Fly ash washing pyrolysis and detoxification process based on composite portland cement system

Abstract

The application provides a fly ash water washing pyrolysis and detoxification process based on a composite Portland cement system and relates to the technical field of solid waste treatment. The process comprises three steps: water washing and chlorine removal of original fly ash, solid-liquid separation to obtain fly ash filter cake with residual moisture; then, the filter cake is sent to a drying unit, and a composite sealing agent composed of cement clinker, gypsum, slag and fly ash is synchronously added, so that the cement clinker is hydrated to release calcium hydroxide, the slag and fly ash are activated to generate a hydrated calcium silicate and aluminate gel network, and the heavy metal is wrapped in the lattice in the semi-dry state drying process; finally, the dried material is sent to a low-temperature pyrolysis unit to decompose and remove dioxins at low temperature, and standard tailings are obtained. The application realizes the co-solids of heavy metals and the degradation of dioxins, does not need additional water, has low energy consumption and cost, has stable detoxification effect, and can meet the harmless disposal and resource utilization requirements of fly ash.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment technology, specifically to a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system. Background Technology

[0002] Municipal solid waste incineration fly ash is fine particulate matter collected by flue gas purification systems during waste-to-energy incineration. It contains dioxins, high concentrations of soluble chlorides, and heavy metals such as Pb, Cd, Zn, and Cr, and is explicitly listed as hazardous waste in the National Hazardous Waste List. With the rapid development of the municipal solid waste incineration power generation industry, the amount of fly ash generated has continued to rise. The Ministry of Ecology and Environment requires a gradual reduction in the amount of fly ash entering municipal solid waste landfills and encourages regions with suitable conditions to achieve zero fly ash landfilling. The harmless disposal and resource utilization of fly ash has become an urgent issue in the field of solid waste treatment.

[0003] Currently, the main technologies for treating incineration fly ash include high-temperature melting, cement kiln co-processing, chelating agent stabilization, water washing + low-temperature pyrolysis, and combinations of these methods. Among these, the water washing + low-temperature pyrolysis process is considered one of the mainstream technologies with significant engineering application prospects due to its remarkable effects on dioxin degradation and chloride removal. The basic idea of ​​this process is as follows: first, most of the soluble chlorides in the fly ash are removed by water washing; then, the washed fly ash is subjected to low-temperature pyrolysis at 300-500℃ in an oxygen-deficient or low-oxygen atmosphere, causing dechlorination of dioxin-like organic pollutants, thereby significantly reducing the toxicity and chloride content of the fly ash. For example, patent number CN118321305A discloses a waste incineration fly ash detoxification process, which includes removing dioxins from waste incineration fly ash through low-temperature pyrolysis to obtain low-temperature pyrolysis fly ash; and removing heavy metals and soluble salts from the fly ash by multi-stage, variable pH, and atmospheric pressure variable temperature immersion. Patent number CN117380711B discloses a low-temperature fly ash resource utilization system and its treatment method, which includes decomposing dioxins in fly ash to prepare low-temperature pyrolysis fly ash, pulping the low-temperature pyrolysis fly ash to obtain fly ash stock solution; and then washing the fly ash stock solution with water to remove chlorine.

[0004] However, although the water washing + low-temperature pyrolysis process can effectively degrade dioxins and significantly reduce the chlorine content of fly ash, it still has significant shortcomings in heavy metal stabilization. Studies have indicated that the migration and transformation mechanisms of heavy metals such as lead, zinc, and copper during subsequent heat treatment after water washing pretreatment are unclear, and commonly used leaching methods underestimate the long-term leaching risk of treated fly ash. In particular, this process is less effective at solidifying low-boiling-point heavy metals such as lead and cadmium, because these heavy metals are highly volatile in the low-temperature pyrolysis temperature range, especially between 300℃ and 400℃. The heat treatment process may actually activate their forms, transforming them from relatively stable compounds into exchangeable or carbonate-bound states, leading to an increase in the leaching concentration of heavy metals in the treated fly ash. Therefore, fly ash treated with water washing and low-temperature pyrolysis still cannot meet the heavy metal leaching limits in the "Identification Standard for Hazardous Waste - Leaching Toxicity Identification," and further solidification and stabilization treatment is required before it can be disposed of or utilized as general industrial solid waste.

[0005] To meet the requirements for the harmless disposal of fly ash, there are two main technical approaches for supplementary post-treatment: cement-based solidification and chelating agent chemical stabilization. Cement solidification involves mixing fly ash with cementitious materials such as silicate cement. The cement hydration products and the alkaline environment promote the formation of insoluble hydroxides or carbonates from heavy metals. Chelating agent stabilization typically uses dithiocarbamate organic chelating agents to form stable coordination compounds with heavy metal ions in the fly ash, reducing their leaching toxicity. However, both cement solidification and chelating agent chemical stabilization methods present significant economic challenges. Cement solidification alone leads to a significant increase in fly ash volume due to the large amount of cement used, while commercially available chemical chelating agents, especially large-molecule organic chelating agents, are expensive, significantly increasing the overall cost of fly ash treatment. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system. The purpose is to solve the problems of poor treatment effect and high cost and energy consumption of traditional water washing and low-temperature pyrolysis processes for low-boiling-point heavy metals. The invention designs a fly ash detoxification process that utilizes the residual moisture in the fly ash filter cake after water washing to generate hydrated calcium aluminosilicate gel in a semi-dry state during the drying process, thereby encapsulating heavy metals in the fly ash. Furthermore, the process achieves synergistic heavy metal solidification and dioxin degradation during low-temperature pyrolysis.

[0007] This invention provides a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system, comprising the following steps: The water washing and dechlorination step involves measuring the basic properties of the original fly ash and then sending it to the water washing unit for washing to remove soluble salts. After solid-liquid separation, a water-washed dechlorinated fly ash filter cake with a moisture content of 25% to 35% is obtained. The moisture retained in the fly ash filter cake obtained after solid-liquid separation in this step can be used as an in-situ water source for the hydration reaction in the subsequent drying reaction step, avoiding the energy consumption and cost of adding water separately. In the drying reaction step, the water-washed dechlorinated fly ash filter cake obtained in the water washing and dechlorination step is sent to the drying unit, and a composite solidifying agent is added to it based on the controlled ratio strategy. The composite solidifying agent is composed of cement clinker, gypsum, slag and fly ash. During the drying process, which utilizes the 25%~35% residual moisture in the water-washed dechlorinated fly ash filter cake to dry it to a moisture content of 5%~15%, the cement clinker in the composite solidifying agent preferentially hydrates and releases calcium hydroxide to activate the pozzolanic activity of slag and fly ash, generating a hydrated calcium aluminosilicate gel network to lattice-encapsulate the heavy metals in the fly ash. This step involves simultaneous hydration and water evaporation under mechanical shearing and warm conditions during drying and crushing. The resulting hydrated calcium aluminosilicate gel has a low calcium-to-silicon ratio and a long-chain, high-polymer structure, resulting in stronger encapsulation of heavy metals. It also simultaneously disposes of industrial solid wastes such as slag, fly ash, and desulfurization gypsum, thus achieving waste treatment with waste. In the low-temperature pyrolysis step, the material obtained from the drying reaction step is sent to the low-temperature pyrolysis unit for low-temperature thermal decomposition to remove dioxins and obtain qualified tailings. This step involves breaking down and dechlorinating dioxin-like organic compounds through low-temperature pyrolysis, converting them into harmless small molecule gases. Furthermore, by utilizing the thermal stability of hydrated calcium aluminosilicate gel with a low calcium-to-silicon ratio, the physical encapsulation and chemical solid solution of heavy metals are further enhanced during the pyrolysis process.

[0008] Furthermore, the amount of the composite sealing agent added is 10% to 15% of the dry weight of the fly ash after washing, and the dry weight of the fly ash after washing refers to the mass of the filter cake of the dechlorinated fly ash after washing and drying to constant weight.

[0009] Determine the total dosage of the composite sealant to prevent excessive dosage from causing cost waste or insufficient dosage from causing inadequate curing.

[0010] Furthermore, based on the total mass of the composite sealant, the mass ratio of cement clinker and gypsum is 50% to 79%, and the mass ratio of slag and fly ash is 21% to 50%.

[0011] The mixing ratio range of the composite sealant is given to ensure that the gel has sufficient aluminosilicate supply and a suitable calcium-silicon ratio, thereby improving the heavy metal fixation rate and thermal stability.

[0012] Furthermore, the specific adjustment and proportioning strategy is as follows: Obtain the values ​​corresponding to each influencing factor that affects the mass ratio of slag and fly ash. The influencing factors include the content of low-boiling-point heavy metals in fly ash, the content of calcium oxide, and the moisture content of the water-washed dechlorinated fly ash filter cake. Based on the mass ratio of slag and fly ash, the baseline value is 21% and the adjustable value is 29%. The values ​​of each influencing factor are normalized and weighted according to the pre-assigned weights. The result is multiplied by the adjustable value and then added to the baseline value to calculate the mass ratio of slag and fly ash, thereby obtaining the mass ratio of cement clinker and gypsum.

[0013] By considering the multidimensional factors affecting the mass ratio of slag and fly ash, the proportion of the composite solidifying agent is changed in real time, and the process formula is changed from a traditional fixed value to an adaptive optimization adjustment mode, thereby improving the robustness of the process.

[0014] Furthermore, a correction strategy is incorporated into the drying reaction step. This correction strategy is used to adjust the dosage of the composite sealing agent, specifically including: The compliant tailings refer to tailings with a heavy metal leaching rate lower than a preset limit. The heavy metal leaching rate of the compliant tailings is tested, and the ratio of the heavy metal leaching rate to the preset limit is calculated. If the ratio is greater than or equal to the preset high threshold, the dosage will be gradually increased; If the ratio is less than the preset high threshold and greater than the preset low threshold, then maintain the dosage. If the ratio is less than or equal to the preset low threshold, the dosage will be gradually reduced.

[0015] Based on the ratio of the heavy metal leaching rate of the qualified tailings to the preset limit, the ratio of the composite solidifying agent is adjusted in different areas. If the ratio is very small, it means that the heavy metal content is very low, and the dosage can be reduced. If the ratio is very large, it means that the heavy metal content is close to the limit, and the dosage is increased. This maintains the process effect and reduces costs while maintaining the process effect.

[0016] Furthermore, the step of obtaining the dry weight of the fly ash after water washing includes: The mass of the original fly ash was obtained and the moisture content of the original fly ash was determined. The dry weight of the original fly ash was then calculated. The elution rate of soluble salts in fly ash was determined in advance, and the dry weight of fly ash after water washing was calculated.

[0017] This invention addresses the problem of not being able to directly weigh the fly ash filter cake after washing during online processing. It indirectly calculates the dry weight of the fly ash after washing by utilizing the original fly ash mass, moisture content, and the soluble salt leaching rate during the washing process. It also enables the automatic batching of composite solidifying agents in continuous processing.

[0018] Furthermore, the drying temperature in the drying reaction step is 80℃~100℃, and the drying time is 15~25 minutes.

[0019] Provide a drying temperature range to avoid excessively high temperatures that cause rapid moisture evaporation and affect condensation formation, and provide a drying time range to ensure that the hydration reaction proceeds fully.

[0020] Furthermore, the pyrolysis temperature in the low-temperature pyrolysis step is 350℃~400℃, and the pyrolysis time is 30~90 minutes.

[0021] The temperature and duration range for low-temperature pyrolysis are given to ensure complete degradation of dioxins while maintaining the stability of the low calcium-to-silicon ratio gel.

[0022] Furthermore, the gypsum is desulfurized gypsum obtained in the pretreatment stage of the washing wastewater generated in the water washing and dechlorination step.

[0023] The gypsum used in the composite sealing agent is desulfurized gypsum produced during the treatment of washing wastewater, which does not need to be purchased externally. This achieves the goal of treating waste with waste, making resource utilization of sulfate ions in washing wastewater, and reducing the cost of purchasing gypsum externally.

[0024] The beneficial effects of this invention are as follows: This invention makes full use of the residual moisture in the fly ash filter cake after washing. The hydration reaction of the composite solidifying agent is completed simultaneously during the semi-dry drying process to a moisture content of 5% to 15%, without the need for additional water. Since the hydration reaction process consumes the residual moisture in the fly ash, there is no need for deep drying, thus saving energy. Furthermore, the spherical fly ash particles fill the voids in the hardened slurry, reducing the formation of cracks and weak areas, and also reducing the water requirement of the reaction process, thus avoiding the problem of insufficient in-situ moisture.

[0025] This invention employs a composite solidifying agent comprising cement clinker, slag, fly ash, and desulfurized gypsum. It utilizes the hydration reaction of cement clinker to release calcium hydroxide, thereby activating the pozzolanic activity of slag and fly ash. This generates a low-calcium-to-silicon ratio, high-polymerization-degree hydrated calcium aluminosilicate gel, which forms a deep lattice encapsulation of low-boiling-point heavy metals such as lead and cadmium. Furthermore, during low-temperature pyrolysis, the gel does not dehydrate or pulverize; instead, it undergoes further condensation to densify its structure, strengthening the solid-solution bond of heavy metals. This results in the treated tailings not only removing dioxins but also achieving a heavy metal leaching rate below the required limit.

[0026] This invention proposes an adaptive ratio adjustment strategy for composite solidifying agents during online treatment, enabling the ratio to adapt to changes in fly ash characteristics from different sources and batches. It also proposes a closed-loop feedback correction strategy based on the detection of heavy metal leaching rate in compliant tailings. By calculating the ratio of the measured leaching rate to the specified limit, the solidification surplus of the current batch is automatically determined, and zone-based decisions are made to increase, maintain, or decrease the dosage of composite solidifying agents, thereby dynamically optimizing the feeding of subsequent batches based on the actual treatment effect. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process for a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to the present invention.

[0028] Figure 2 This is a schematic diagram showing the component ratio of the composite solidifying agent in a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to the present invention.

[0029] Figure 3 This is a table showing the determination of heavy metal leaching toxicity in the original fly ash in Example 2.

[0030] Figure 4 This is a table showing the leaching toxicity test of heavy metals in fly ash after water washing in Example 2.

[0031] Figure 5 The fly ash in Example 2 is the fly ash after the drying reaction step.

[0032] Figure 6 This is a table showing the leaching toxicity determination of heavy metals in the tailings after low-temperature pyrolysis in Example 2. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] It should be noted that when a component is described as "fixed to" another component, it can be directly on the other component or may have a component in between. When a component is considered "connected to" another component, it can be directly connected to the other component or may have a component in between. When a component is considered "set on" another component, it can be directly set on the other component or may have a component in between. The terms "vertical," "horizontal," "left," "right," and similar expressions used in this document are for illustrative purposes only.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0036] Example 1 This invention provides a fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system, the specific steps of which are as follows: Water washing dechlorination steps The basic properties of raw fly ash were determined, including quantifiable physicochemical indicators such as moisture content, chloride content, heavy metal content, particle size distribution, density, and soluble salt composition and content. Methods included: determining the moisture content of raw fly ash by drying and weighing; determining the soluble chloride content by ion chromatography; determining the heavy metal content (lead, cadmium, zinc, chromium, etc.) by inductively coupled plasma mass spectrometry; and determining the particle size distribution by sieving.

[0037] After determining the basic properties of the raw fly ash, the raw fly ash is sent to the washing unit for washing. The washing unit can be a stirred washing vessel with a stirring function, so that the fly ash and water are fully mixed and washed. During the washing process, the fly ash and water are fully mixed to form a uniform suspension. The soluble chloride salts in the fly ash are mainly sodium chloride and potassium chloride, etc. These compounds are easily soluble in water. Therefore, during the stirring process, the high content of soluble salts such as sodium chloride and potassium chloride in the fly ash gradually dissolves in the water.

[0038] After stirring and washing, the fly ash suspension in the stirring and washing vessel is transported to a plate and frame filter press for solid-liquid separation. Under pressure, water passes through the filter cloth to form chlorine-containing wastewater, which is discharged from the drain. The fly ash solid particles are trapped on the surface of the filter cloth to form a filter cake. During the solid-liquid separation process, the moisture content of the filter cake is controlled by adjusting the pressure, filtration time, and filter cloth pore size of the plate and frame filter press. This ensures that the moisture content of the final water-washed dechlorinated fly ash filter cake is stably within the range of 25% to 35%. The water remaining in the water-washed dechlorinated fly ash filter cake serves as the in-situ water source for the hydration reaction in the subsequent drying reaction step.

[0039] Drying reaction steps The dechlorinated fly ash filter cake obtained from the water washing and dechlorination step, along with the composite solidifying agent with a ratio determined based on the control ratio strategy, are simultaneously fed into the drying unit. The drying unit adopts an integrated equipment that combines drying and mechanical crushing functions, which can shear and disperse the material during the drying process to prevent the filter cake from agglomerating and caking.

[0040] Among them, the composite solidifying agent is a multi-component cementitious material used to stabilize heavy metals in fly ash. It is composed of four industrial raw materials: cement clinker, gypsum, slag, and fly ash, blended in a specific ratio. The main mineral components of cement clinker are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. Therefore, it can undergo rapid hydration reaction upon contact with water. The residual moisture in the fly ash filter cake promotes the preferential hydration and release of Ca(OH)2 from the cement clinker. Slag and fly ash possess potential pozzolanic activity, which is activated under alkaline activators. Therefore, when the cement clinker hydration reaction releases Ca(OH)2, it activates the pozzolanic activity of the active mixture composed of slag and fly ash. The active mixture continuously dissolves silicon-oxygen tetrahedra or aluminum-oxygen tetrahedra, consuming Ca. 2+ and OH - This process promotes the targeted generation of C-(A)-SH gel with a low calcium-to-silicon ratio, namely a hydrated calcium aluminosilicate gel network. This gel has long chains and a high degree of polymerization, exhibiting excellent thermal stability in the pyrolysis range of 350℃ to 400℃. Pyrolysis not only does not destroy the crystal lattice but also strengthens the solid-solution bonding of heavy metals. The gypsum used is desulfurized gypsum produced during the pretreatment stage of the dechlorination step in the wastewater treatment process, which can be used to adjust the hydration rate of cement clinker and improve the performance of the cementitious system.

[0041] The dosage of the composite solidifying agent is 10% to 15% of the dry weight of the fly ash after washing. The dry weight of the fly ash after washing refers to the solid mass obtained after the filter cake of the dechlorinated fly ash after washing is dried to constant weight. Therefore, before starting the drying reaction, the dry weight of the fly ash after washing must be obtained to calculate the dosage of the composite solidifying agent required for the drying stage.

[0042] In one embodiment of the present invention, the calculation method is as follows: The dry weight of the original fly ash added to the washing tank can be calculated by obtaining the mass of the original fly ash added to the washing tank and the moisture content of the original fly ash obtained by measuring the basic properties before addition. The content of soluble salts in this batch of fly ash is then obtained through basic property determination, and the actual washing rate after water washing is calculated. The dry weight of the fly ash after water washing is calculated by subtracting the weight of the soluble salts removed by water washing from the original dry weight of fly ash.

[0043] The dosage of the composite sealant was determined, and its components were proportioned. The proportions were based on the total mass of the composite sealant, with cement clinker and gypsum accounting for 50%–79% of the mass, and slag and fly ash accounting for 21%–50% of the mass. (See attached figure.) Figure 2 The table is shown below.

[0044] The drying reaction stage involves drying the fly ash filter cake containing 25% to 35% residual moisture to a moisture content of 5% to 15% by using water-washed dechlorinated fly ash. During this process, the moisture in the material is evaporated, which causes the cement clinker in the composite solidifying agent to undergo a hydration reaction and release calcium hydroxide. This process activates the pozzolanic activity of the slag and fly ash, generating a hydrated calcium aluminosilicate gel network. This network encapsulates the heavy metals in the fly ash and physically isolates and chemically dissolves the heavy metal ions or compounds in the fly ash.

[0045] Due to differences in the original fly ash from different batches, the content of low-boiling-point heavy metals, calcium oxide, and moisture content in the fly ash filter cake after water washing and dechlorination vary. The content of low-boiling-point heavy metals determines the required lattice encapsulation capacity of the composite solidifying agent; higher content necessitates increasing the ratio of slag and fly ash to enhance pozzolanic gelation. The calcium oxide content reflects the alkalinity of the fly ash itself; higher content indicates stronger alkalinity of the surface fly ash, allowing for a reduction in cement clinker usage and a corresponding increase in the slag and fly ash ratio to balance the hydration reaction rate. Furthermore, the moisture content of the water-washed dechlorinated fly ash filter cake directly affects the rate of water evaporation during drying and the degree of cement clinker hydration. Higher moisture content provides more in-situ moisture for hydration, allowing for a reduction in the cement clinker ratio and an increase in the slag and fly ash ratio. Conversely, lower moisture content requires an increase in the cement clinker ratio to ensure sufficient hydration.

[0046] However, a fixed ratio of composite solidifying agent cannot match the actual stabilization requirements of each batch of fly ash. When the content of low-boiling-point heavy metals is too high and the proportion of slag and fly ash is insufficient, the heavy metal leaching rate will exceed the standard. When the content of calcium oxide is too high and the proportion of cement clinker is too high, the hydration will be too fast, causing the material to agglomerate and crack. When the moisture content of the filter cake is too low and the proportion of slag and fly ash is too high, the pozzolanic reaction will be insufficiently activated and the solidification effect will decrease.

[0047] Therefore, this invention proposes a proportioning control strategy to optimize the component ratio in the composite sealing agent in real time based on the characteristics of fly ash raw materials, specifically as follows: Based on the mass ratio of slag and fly ash ranging from 21% to 50%, it can be known that the minimum fixed value of the mass ratio of slag and fly ash is 21%, which is calibrated as the benchmark value, while the maximum adjustable range of the mass ratio of slag and fly ash is 29%, which is calibrated as the adjustable value. The numerical values ​​of factors affecting the ratio adjustment are obtained. These factors include the low-boiling-point heavy metal content in fly ash, the calcium oxide content, and the moisture content of the dechlorinated fly ash filter cake after washing. Specifically, the low-boiling-point heavy metal content in the washed fly ash can be calculated using the basic property measurement data of the original fly ash before entering the washing unit and the online detection of heavy metal content in the washing wastewater. The calcium oxide content in the fly ash is obtained from the basic property measurement of the original fly ash. The moisture content of the dechlorinated fly ash filter cake after washing can be obtained through real-time online detection using a monitoring sensor. The monitoring sensor can be a non-contact or semi-contact capacitive moisture sensor, a microwave moisture sensor, or a near-infrared moisture sensor. The values ​​of the three influencing factors were normalized. The normalization process can be carried out by taking the ratio of the difference between the obtained value and the minimum value of the influencing factor to the difference between the maximum value and the minimum value of the influencing factor. The influence of the three factors is pre-assigned corresponding weights, and the mass ratio of slag and fly ash is obtained using the calculation formula. The calculation formula is: ,in, As the baseline value; It is an adjustable value; This represents the normalized value for the content of low-boiling-point heavy metals. This represents the normalized value of calcium oxide content; Normalized value of moisture content in water-washed dechlorinated fly ash filter cake; , and Let be the weights corresponding to each impact factor, and satisfy . .

[0048] After obtaining the mass ratio of slag and fly ash, the mass ratio of cement clinker and gypsum is calculated by using the fact that the sum of the mass ratios of slag and fly ash, fly ash, cement clinker, and gypsum is 100%.

[0049] The washed fly ash filter cake and the metered composite solidifying agent are simultaneously fed into the drying equipment. The drying temperature is set at 80℃~100℃, and the drying time is 15~25 minutes. During the drying process, the residual moisture in the filter cake gradually evaporates, and the moisture content slowly decreases from 25% to 35% to 5% to 15%. During this process, the cement clinker in the composite solidifying agent preferentially undergoes a hydration reaction with the residual moisture. The tricalcium silicate and dicalcium silicate in the cement clinker rapidly hydrate and release calcium hydroxide, forming initial hydrated calcium silicate gel. The alkaline environment in the system is continuously enhanced, thereby activating the pozzolanic activity of the slag and fly ash. The active silica and alumina in the slag react with the hydroxide... Calcium reacts to form hydrated calcium aluminosilicate. The glass phase structure in fly ash, after being destroyed by alkaline substances, also participates in the hydration reaction to form hydrated calcium aluminosilicate. Various hydration products cross-link to form a dense hydrated calcium aluminosilicate gel network. This gel network has the structural characteristics of low calcium-to-silicon ratio and long chain with high polymerization degree. It can embed heavy metal ions or heavy metal compounds such as lead, cadmium, zinc, and chromium in fly ash into the crystal structure to achieve lattice encapsulation and prevent heavy metals from dissolving and migrating in the subsequent environment. At the same time, the mechanical shearing action of the drying and crushing equipment can prevent material agglomeration, ensure that the composite solidifying agent is in full contact with fly ash and that the hydration reaction proceeds uniformly, and avoid insufficient local solidification effect.

[0050] Furthermore, due to the spherical structure of the fly ash particles used, during the drying reaction step, the solid cementitious matrix formed after the composite solidifying agent reacts with the residual moisture in the washed fly ash filter cake to form a hardened slurry is achieved. The spherical fly ash particles can be evenly dispersed inside the hardened slurry, filling the gaps between large particles and capillary voids formed by the hydration products of cement clinker. This effectively reduces the overall porosity of the slurry, blocks the interconnected channels of pores, reduces the water penetration path, lowers the capillary water demand inside the slurry, and reduces the amount of additional water required for cement clinker hydration. Moreover, the spherical particles act as physical ball bearings during the slurry hardening process, dispersing the shrinkage stress inside the slurry and preventing stress concentration that could lead to micro-cracks. At the same time, they fill weak and loose areas, making the slurry structure dense and uniform, significantly reducing the formation of cracks and weak areas.

[0051] like Figure 1As shown, the drying reaction step not only utilizes the residual moisture in the fly ash filter cake after dechlorination as the in-situ water source for the hydration reaction of the composite solidifying agent, eliminating the need for additional fresh water and effectively reducing overall water consumption, but also allows the water evaporated from the fly ash filter cake during the drying process to be collected by a recovery device and recycled to the washing unit for use in the original fly ash agitation and washing operations. Additionally, the high-chlorine wastewater generated after the washing and dechlorination of the original fly ash in the washing unit, after pretreatment to remove suspended solids, heavy metal ions, and some organic pollutants, enters the evaporation crystallization system for concentration and crystallization. During the evaporation crystallization process, water vaporizes upon heating, forming water vapor, which is then recovered using a condensation recovery device. This recovered water is also transported to the washing unit via a dedicated pipeline for reuse, used for fly ash washing or system equipment rinsing. This entire water recovery and reuse system significantly reduces the amount of fresh water needed, lowers water consumption and wastewater discharge in the fly ash treatment process, and improves water resource utilization efficiency.

[0052] Low-temperature pyrolysis steps The material obtained from the drying reaction step is fed into a low-temperature pyrolysis unit. The low-temperature pyrolysis unit can be a special pyrolysis equipment with a closed rotary or vertical furnace structure that can precisely control the temperature, atmosphere and residence time. The thermal decomposition reaction is carried out in a low-temperature and oxygen-deficient environment. The thermal energy destroys the molecular structure of dioxin-like organic compounds, causing them to undergo dechlorination, chain breaking and ring opening reactions and decompose into harmless small molecule gases.

[0053] The low-temperature pyrolysis temperature was set at 350℃~400℃, the pyrolysis time was set at 30 minutes~90 minutes, and the oxygen concentration in the environment was controlled during the pyrolysis process to ensure an oxygen-deficient atmosphere. During the pyrolysis of dioxin-like organic compounds, the hydrated calcium aluminosilicate gel network formed by the composite sealing agent not only did not undergo dehydration and pulverization at temperatures of 350℃ to 400℃, but also became more compact due to thermal condensation reaction. This further enhanced the lattice encapsulation and chemical solid solution of low-boiling-point heavy metals such as lead and cadmium, preventing the volatilization and migration of heavy metals during pyrolysis.

[0054] After pyrolysis, the tailings obtained need to be naturally cooled to room temperature. Then, the dioxin content and heavy metal leaching rate of the tailings need to be tested. If both are lower than the standard limits specified in the national standard "Identification Standard for Hazardous Waste - Leaching Toxicity Identification", the tailings are considered compliant to ensure that they meet the requirements for the disposal of general industrial solid waste.

[0055] To achieve both optimal tailings leaching compliance and cost-effectiveness, this application proposes a modification strategy: a closed-loop dynamic adjustment of the composite solidifying agent dosage based on the heavy metal leaching rate test results of the compliant tailings leaching. Specifically: The concentration of heavy metals leaching in tailings measured according to the leaching method specified in the "Identification Standard for Hazardous Waste - Leaching Toxicity Identification" is compared with the upper limit of heavy metal leaching concentration specified in the current national standard. The ratio of the measured heavy metal leaching concentration in tailings to the corresponding standard limit directly reflects the excess level of the composite solidification agent's ability to stabilize heavy metals, that is, it shows the degree of excess solidification effect. The higher the ratio, the closer the heavy metal leaching rate is to the standard limit, indicating that the smaller the solidification margin, the higher the risk; the lower the ratio, the much smaller the heavy metal leaching rate is to the standard limit, indicating that the larger the solidification margin, the higher the safety. Based on the degree of curing effect, a zoning decision is made, with high and low thresholds pre-set. When the ratio is greater than or equal to the high threshold, it is determined that the current batch has insufficient curing effect and the heavy metal stabilization effect is close to the critical value, and the amount of composite sealant needs to be gradually increased. When the ratio is less than the high threshold and greater than the low threshold, it is determined that the current batch has a moderate degree of curing excess and the heavy metal stabilization effect is stable and controllable. No parameter adjustment is required. The current amount of composite sealant added should be kept unchanged to maintain stable process operation. When the ratio is less than or equal to the low threshold, it is determined that the current batch has sufficient curing margin and a large margin for heavy metal stabilization, and the dosage of composite sealant can be gradually reduced.

[0056] A closed-loop feedback correction strategy is adopted to dynamically optimize the feeding parameters based on the actual treatment effect during the process, so as to achieve both harmlessness and economic cost. In addition, the dosage is 10% to 15% of the dry weight of the fly ash after washing, and the dosage must still be between 10% and 15% after each adjustment.

[0057] Example 2 The basic properties of the raw fly ash were determined, and the results of the heavy metal leaching toxicity determination in the raw fly ash are attached. Figure 3 As shown, the raw fly ash is fed into a stirred washing tank and thoroughly mixed with water for washing and dechlorination. Then, it is separated into solid and liquid components using a plate and frame filter press to obtain a water-washed dechlorinated fly ash filter cake with a moisture content of approximately 30%. The leaching toxicity of heavy metals from the fly ash after water washing and dechlorination is shown in the attached figure. Figure 4 As shown; Based on the dry weight of the washed fly ash, the heavy metal content and moisture content of the washed fly ash, and the calcium oxide content in the fly ash, the dosage of the composite solidifying agent, which is 12% of the dry weight of the washed fly ash, was calculated. The amount of cement and the proportion of gypsum were also calculated to be 68%, and the proportion of the mixture of slag and fly ash was 32%. The fly ash filter cake and the added materials are fed into a reactor equipped with a stirring dryer. The drying temperature is set at 90℃, and the drying and stirring time is 20 minutes. Under mechanical shearing and warm drying conditions, the residual moisture in the fly ash filter cake promotes the preferential hydration of cement clinker, releasing Ca(OH)2. This in-situ activates the pozzolanic activity of the fly ash itself and the active admixtures. The reaction process not only consumes a large amount of residual moisture in the fly ash, thus eliminating the need for deep drying and saving energy, but also allows the fly ash and slag to react with the released Ca during the reaction. 2+ and OH - The reaction generates a hydrated calcium aluminosilicate (C-(A)-SH) gel network with a low calcium-to-silicon ratio (Ca / Si) and high degree of polymerization, which deeply encapsulates the heavy metals. The fly ash after drying is shown in the attached figure. Figure 5 As shown.

[0058] The dried fly ash particles, densely coated with heavy metals, are fed into a low-temperature pyrolysis unit. The pyrolysis temperature is set at 350℃, and the pyrolysis time is set at 60 minutes. At this temperature, residual dioxins in the fly ash undergo thermal desorption and chain-breaking degradation. Furthermore, the hydrated calcium aluminosilicate gel long chains obtained through the reaction with cement clinker and admixtures do not exhibit high-temperature dehydration and pulverization at this temperature; instead, they undergo high polymerization under thermal shrinkage, resulting in a denser gel skeleton and more stable coating. The final product is tailings that meet the standards below the limits set in the national standard "Identification Standard for Hazardous Waste - Leaching Toxicity Identification". The heavy metal leaching toxicity of the pyrolyzed fly ash is shown in the attached figure. Figure 6 As shown, the qualified tailings can be used as general solid waste for subsequent resource utilization.

[0059] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system, characterized in that, Includes the following steps: The water washing and dechlorination step involves measuring the basic properties of the original fly ash and then sending it to the water washing unit for washing to remove soluble salts. After solid-liquid separation, a water-washed dechlorinated fly ash filter cake with a moisture content of 25% to 35% is obtained. In the drying reaction step, the water-washed dechlorinated fly ash filter cake obtained in the water washing and dechlorination step is sent to the drying unit, and a composite solidifying agent is added to it based on the controlled ratio strategy. The composite solidifying agent is composed of cement clinker, gypsum, slag and fly ash. During the drying process, which utilizes the 25%~35% residual moisture in the water-washed dechlorinated fly ash filter cake to dry it to a moisture content of 5%~15%, the cement clinker in the composite solidifying agent preferentially hydrates and releases calcium hydroxide to activate the pozzolanic activity of slag and fly ash, generating a hydrated calcium aluminosilicate gel network to lattice-encapsulate the heavy metals in the fly ash. In the low-temperature pyrolysis step, the material obtained from the drying reaction step is sent to the low-temperature pyrolysis unit for low-temperature thermal decomposition to remove dioxins and obtain qualified tailings.

2. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 1, characterized in that, The amount of the composite sealing agent added is 10% to 15% of the dry weight of the fly ash after washing. The dry weight of the fly ash after washing refers to the mass of the filter cake of the dechlorinated fly ash after washing and drying to constant weight.

3. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 2, characterized in that, Based on the total mass of the composite sealant, the cement clinker and gypsum account for 50% to 79% of the mass, and the slag and fly ash account for 21% to 50% of the mass.

4. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 3, characterized in that, The specific adjustment and proportioning strategy is as follows: Obtain the values ​​corresponding to each influencing factor that affects the mass ratio of slag and fly ash. The influencing factors include the content of low-boiling-point heavy metals in fly ash, the content of calcium oxide, and the moisture content of the water-washed dechlorinated fly ash filter cake. Based on the mass ratio of slag and fly ash, the baseline value is 21% and the adjustable value is 29%. The values ​​of each influencing factor are normalized and weighted according to the pre-assigned weights. The result is multiplied by the adjustable value and then added to the baseline value to calculate the mass ratio of slag and fly ash, thereby obtaining the mass ratio of cement clinker and gypsum.

5. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 4, characterized in that, The drying reaction step includes a correction strategy for adjusting the dosage of the composite sealing agent, specifically including: The compliant tailings refer to tailings with a heavy metal leaching rate lower than a preset limit. The heavy metal leaching rate of the compliant tailings is tested, and the ratio of the heavy metal leaching rate to the preset limit is calculated. If the ratio is greater than or equal to the preset high threshold, the dosage will be gradually increased; If the ratio is less than the preset high threshold and greater than the preset low threshold, then maintain the dosage. If the ratio is less than or equal to the preset low threshold, the dosage will be gradually reduced.

6. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 2, characterized in that, The steps for obtaining the dry weight of the fly ash after water washing include: The mass of the original fly ash was obtained and the moisture content of the original fly ash was determined. The dry weight of the original fly ash was then calculated. The elution rate of soluble salts in fly ash was determined in advance, and the dry weight of fly ash after water washing was calculated.

7. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 1, characterized in that, The drying temperature in the drying reaction step is 80℃~100℃, and the drying time is 15~25 minutes.

8. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 1, characterized in that, The pyrolysis temperature in the low-temperature pyrolysis step is 350℃~400℃, and the pyrolysis time is 30~90 minutes.

9. The fly ash water washing and pyrolysis synergistic detoxification process based on a composite silicate cement system according to claim 1, characterized in that, The gypsum is desulfurized gypsum obtained from the pretreatment stage of the washing wastewater generated in the water washing and dechlorination step.

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