and difficulties in subsequent treatment. Second, it also involves complex processes and high energy consumption. Summary of the Invention

[0005] Based on the above problems, this application provides a method for preparing geopolymers using low-temperature pyrolysis-water washing of waste incineration fly ash as raw material. The purpose is to reduce the high-temperature activation process, thereby reducing environmental pollution, reducing energy consumption, and improving economic efficiency.

[0006] This application provides a method for preparing geopolymers using low-temperature pyrolysis-water washing of waste incineration fly ash as raw material, comprising the following steps: Low-temperature pyrolysis: Pyrolysis of waste ash at 350–500℃ to remove dioxins; Water washing and desalination: Soluble salts in fly ash are removed by water washing, ensuring that the soluble chloride ion content in fly ash does not exceed 1.5 wt%. Activation: The washed fly ash is dried, and then 1-3 wt% of an activating agent is added to the fly ash. The mixture is then activated by grinding in a ball mill. The activating agent contains at least the following components by weight: One part of aluminum-based nanomaterials; Citric acid 0.2–0.5 parts; Polymerization: The treated fly ash is mixed according to the following mass ratio: 100 copies of ash 40-60 parts of reinforcing material 30-50 parts of stimulant The activator comprises at least an alkaline aqueous solution and water glass; After the above materials are mixed in a mixer, vacuum forming is performed to remove air bubbles, followed by curing to obtain geopolymer.

[0007] In the above technical solution, dioxins and soluble salts in the system are first removed as much as possible through low-temperature pyrolysis and water washing desalination. Then, a grinding activation method is used, and aluminum-based nanomaterials and citric acid are added to activate the system. In these steps, grinding, aluminum-based nanomaterials, and citric acid have a synergistic effect. The purpose of grinding is to break the originally inert silicon-oxygen bonds and aluminum-oxygen bonds, forming active groups and thus improving the reactivity. Based on this, citric acid can selectively dissolve and etch the free calcium components on the fly ash surface during grinding, forming calcium citrate complexes. This stabilizes the calcium content, reduces the runaway solidification time caused by rapid calcium ion dissolution, and also compensates for the poor flowability of the fly ash. The citric acid forms a certain degree of organic film layer on the fly ash surface, ensuring powder flowability. Simultaneously, based on the partial etching by citric acid, aluminum-based nanomaterials are used to fill defect sites and also act as crystal nuclei, thereby controlling the overall reaction kinetics and achieving a significant reinforcing effect.

[0008] Preferably, after activation, the ash treatment also includes the following steps: Hydroxyapatite was added to the fly ash at a mass fraction of 1–5 wt%, and then mixed thoroughly.

[0009] In the above scheme, hydroxyapatite can further stabilize heavy metals by reducing their leaching through its stronger complexing ability. At the same time, hydroxyapatite can also play a reinforcing role in the system.

[0010] Preferably, in the activation step, the activator further comprises 0.5 to 1 part by weight of tartaric acid.

[0011] Tartaric acid provides stronger complexing ability in the system, allowing for better control over the concentration of calcium ions. It can also further disrupt the inert layer on the fly ash surface, forming more active sites, building upon the initial treatment with citric acid. Tartaric acid also provides a retarding effect, resulting in better density and later-stage strength. Furthermore, due to its rigid molecular structure, tartaric acid can form a microfiber-reinforcing phase in the hardened structure, thus improving overall mechanical strength. Overall, under the combined action of citric acid and tartaric acid, the early and later-stage strength of the system are well-preserved, and the curing process is relatively smooth.

[0012] Preferably, the aluminum-based nanomaterial is a combination of nano-alumina or nano-metakaolin and polyaluminum chloride, wherein the mass of the polyaluminum chloride accounts for 10-25% of the mass of the aluminum-based nanomaterial.

[0013] In the above system, nano-alumina and polyaluminum chloride (PAC) can form a dual-channel aluminum supply system. Simultaneously, as a soluble electrolyte, it can further increase the specific surface area by utilizing the surface energy of the ash particles during ball milling. Furthermore, the Al(OH)₂ produced by its hydrolysis... + Adsorbed on the particle surface, it further reduces the aggregation of fly ash particles.

[0014] Further preferred, after grinding and activation, the fly ash particle size D50 ≤ 40 μm and specific surface area ≥ 400 m² / kg.

[0015] After grinding to the above-mentioned particle size and specific surface area range, the overall reaction performance and flowability can be well balanced, which has a certain improvement effect on the subsequent strength.

[0016] Preferably, the reinforcing material comprises the following components by mass fraction: Fly ash 40-60% Metakaolin 30-35% Steel slag powder ≤20%.

[0017] The above scheme uses a combination of fly ash, metakaolin, and steel slag powder. Fly ash and metakaolin provide a continuous silica-alumina skeleton, which forms an extruded framework structure with the fly ash. Meanwhile, metakaolin, with its amorphous aluminum silicate and high reactivity, can efficiently initiate the polymerization reaction. Its sheet-like structure can fill the voids, improving the material's strength and bonding performance. Steel slag powder can provide a calcium source in the system and can activate the reaction in an aqueous system. Overall, it can effectively promote the polymerization process.

[0018] Preferably, in the water washing and desalination step, the water washing and desalination wastewater is filtered through a 0.22 μm membrane and then used to prepare the activator in the polymerization step.

[0019] In the above process, the wastewater from the washing process can be recycled and treated. After being filtered through a filter membrane, it can be used to prepare an activator, thereby reducing the amount of deionized water used and lowering the wastewater treatment cost. Furthermore, after desalination, the presence of a certain amount of sodium and potassium ions in the wastewater can improve the activation efficiency.

[0020] Preferably, the activator comprises the following components by mass fraction: Sodium hydroxide solution with a concentration of 10–14 mol / L, 20–40% Water glass 40-60% Remaining amount of wastewater from water washing and desalination.

[0021] Overall, the above-mentioned activators, with the participation of water washing and desalination wastewater, achieved a good activation effect by relying on the sodium hydroxide solution and water glass system.

[0022] Preferably, in the activation step, the drying temperature is controlled to be no higher than 80°C.

[0023] In the above process, the main purpose of low-temperature drying is to inhibit the resynthesis of dioxins. By performing activation pretreatment through low-temperature drying, premature reaction caused by the hydrolysis of citric acid in the system due to excessive water content can be avoided, and the formation of dioxins in the system can be further reduced.

[0024] Preferably, in the polymerization step, a gradient stirring method is used for mixing, as follows: Pour the pretreated washed fly ash and reinforcing material into the mixer and dry mix for 5-15 minutes. Then add 50% of the activator and wet mix for 3-10 minutes. Add the remaining 50% of the activator and continue wet mixing for 10-15 minutes. Then discharge the material. And / or, Maintenance specifically includes the following steps: S1. Pre-curing: Place in an environment with a temperature of 25-30℃ and a relative humidity of ≥95% for 6-8 hours; S2. Heating and curing: Increase the temperature to 60-80℃ at a rate of 2-8℃ / h, maintain relative humidity ≥85%, and cure for 24-36 hours; S3. Cooling and stabilizing: Cool to 20-25℃ at a rate of 1-5℃ / h, and continue to place to eliminate internal stress.

[0025] In the above scheme, a gradient mixing method is used. First, dry mixing ensures uniform powder mixing and avoids clumping. Then, wet mixing is performed in stages. An activator is added initially to initiate the reaction, followed by additional additions to adjust the flowability to the target level. This prevents localized violent reactions or uneven moisture distribution, ensuring a uniform, stable, and dense slurry. During the curing process, a pre-curing step is used for initial solidification, followed by heated curing to improve the degree of internal solidification. This approach balances early and late-stage strength and avoids excessive stress that could lead to material cracking.

[0026] In summary, this application provides a method for activating waste fly ash after treatment. It adopts a system of low-temperature water washing and desalination + nano-aluminum-based materials + citric acid composite activation, which not only breaks the agglomeration and clay coating of water-washed fly ash, but also achieves deep fixation of heavy metals, solving the dual problems of "decreased activity after desalination" and "dissolution of heavy metals".

[0027] Furthermore, in this application, hydroxyapatite is added to further fix the heavy metals, significantly reducing their leaching. Additionally, this application innovatively employs a water washing saline wastewater recycling system: the filtered water washing saline wastewater is used to prepare an alkaline activator, replacing 50-70% of deionized water, reducing wastewater treatment costs (reducing wastewater discharge by more than 80%), and the Na⁺ and K⁺ in the wastewater can further enhance the activation efficiency. Detailed Implementation

[0028] The technical solutions in this application will be further described through the following specific embodiments.

[0029] Example 1: This example provides a geopolymer, which is obtained through low-temperature pyrolysis, water washing and desalination, activation, and polymerization of waste ash. The specific steps are as follows: Low-temperature pyrolysis: The waste ash was pyrolyzed at 350℃ to remove dioxins. The composition after pyrolysis was determined as follows: SiO2 30%, Al2O3 11%, CaO 23%, Cl⁻ 8.5%, Pb 680mg / kg, Cd 7.5mg / kg.

[0030] Water washing and desalination: Add deionized water to the pyrolysis fly ash at a liquid-to-solid ratio of 2.5:1, stir at 45℃ and 250 rpm for 40 minutes, filter, and repeat water washing twice. The chloride ion content of the washed fly ash is 1.2 wt%. Activation: The washed fly ash is dried at 70℃ to reduce its moisture content to below 5% (actual product measurement was 4.2%). Then, a composite activator is added, and the mixture is ground in a grinder at 450 rpm for 60 minutes. After grinding, the D50 is 25 μm, and the specific surface area is 420 m² / kg. Specifically, per 100 parts by weight of waste fly ash (excluding water), the composite activator contains the following components: Nano-alumina (average particle size 80nm) 1.35 parts; Polyaluminum chloride (powder, density 1.36 g / cm³) 0.15 parts; Citric acid 0.5 parts; Tartaric acid 0.2 parts.

[0031] After activation, hydroxyapatite (average particle size 40 μm) was added at 3 wt% and stirred for 18 min.

[0032] polymerization: First, prepare the materials according to the following mass proportions: 100 copies of ash 50 parts of reinforcing material, details as follows: Fly ash (Grade 1) 25 parts; 18 parts of metakaolin; 7 parts steel slag powder.

[0033] 40 parts of activator, specifically comprising the following: 12 parts of 12 mol / L sodium hydroxide aqueous solution Water glass (modulus 2.2, solids content 43.5%), 20 parts Eight portions of water washing and desalination wastewater.

[0034] Among them, the water washing and desalination wastewater is treated by filtration through a 2.2μm microfiltration membrane.

[0035] The material mixing method is as follows: Add fly ash and reinforced ash to the mixer and dry mix for 9 minutes. Then add 50% of the activator and wet mix for 7 minutes. Add the remaining activator and continue wet mixing for 11 minutes to obtain a wet material with a flowability of 210 mm.

[0036] The wet material is first degassed under vacuum at -0.085MPa, then statically pressed at 30MPa, and subsequently cured. The specific curing steps are as follows: S1. Pre-curing at 28℃ / 95%RH for 7 hours; S2, heat to 75℃ / 85%RH at 5℃ / h, and cure for 30 hours; S3, 3℃ / h cooling down to 20℃, and placed for 7 days.

[0037] Examples 2-14: The composition and ratio of the activator were adjusted in the above examples, as shown in Table 1.

[0038]

[0039] Example 15 differs from Example 1 in that hydroxyapatite is not added after activation.

[0040] Example 16 differs from Example 1 in that the mass of hydroxyapatite added is 1 wt% of fly ash.

[0041] Example 17 differs from Example 1 in that the added mass of hydroxyapatite is 5 wt% of fly ash.

[0042] Example 18 differs from Example 1 in that deionized water is used instead of recycled wastewater during the preparation of the activator.

[0043] Example 19 differs from Example 1 in that, in the mixing step, all components are directly mixed together in a mixer for 30 minutes.

[0044] Example 20 differs from Example 1 in that nano-alumina in the activator is replaced by nano-metakaolin in equal mass.

[0045] Compared with Example 1, the difference is that no activator is added in the activation step.

[0046] The above embodiments and control examples were verified using the following testing standards: The 7-day compressive strength and 28-day compressive strength were determined in accordance with GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)".

[0047] The leaching concentrations of lead, cadmium, and chromium were determined in accordance with HJ / T 299-2007 "Leaching Toxicity Methods for Solid Waste / Sulfuric Acid and Nitric Acid Method".

[0048] The compressive strength loss rate before and after freeze-thaw was determined according to GB / T 50082-2009 "Standard for Test Methods of Long-Term Performance and Durability of Concrete" (Section 4.3 Single-sided freeze-thaw method (or salt freezing method), 28 freeze-thaw cycles) and shrinkage rate (Section 8.1, non-contact method).

[0049] Flowability: The test was conducted in accordance with JGJ / T 70-2009 "Standard for Test Methods of Basic Performance of Building Mortar".

[0050] The experimental results for the above-described embodiments and control examples are shown in Table 2.

[0051]

[0052] The comparison of the above experimental data shows that activation significantly improves the compressive strength of the geopolymer in this application. Comparing Examples 1-14, the composition of the activator has a significant impact on the system's performance. For example, in Example 2, without the addition of polyaluminum chloride, the 7-day strength after curing is significantly reduced, indicating that polyaluminum chloride significantly improves the early strength performance of the system. In Example 4, however, excessive addition of polyaluminum chloride leads to premature curing of the system, potentially reducing fluidity and causing insufficient gelation due to its strong adsorption properties, thus reducing overall strength.

[0053] In Example 8, the addition of excessive citric acid resulted in a significant decrease in strength and an increase in the freeze-thaw resistance loss rate. In Example 10, insufficient citric acid failed to provide etching during activation, leading to both reduced system strength and decreased fluidity. In Example 12, the absence of tartaric acid not only facilitated the dissolution of metal ions but also resulted in a significant reduction in early system strength and fluidity. In Example 13, the addition of excessive tartaric acid resulted in overly strong complexing properties, significantly impacting the hardening process and also leading to a marked decrease in system strength.

[0054] In Example 15, the absence of hydroxyapatite significantly reduced the fixation performance of heavy metals and led to a marked increase in the leaching of various heavy metals. Furthermore, comparing Example 18 and Example 1, it can be seen that the wastewater used in this application, after recycling, showed little overall difference compared to using deionized water, and even exhibited a certain improvement in early strength, significantly reducing the production cost of the geopolymer.

[0055] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing geopolymer using low-temperature pyrolysis-water washed waste incineration fly ash as raw material, characterized in that, Includes the following steps: Low-temperature pyrolysis: Pyrolysis of waste ash at 350–500℃ to remove dioxins; Water washing and desalination: Soluble salts in fly ash are removed by water washing, ensuring that the soluble chloride ion content in fly ash does not exceed 1.5 wt%. Activation: The washed fly ash is dried, and then 1-3 wt% of an activating agent is added to the fly ash. The mixture is then activated by grinding in a ball mill. The activating agent contains at least the following components by weight: One part of aluminum-based nanomaterials; Citric acid 0.2–0.5 parts; Polymerization: The treated fly ash is mixed according to the following mass ratio: 100 copies of ash 40-60 parts of reinforcing material 30-50 parts of stimulant The activator comprises at least an alkaline aqueous solution and water glass; After the above materials are mixed in a mixer, vacuum forming is performed to remove air bubbles, followed by curing to obtain geopolymer.

2. The method according to claim 1, wherein the method is characterized by, After activation, the ash disposal process includes the following steps: Hydroxyapatite was added to the fly ash at a mass fraction of 1–5 wt%, and then mixed thoroughly.

3. The method according to claim 1, wherein the method is characterized by, In the activation step, the activator further comprises 0.1 to 0.2 parts by weight of tartaric acid.

4. The method according to claim 1, wherein the method is characterized by, The aluminum-based nanomaterial is a combination of nano-alumina or nano-metakaolin and polyaluminum chloride, wherein the mass of the polyaluminum chloride accounts for 10-25% of the mass of the aluminum-based nanomaterial.

5. The method according to claim 3 or 4, wherein the method is characterized by, After grinding and activation, the fly ash particle size D50 ≤ 40 μm and specific surface area ≥ 400 m² / kg.

6. The method according to claim 1, wherein the method is characterized by, The reinforcing material comprises the following components by mass fraction: Fly ash 40-60% Metakaolin 30-35% Steel slag powder ≤20%.

7. The method according to claim 1, wherein the method is characterized by, In the water washing and desalination step, the water washing and desalination wastewater is filtered through a 0.22μm membrane and then used to prepare the activator in the polymerization step.

8. The method according to claim 7, wherein the method is characterized by, The activator comprises the following components by mass fraction: Sodium hydroxide solution with a concentration of 10–14 mol / L, 20–40% Water glass 40-60% Remaining amount of wastewater from water washing and desalination.

9. The method for preparing geopolymers using low-temperature pyrolysis-washed waste incineration fly ash as raw material according to claim 1, characterized in that, During the activation step, the drying temperature is controlled to be no higher than 80°C.

10. A method for preparing geopolymers using low-temperature pyrolysis-washed waste incineration fly ash as raw material, as described in claim 1, characterized in that, In the polymerization step, a gradient stirring method is used for mixing, as follows: Pour the pretreated washed fly ash and reinforcing material into the mixer and dry mix for 5-15 minutes. Then add 50% of the activator and wet mix for 3-10 minutes. Add the remaining 50% of the activator and continue wet mixing for 10-15 minutes. Then discharge the material. And / or, Maintenance specifically includes the following steps: S1. Pre-curing: Place in an environment of 25-30℃ and relative humidity ≥95% for 6-8 hours; S2. Heating and curing: Increase the temperature to 60-80℃ at a rate of 2-8℃ / h, maintain relative humidity ≥85%, and cure for 24-36 hours; S3. Cooling and stabilizing: Cool to 20-25℃ at a rate of 1-5℃ / h, and continue to place to eliminate internal stress.

Images