Process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash

Abstract

The invention relates to the technical field of solid waste resource utilization and adsorption material preparation, in particular to a process system for subcritical hydrothermal preparation of an activated carbon-zeolite composite molecular sieve from fly ash. The process system comprises a raw material pretreatment unit, a homogenizing and blending unit, a pressurization and heat compensation device, a subcritical hydrothermal reaction device, a heat recovery and pressure regulation device, a pressure self-driven separation device, a purification and drying device, an inorganic salt separation device and a wastewater neutralization unit. The fly ash is used as a raw material, the subcritical hydrothermal reaction device is combined with the characteristics of subcritical water and the synergistic effect of an oxidizing agent, conversion of silicon-aluminum components in the fly ash into zeolite and in-situ activation of carbon residues into activated carbon are synchronously achieved, and the problems that in the technology for preparing the zeolite molecular sieve from the fly ash, carbon residue resources are wasted, the waste liquid generation amount is large, and the cost is low are solved. The treatment cost is high; and the reaction energy consumption is high.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization and adsorption material preparation technology, specifically to a process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal methods. Background Technology

[0002] Fly ash is a major industrial solid waste generated by coal-fired power plants and industrial boilers. Long-term stockpiling not only occupies land resources, but its heavy metals and dust content can also easily cause soil and groundwater pollution. Therefore, promoting the transformation of fly ash from low-value disposal to high-value conversion has become a core industry demand. The abundant silica and alumina resources in fly ash can serve as a key raw material for the synthesis of zeolite molecular sieves. Zeolite molecular sieves, as a type of aluminosilicate crystal with a regular microporous structure, are widely used in wastewater treatment and exhaust gas purification due to their excellent adsorption and ion exchange properties. However, commercially available zeolites are mostly synthesized using high-purity chemical raw materials such as silica sol and sodium aluminate, with raw material costs accounting for more than 60% of the total preparation cost, and the production process is energy-intensive.

[0003] To reduce the preparation cost of zeolite molecular sieves, researchers have attempted to use fly ash as a silicon-aluminum source for synthesis. Existing processes for preparing zeolite molecular sieves mainly consist of a core reaction unit and a post-processing unit. The core reaction unit primarily utilizes traditional hydrothermal methods, alkali fusion methods, and two-step hydrothermal methods. However, in the traditional hydrothermal method, the inert crystalline phases such as mullite and quartz in fly ash are difficult to fully dissolve, resulting in a silicon-aluminum utilization rate of less than 50%. The alkali fusion method has high energy consumption and cost. The two-step hydrothermal method suffers from cumbersome operation, large wastewater generation, high treatment costs, and difficulty in large-scale application. Furthermore, since fly ash typically contains 5%–10% residual carbon (unburned carbon), existing processes generally treat this residual carbon as an impurity, typically removing it beforehand in the post-processing unit through acid washing or high-temperature calcination. This not only wastes carbon resources but also increases energy consumption. Therefore, the existing process system for synthesizing zeolite molecular sieves using fly ash as raw material has significant technical defects in terms of raw material utilization, energy consumption, waste liquid treatment, and residual carbon resource utilization. Summary of the Invention

[0004] To address the problems of waste of residual carbon resources, large amount of waste liquid generation, high treatment cost, and high reaction energy consumption in the process system for preparing zeolite molecular sieves from fly ash, this invention provides a process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal heating.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows.

[0006] This invention provides a process system for preparing activated carbon-zeolite composite molecular sieves from fly ash using subcritical hydrothermal methods. The system includes a pretreatment and blending device, a pressurization and heating device, a subcritical hydrothermal reaction device, a pressure regulating and separation device, and a post-treatment unit. The pretreatment and blending device is used to dissolve the silica-alumina components in the fly ash and form a slurry with a silica-alumina ratio of 1:0.5-3. The outlet of the pretreatment and blending device is connected to the pressurization and heating device to adjust the temperature and pressure of the slurry to the conditions for subcritical hydrothermal crystallization reaction. The outlet of the pressurization and heating device is connected to the subcritical hydrothermal reaction device. The oxidant inlet of the subcritical hydrothermal reaction device is connected to a carbon activation enhancer supply unit to provide trace amounts of oxidant, which reacts with the slurry. A subcritical hydrothermal crystallization reaction is carried out to simultaneously convert the silica-alumina components in fly ash into zeolite molecular sieves and activate residual carbon into activated carbon in situ, forming a mixed liquid of activated carbon-zeolite composite molecular sieves. The outlet of the subcritical hydrothermal reactor is connected to a pressure regulating and separation device. The post-treatment unit includes a purification and drying device, an inorganic salt separation device, and a wastewater neutralization unit. The solid phase outlet of the pressure regulating and separation device is connected to the purification and drying device. The liquid phase outlet of the pressure regulating and separation device is connected to the inorganic salt separation device. The alkali outlets of the purification and drying device and the inorganic salt separation device are both connected to the wastewater neutralization unit. The acid outlet of the pretreatment and blending device is connected to the wastewater neutralization unit.

[0007] Preferably, the pretreatment and blending device includes a raw material pretreatment unit and a homogenization and blending unit; the raw material pretreatment unit includes a crushing and screening unit and an acid washing and impurity removal unit; the outlet of the crushing and screening unit is connected to the inlet of the acid washing and impurity removal unit, and is used to perform acid washing treatment on the pretreated fly ash at a temperature of 60℃~90℃ to remove ferric ions from the fly ash; the acid outlet of the acid washing and impurity removal unit is connected to the inlet of the wastewater neutralization unit; the discharge port of the acid washing and impurity removal unit is connected to the inlet of the homogenization and blending unit.

[0008] Preferably, the fly ash inlet of the homogenizing and blending unit is connected to the outlet of the raw material pretreatment unit, the alkali source inlet of the homogenizing and blending unit is connected to the outlet of the alkali source addition unit, the silicon-aluminum component inlet of the homogenizing and blending unit is connected to the outlet of the silicon-aluminum component addition unit, and the outlet of the homogenizing and blending unit is connected to the inlet of the pressurization and heating device.

[0009] Preferably, the pressurization and heating device is equipped with a first heat exchange component and a pressurization component, which are used to adjust the crystallization temperature of the slurry entering the subcritical hydrothermal reactor to 120°C to 250°C and the pressure to 0.3MPa to 5MPa.

[0010] Preferably, the oxidant is a hydrogen peroxide solution, ozone, or oxygen; wherein the mass concentration of the hydrogen peroxide solution is 5% to 20%; and the amount of oxidant added is 1% to 5% of the mass of fly ash.

[0011] Preferably, the pressure regulating and separation device includes a heat recovery pressure regulating device and a pressure self-driven separation device; the outlet of the subcritical hydrothermal reactor is connected to the inlet of the heat recovery pressure regulating device; the outlet of the heat recovery pressure regulating device is connected to the pressure self-driven separation device; the solid phase outlet of the pressure self-driven separation device is connected to the purification and drying device; and the liquid phase outlet of the pressure self-driven separation device is connected to the inorganic salt separation device.

[0012] Preferably, the heat recovery pressure regulating device is provided with a second heat exchange component and a pressure regulating component, which are used to regulate the temperature of the mixture to 60℃~80℃ and the pressure to 0.1MPa~0.3MPa; the outlet of the second heat exchange component is connected to the inlet of the heat exchange unit of the inorganic salt separation device through a heat transfer oil pipeline.

[0013] Preferably, the purification and drying device includes a washing unit and a drying unit connected in sequence, wherein the alkali outlet of the washing unit is connected to the wastewater neutralization unit; and the outlet of the drying unit is connected to a storage silo for storing activated carbon-zeolite composite molecular sieves.

[0014] Preferably, the salt outlet of the inorganic salt separation device is connected to the salt recovery unit; the mother liquor outlet of the inorganic salt separation device is connected to the mother liquor recovery unit; and the mother liquor reflux port of the mother liquor recovery unit is connected to the feed end of the pretreatment and blending device and the wastewater neutralization unit.

[0015] The beneficial effects of this invention are: 1. The process system of the present invention, through the cooperation of a pretreatment and blending device, a pressurization and heating device, a subcritical hydrothermal reaction device, a pressure regulating and separation device, and a post-treatment unit, uses fly ash as raw material. Through the synergy of subcritical hydrothermal reaction and carbon activation enhancer, it simultaneously completes the crystallization transformation of silicon and aluminum components in fly ash into zeolite and the in-situ activation of residual carbon into activated carbon, avoiding the waste of residual carbon resources. The final product is an activated carbon-zeolite composite molecular sieve, which solves the problems of waste of residual carbon resources, large amount of waste liquid generated, high treatment cost, and high reaction energy consumption in the process system for preparing zeolite molecular sieves from fly ash.

[0016] 2. The subcritical hydrothermal reactor of the present invention mainly relies on the strong alkaline activation effect of high concentration of hydroxide ions in a subcritical water environment to achieve efficient activation of fly ash silicon-aluminum components without the need for large-scale addition of external alkaline sources. At the same time, combined with the mother liquor recovery unit, the unreacted alkaline source in the waste liquid is returned to the homogenization and blending unit for recycling, realizing the dual optimization of "less addition + full reuse" of alkaline source, which reduces the total amount of alkaline source used, avoids the waste of alkaline source, and significantly reduces the cost of raw materials.

[0017] 3. This invention utilizes a heat recovery and pressure regulation device to recover the waste heat of the mixed liquid after the subcritical hydrothermal reaction, providing a heat source for the salt evaporation process of the inorganic salt separation device, replacing the purchased heat source, reducing additional energy consumption, and meeting the requirements of low-carbon production. On the other hand, by neutralizing the pickling waste liquid and washing wastewater through the wastewater neutralization unit, and in conjunction with the inorganic salt separation device to recover the salts in the waste liquid, the direct discharge of acid and alkaline wastewater and the problem of water salinization are avoided, solving the pain point of waste liquid pollution in traditional process systems and realizing clean production throughout the entire process.

[0018] 4. The present invention uses a raw material pretreatment unit for crushing and screening, and acid washing to remove impurities. Combined with a homogenization and blending unit to precisely control the silicon-aluminum ratio, it reduces the formation of impurity crystals and ensures the purity and crystallinity of the composite molecular sieve. The various units work together to form a complete process system. The process is controllable and stable, which can meet the needs of large-scale production and has broad prospects for industrialization. Attached Figure Description

[0019] Figure 1 A flow chart of the process system for preparing activated carbon-zeolite composite molecular sieves from fly ash using subcritical hydrothermal methods, as provided in an embodiment of the present invention.

[0020] Explanation of reference numerals in the attached figures: 1. Raw material pretreatment unit; 1a. Crushing and screening unit; 1b. Acid washing and impurity removal unit; 2. Homogenization and blending unit; 2a. Alkali source addition unit; 2b. Silicon and aluminum component addition unit; 3. Pressurization and heating device; 4. Subcritical hydrothermal reactor; 4a. Carbon activation enhancer supply unit; 5. Heat recovery and pressure regulation device; 6. Pressure self-driven separation device; 7. Purification and drying device; 7a. Washing unit; 7b. Drying unit; 8. Inorganic salt separation device; 8a. Salt recovery unit; 8b. Mother liquor recovery unit; 9. Storage silo; 10. Wastewater neutralization unit. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0022] 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.

[0023] Traditional hydrothermal methods involve mixing fly ash with alkaline solution and crystallizing at 60℃–100℃ for 24–120 hours. However, due to the difficulty in fully dissolving inert crystalline phases such as mullite and quartz in fly ash, the utilization rate of silicon and aluminum is less than 50%, and other crystalline phases are easily mixed into the product. Alkali fusion methods require calcining fly ash with a strong alkali at 550℃–800℃, which can increase the utilization rate of silicon and aluminum to over 80%, but the high-temperature calcination energy consumption accounts for 40%–50% of the total cost, and it places stringent requirements on the corrosion resistance and high-temperature resistance of the equipment. Two-step hydrothermal methods extract silicon and aluminum components through two separate crystallization processes, which helps improve product purity, but the operation is cumbersome, the water consumption is 2–3 times that of traditional hydrothermal methods, the waste liquid generation is large, and the treatment cost is high, making large-scale application difficult. Furthermore, existing technologies generally suffer from excessive alkali consumption, which not only increases raw material costs but also burdens subsequent waste liquid treatment.

[0024] Furthermore, fly ash typically contains 5%–10% residual carbon (unburned carbon), which is often treated as an impurity by existing technologies and removed through acid washing or high-temperature calcination. This not only wastes carbon resources but also increases energy consumption. Currently, residual carbon treatment mainly revolves around two methods: "additional activation steps" or "optimization of crystals relying on auxiliary materials." Some studies require adding a separate pre-activation process for residual carbon, such as pre-treating the residual carbon with steam before zeolite synthesis, which is cumbersome and increases energy consumption. Other studies retain residual carbon in situ, but require external substances such as directing agents and seed crystals to mitigate the impact of residual carbon on zeolite crystallinity, which not only increases raw material costs but also makes it difficult to achieve efficient and coordinated processes throughout the entire process.

[0025] In this invention, relying on a subcritical hydrothermal system and utilizing the high hydroxide concentration of subcritical water, the silicon and aluminum components in fly ash are dissolved and crystallized into zeolite. At the same time, a trace amount of oxidant is added as needed through a carbon activation enhancer supply unit, and the residual carbon is directly activated into activated carbon in situ within the same reaction device. The entire process does not require a separate residual carbon removal or activation step. The conversion of silicon and aluminum to zeolite and the activation of residual carbon to activated carbon are completed simultaneously, which greatly simplifies the process flow and reduces equipment investment and operating costs.

[0026] Furthermore, by leveraging the characteristics of the subcritical hydrothermal environment to accelerate the dissolution rate and crystallization process of the silicon-aluminum components, the interference of residual carbon on zeolite crystallization is avoided. This eliminates the need for auxiliary substances such as directing agents and seed crystals as described in the literature, thereby reducing raw material consumption and ensuring product quality stability.

[0027] Moreover, the activated carbon formed after the residual carbon is activated has a much more developed porous structure than the original residual carbon, and its specific surface area is significantly increased, which can give full play to the advantages of physical adsorption. The present invention forms activated carbon by activating residual carbon, so that the adsorption performance of the obtained activated carbon-zeolite composite molecular sieve is better than that of composite materials that simply retain residual carbon.

[0028] In summary, the carbon residue treatment method of the present invention not only solves the defects of the existing technology, such as cumbersome process and dependence on auxiliary materials, but also achieves the integration of high-value carbon residue and efficient zeolite preparation through simultaneous activation, accelerated crystallization and structural optimization, which better meets the industrial production requirements for high efficiency, low cost and high quality.

[0029] The technical solution of the present invention will be further described below through specific embodiments. Unless otherwise specified, the methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0030] See Figure 1 A process system for preparing activated carbon-zeolite composite molecular sieves from fly ash using subcritical hydrothermal methods includes a raw material pretreatment unit 1, a homogenization and blending unit 2, a pressurization and heating device 3, a subcritical hydrothermal reaction device 4, a heat recovery and pressure regulation device 5, a pressure self-driven separation device 6, a purification and drying device 7, an inorganic salt separation device 8, a storage silo 9, and a wastewater neutralization unit 10. Each unit is connected sequentially through pipelines to form a complete process link.

[0031] The subcritical hydrothermal reactor 4 is equipped with a carbon activation enhancer supply unit 4a to provide trace amounts of oxidant required for the reaction; the homogenization and mixing unit 2 is equipped with an alkali source addition unit 2a and a silicon-aluminum component addition unit 2b, which are used for alkali source supply and silicon-aluminum ratio control, respectively; the purification and drying unit 7 includes a washing unit 7a and a drying unit 7b connected in sequence to achieve purification and drying of solid products; the inorganic salt separation unit 8 includes a salt recovery unit 8a and a mother liquor recovery unit 8b to complete the recovery of salts in waste liquid and the recycling of mother liquor; and the wastewater neutralization unit 10 receives the acidic and alkaline wastewater generated throughout the process to achieve environmentally friendly treatment.

[0032] The feed inlet of raw material pretreatment unit 1 is connected to a fly ash silo to receive fly ash raw materials discharged from the power plant. The raw material pretreatment unit 1 internally includes a crushing and screening unit 1a and an acid washing and impurity removal unit 1b. The outlet of the crushing and screening unit 1a is connected to the inlet of the acid washing and impurity removal unit 1b. The mechanical action of the crushing and screening unit 1a refines the particle size of the fly ash raw materials, and then the acid washing and impurity removal unit 1b removes impurities through chemical treatment. The solid discharge outlet of the raw material pretreatment unit 1 is connected to a homogenization and blending unit 2 to transport the treated fly ash to the homogenization and blending unit 2. The waste liquid outlet of the raw material pretreatment unit 1 is connected to a wastewater neutralization unit 10 via a pipeline to transport the acidic waste liquid generated from acid washing to the wastewater neutralization unit 10.

[0033] The first addition port of the homogenizing and blending unit 2 is connected to the alkali source addition unit 2a for alkali source supply; the first addition port of the homogenizing and blending unit 2 is also connected to the silicon-aluminum component addition unit 2b for silicon-aluminum ratio control. Specifically, the homogenizing and blending unit 2 receives fly ash after impurity removal, and simultaneously adds alkali source solution through the alkali source addition unit 2a, and adds silicon and aluminum sources through the silicon-aluminum component addition unit 2b. The mixture is then stirred by a built-in stirring mechanism to form a homogeneous initial slurry. The discharge port of the homogenizing and blending unit 2 is connected to the pressurization and heating device 3 for conveying the initial slurry to the pressurization and heating device 3.

[0034] The pressurization and heating device 3 is equipped with a first heat exchange component and a pressurization component. The pressurization and heating device 3 uses the first heat exchange component and the pressurization component to work together to adjust the temperature and pressure of the initial slurry to the initial state required for the subcritical hydrothermal reaction. Then, the adjusted slurry is transported to the subcritical hydrothermal reaction device 4.

[0035] The outlet of the pressurization and heating device 3 is connected to the subcritical hydrothermal reactor 4; the additive inlet of the subcritical hydrothermal reactor 4 is connected to the carbon activation enhancer supply unit 4a. Specifically, the subcritical hydrothermal reactor 4 is equipped with a carbon activation enhancer supply unit 4a, which is used to provide trace amounts of oxidant required for the reaction. In this embodiment of the invention, trace amounts of oxidant, such as hydrogen peroxide, ozone, and industrial oxygen, are added to the reaction system in the subcritical hydrothermal reactor 4 as needed through the carbon activation enhancer supply unit 4a. With the assistance of the stirring mechanism, the slurry in the subcritical hydrothermal reactor 4 undergoes a subcritical hydrothermal crystallization reaction, simultaneously realizing the conversion of the silicon-aluminum components in fly ash to zeolite molecular sieves and the in-situ activation of residual carbon to activated carbon, forming a composite molecular sieve mixture. The composite molecular sieve mixture is then transported to the heat recovery and pressure regulating device 5.

[0036] The outlet of the subcritical hydrothermal reactor 4 is connected to a heat recovery and pressure regulating device 5. A second heat exchange component and a pressure regulating component are fixedly installed inside the heat recovery and pressure regulating device 5. The pressure regulating component adjusts the pressure of the mixture to a suitable separation range. The outlet of the second heat exchange component is connected to the heat exchange unit of the inorganic salt separation device 8 via a heat transfer oil pipeline, providing the inorganic salt separation device 8 with the heat required for salt evaporation, thus utilizing waste heat. In this embodiment of the invention, the heat recovery and pressure regulating device 5, on the one hand, recovers waste heat from the composite molecular sieve mixture through the second heat exchange component, which can be transported to the inorganic salt separation device 8 via a heat transfer oil pipeline for salt evaporation; on the other hand, it regulates the pressure of the mixture to a suitable separation range. The treated mixture is then transported to a pressure-driven separation device 6.

[0037] The outlet of the heat recovery pressure regulating device 5 is connected to the pressure self-driven separation device 6. The solid phase outlet of the pressure self-driven separation device 6 is connected to the purification and drying device 7, and the liquid phase outlet of the pressure self-driven separation device 6 is connected to the inorganic salt separation device 8. Specifically, the pressure self-driven separation device 6 uses the residual pressure of the mixture itself to achieve solid-liquid separation. The separated solid phase product is transported to the purification and drying device 7, and the liquid phase waste liquid is transported to the inorganic salt separation device 8.

[0038] The purification and drying device 7 includes a washing unit 7a and a drying unit 7b connected in sequence to achieve the purification and drying of the solid product. The waste liquid outlet of the washing unit 7 is connected to the wastewater neutralization unit 10; the discharge port of the drying unit 7b is connected to the storage silo 9. Specifically, the washing unit 7a of the purification and drying device 7 uses deionized water to spray and wash the solid product to remove impurities and excess alkali solution adhering to the surface. The alkaline wastewater generated during washing is transported to the wastewater neutralization unit 10. The washed solid product enters the drying unit 7b for drying treatment to obtain a composite molecular sieve product that meets the requirements, which is then stored in the storage silo 9.

[0039] The outlet of the second heat exchange component of the heat recovery pressure regulating device 5 is connected to the heat exchange unit of the inorganic salt separation device 8 via a heat transfer oil pipeline. This provides the inorganic salt separation device 8 with the heat required for salt evaporation, thus utilizing waste heat. The inorganic salt outlet of the inorganic salt separation device 8 is connected to the salt recovery unit 8a, and the mother liquor outlet of the inorganic salt separation device 8 is connected to the mother liquor recovery unit 8b, completing the recovery of salts from the waste liquid and the recycling of the mother liquor. The liquid outlet of the mother liquor recovery unit 8b is connected to the homogenization and blending unit 2 and the wastewater neutralization unit 10, respectively. Specifically, the inorganic salt separation device 8 uses the waste heat supplied by the heat recovery pressure regulating device 5 to heat and evaporate the liquid phase waste liquid separated by the pressure self-driven separation device 6, recovering salt by-products, or separating inorganic salts through membrane separation technology. The separated inorganic salts are then recovered to the salt recovery unit 8a, and the separated mother liquor enters the mother liquor recovery unit 8b. After concentration detection and adjustment, it is returned to the homogenization and blending unit 2 for slurry preparation. Unqualified mother liquor is directly discharged to the wastewater neutralization unit 10.

[0040] Wastewater neutralization unit 10 accepts both acidic and alkaline wastewater generated throughout the entire process, achieving environmentally friendly treatment. Specifically, wastewater neutralization unit 10 uses a built-in stirring mechanism and an online pH monitor to mix and adjust the acidic wastewater to neutral. The treated wastewater is then discharged as needed or transported to the plant's circulating water system for reuse.

[0041] To further optimize process performance, improve product quality and resource utilization efficiency, the present invention provides the following preferred embodiments. The settings of each preferred parameter are based on extensive experimental verification and can be flexibly adjusted according to actual production needs and raw material characteristics to ensure the practicality and stability of the process.

[0042] In a preferred embodiment, the raw material pretreatment unit 1 controls the target particle size D of the crushed and screened fly ash to be 20μm to 50μm, and the qualified particle size rate of the screened fly ash is ≥90%. By refining the fly ash to this particle size range, its specific surface area and reactive sites can be significantly increased, promoting sufficient contact between the silicon-aluminum components and the alkali source in the subsequent homogenization and blending unit 2. At the same time, it avoids the problems of increased crushing energy consumption and particle agglomeration caused by excessively small particle size, or insufficient reaction and low silicon-aluminum utilization caused by excessively large particle size. The setting of this particle size range can achieve an optimal balance between "reactivity" and "processing cost".

[0043] In a preferred embodiment, the acid source in the acid washing and impurity removal unit 1b is a hydrochloric acid or sulfuric acid solution with a concentration of 3 mol / L to 5 mol / L, which is used to treat the pretreated fly ash at 60℃ to 90℃ for 1 to 2 hours. This combination of parameters ensures that the removal rate of ferric ions in the fly ash is ≥90%, effectively avoiding interference from residual iron ions on the subsequent zeolite crystal structure. At the same time, the acid concentration and treatment conditions can strictly control the acid washing intensity, avoiding excessive acid washing that could lead to the loss of aluminum components in the fly ash, and ensuring the stability of the silicon-aluminum ratio in the subsequent slurry.

[0044] In a preferred embodiment, the stirring speed of the homogenization and mixing unit 2 is 300 rpm to 500 rpm, and the mixing time is 2 min to 30 min. These stirring parameters ensure that the fly ash, alkali source solution, and silicon-aluminum components are fully mixed to form a uniform initial slurry, avoiding excessive local differences in alkali concentration due to uneven mixing, thus laying the foundation for the uniformity of the subsequent subcritical hydrothermal reaction.

[0045] In a preferred embodiment, the alkali source in the alkali source addition unit 2a is a NaOH solution or KOH solution with a concentration of 1 mol / L to 3 mol / L. A small amount of alkali solution is added to adjust the pH value of the slurry to 7 to 10. This pH range of the slurry can adapt to the subsequent subcritical hydrothermal environment, ensuring that the dissolution rate of silicon and aluminum components in fly ash reaches more than 80%, while effectively inhibiting the formation of impurities such as hydroxyl sodalite.

[0046] In a preferred embodiment, the silicon-aluminum component addition unit 2b contains a silicon source and an aluminum source, wherein the added silicon source is sodium silicate and the added aluminum source is sodium aluminate, used to adjust the silicon-aluminum ratio (SiO2 / Al2O3) of the initial slurry to be 1:0.5 to 3. This silicon-aluminum ratio range can be flexibly adjusted according to the application scenario of the target product. For example, when it is necessary to prepare X-type zeolite suitable for heavy metal adsorption, the silicon-aluminum ratio can be controlled at 1:0.5 to 1.5; when it is necessary to prepare Y-type zeolite suitable for VOCs adsorption, the silicon-aluminum ratio can be adjusted to 1:1.5 to 3, significantly broadening the applicability of the product.

[0047] In a preferred embodiment, the crystallization temperature T of the subcritical hydrothermal reactor 4 is adjustable within the range of 120℃ to 250℃, and the crystallization time t is controlled within the range of 8h to 36h; wherein, the crystallization temperature T is further preferably 130℃ to 180℃, and the crystallization time t is further preferably 12h to 28h. For example, the crystallization temperature T can be 120℃, 130℃, 140℃, 150℃, 160℃, 180℃, 200℃, 220℃, and 250℃, etc.

[0048] In this embodiment of the invention, the crystallization temperature T parameter range can precisely control the reaction process. For example, when the temperature is below 120°C, the characteristics of subcritical water, namely "low dielectric constant and high ion product", cannot be fully utilized, the silica-alumina minerals dissolve slowly, and the crystallization time needs to be extended to more than 72 hours, which seriously affects production efficiency. When the temperature is above 250°C, the pressure resistance of the equipment needs to be increased from 0.3MPa to 5MPa to 5MPa to 10MPa, which increases the equipment cost and easily leads to excessive growth of zeolite crystals, shrinkage of pore size, and reduction of adsorption performance.

[0049] Meanwhile, controlling the time parameter is equally crucial. Specifically, if the time is less than 8 hours, the zeolite crystals will not develop completely, resulting in low crystallinity; if the time is longer than 36 hours, zeolite crystal transformation will occur, leading to a decrease in the product's adsorption performance. The optimal combination of a temperature of 130℃ to 180℃ and an optimal time of 12 to 28 hours can ensure that the zeolite crystallinity is ≥85%, while controlling the reaction cycle within a reasonable range, thus balancing product quality and production efficiency.

[0050] In a preferred embodiment, the oxidant provided by the carbon activation enhancer supply unit 4a is a hydrogen peroxide solution with a mass concentration of 5%–20%, and the amount of oxidant added is 1%–5% of the fly ash mass. This oxidant parameter can match the 5%–10% residual carbon content in the fly ash, ensuring a residual carbon activation rate ≥90%, simultaneously forming a "zeolite-activated carbon" composite structure, resulting in a significantly improved specific surface area compared to single zeolite. If the hydrogen peroxide concentration is too high, it is prone to violent decomposition, wasting residual carbon resources and increasing raw material costs. The mass concentration of the hydrogen peroxide solution can be 5%, 10%, 15%, and 20%, etc. The amount of oxidant added is 1%, 2%, 3%, 4%, and 5% of the fly ash mass, etc.

[0051] In a preferred embodiment, the heat recovery pressure regulating device 5 reduces the pressure of the mixed liquid from 1MPa to 5MPa to 0.1MPa to 0.3MPa, and the temperature from 120℃ to 200℃ to 60℃ to 80℃. When the pressure is higher than 0.3MPa, the pressure-driven separation device 6 needs to use high-pressure separation equipment, increasing equipment investment costs; when the pressure is lower than 0.1MPa, the separation efficiency decreases, and the water content of the solid product exceeds 40%, significantly increasing the energy consumption of the subsequent drying process; the temperature of 60 to 80℃ ensures the stability of salt solubility in the liquid waste liquid, preventing premature precipitation and pipe blockage due to excessively low temperatures, while providing sufficient waste heat for the inorganic salt separation device, reducing the consumption of external heat sources.

[0052] In a preferred embodiment, the separation speed of the pressure self-driven separation device 6 is set to 400 rpm, and the separation time is controlled to be 15 min to 20 min. These parameters ensure that the water content of the solid product after separation is <30%, the energy consumption of the drying process is reduced by more than 25%, and the liquid-solid content of the liquid waste is <0.5%, preventing solid particles from entering the inorganic salt separation device and causing scaling in the salt recovery unit, thus ensuring the long-term stable operation of the equipment.

[0053] In a preferred embodiment, the washing unit 7a of the purification and drying apparatus 7 washes the solid product to pH 6-7. This endpoint pH ensures the removal of excess alkali and soluble salts remaining on the surface of the solid product, avoiding acid residue corrosion of the equipment when pH < 6, or alkali residue causing a decrease of more than 15% in the adsorption capacity of the product for heavy metals when pH > 7, while maintaining the adsorption active sites of the product.

[0054] In a preferred embodiment, the drying temperature of the drying unit 7b of the purification drying device 7 is 80℃~120℃, and the drying time is 4h~6h. When the temperature is below 80℃, the moisture content of the dried product exceeds 15%, which easily absorbs moisture and clumps, affecting storage and use; when the temperature is above 120℃, the zeolite pore structure is prone to collapse; the combination of 80℃~120℃ and 4h~6h can ensure that the product moisture content is ≤10%, while completely preserving the pore structure of zeolite and activated carbon, ensuring stable adsorption performance.

[0055] In a preferred embodiment, the salt recovery unit 8a of the inorganic salt separation device 8 heats the liquid waste liquid to 80°C–120°C, and the evaporation time is 2–3 hours. This parameter ensures that the salts are fully separated, with a salt recovery rate of ≥90%. The recovered salts are mainly Na2SO4 and / or K2SO4, which can be reused as industrial auxiliary materials or sold externally. When the temperature is below 80°C, the evaporation rate is slow and the processing efficiency is low; when the temperature is above 120°C, the alkaline components in the mother liquor are lost, increasing the cost of replenishing the alkali source.

[0056] In a preferred embodiment, the mother liquor recovery unit 8b controls the number of mother liquor recycling cycles to ≤3 times, and maintains an alkali concentration of 1 mol / L to 3 mol / L during the recycling process. Experimental results show that within 3 cycles, the silica-alumina concentration in the mother liquor still meets the requirements for slurry preparation; maintaining an alkali concentration of 1 mol / L to 3 mol / L can prevent a decrease in the silica-alumina dissolution rate in subsequent reactions due to insufficient alkali, ensuring process stability. When the mother liquor cannot be recycled, it is discharged to the wastewater neutralization unit 10 for neutralization treatment.

[0057] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash, characterized in that, Includes a pretreatment and blending device, a pressurization and heating device (3), a subcritical hydrothermal reactor (4), a pressure regulating and separation device, and a post-treatment unit; The pretreatment and blending device is used to dissolve the silicon and aluminum components in fly ash and form a slurry with a silicon-to-aluminum ratio of 1:0.5 to 3; The outlet of the pretreatment and blending device is connected to the pressurization and heating device (3) to adjust the temperature and pressure of the slurry to the conditions of subcritical hydrothermal crystallization reaction. The outlet of the pressurizing and heating device (3) is connected to the subcritical hydrothermal reaction device (4). The oxidant inlet of the subcritical hydrothermal reaction device (4) is connected to a carbon activation enhancer supply unit (4a) to provide oxidant and carry out subcritical hydrothermal crystallization reaction with the slurry. Simultaneously, the silicon and aluminum components in the fly ash are converted into zeolite molecular sieves and the residual carbon is activated into activated carbon in situ, forming a mixture of activated carbon-zeolite composite molecular sieve. The outlet of the subcritical hydrothermal reactor (4) is connected to the pressure regulating and separation device; The post-treatment unit includes a purification and drying device (7), an inorganic salt separation device (8), and a wastewater neutralization unit (10). The solid phase outlet of the pressure regulating and separation device is connected to the purification and drying device (7); the liquid phase outlet of the pressure regulating and separation device is connected to the inorganic salt separation device (8). The alkaline outlet of the purification and drying device (7) and the alkaline outlet of the inorganic salt separation device (8) are both connected to the wastewater neutralization unit (10), and the acid outlet of the pretreatment and blending device is connected to the wastewater neutralization unit (10).

2. The process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash according to claim 1, characterized in that, The pretreatment and blending device includes a raw material pretreatment unit (1) and a homogenization blending unit (2); The raw material pretreatment unit (1) includes a crushing and screening unit (1a) and an acid washing and impurity removal unit (1b); the outlet of the crushing and screening unit (1a) is connected to the inlet of the acid washing and impurity removal unit (1b) for acid washing of the pretreated fly ash at 60℃~90℃ to remove ferric ions from the fly ash; the acid outlet of the acid washing and impurity removal unit (1b) is connected to the inlet of the wastewater neutralization unit (10); the discharge port of the acid washing and impurity removal unit (1b) is connected to the inlet of the homogenization and blending unit (2).

3. The process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash according to claim 2, characterized in that, The fly ash inlet of the homogenizing and blending unit (2) is connected to the outlet of the raw material pretreatment unit (1), the alkali source inlet of the homogenizing and blending unit (2) is connected to the outlet of the alkali source addition unit (2a), the silicon-aluminum component inlet of the homogenizing and blending unit (2) is connected to the outlet of the silicon-aluminum component addition unit (2b), and the outlet of the homogenizing and blending unit (2) is connected to the inlet of the pressurization and heating device (3).

4. The process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash according to claim 1, characterized in that, The pressurization and heating device (3) is equipped with a first heat exchange component and a pressurization component, which are used to adjust the crystallization temperature of the slurry entering the subcritical hydrothermal reactor (4) to 120℃~250℃ and the pressure to 0.3MPa~5MPa.

5. The process system for subcritical hydrothermal preparation of activated carbon-zeolite composite molecular sieve from fly ash according to claim 1, characterized in that, The oxidant is a hydrogen peroxide solution, ozone, or oxygen; wherein the mass concentration of the hydrogen peroxide solution is 5% to 20%; and the amount of oxidant added is 1% to 5% of the mass of fly ash.

6. The process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal treatment according to claim 1, characterized in that, The pressure regulating and separation device includes a heat recovery pressure regulating device (5) and a pressure self-driven separation device (6); the outlet of the subcritical hydrothermal reactor (4) is connected to the inlet of the heat recovery pressure regulating device (5); the outlet of the heat recovery pressure regulating device (5) is connected to the pressure self-driven separation device (6); the solid phase outlet of the pressure self-driven separation device (6) is connected to the purification and drying device (7); and the liquid phase outlet of the pressure self-driven separation device (6) is connected to the inorganic salt separation device (8).

7. The process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal treatment according to claim 6, characterized in that, The heat recovery pressure regulating device (5) is equipped with a second heat exchange component and a pressure regulating component, which are used to adjust the temperature of the mixture to 60℃~80℃ and the pressure to 0.1MPa~0.3MPa; the outlet of the second heat exchange component is connected to the inlet of the heat exchange unit of the inorganic salt separation device (8) through a heat transfer oil pipeline.

8. The process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal treatment according to claim 1, characterized in that, The purification and drying device (7) includes a washing unit (7a) and a drying unit (7b) connected in sequence. The alkaline outlet of the washing unit (7a) is connected to the wastewater neutralization unit (10). The outlet of the drying unit (7b) is connected to a storage bin (9) for storing activated carbon-zeolite composite molecular sieves.

9. The process system for preparing activated carbon-zeolite composite molecular sieves from fly ash via subcritical hydrothermal treatment according to claim 1, characterized in that, The salt outlet of the inorganic salt separation device (8) is connected to the salt recovery unit (8a); the mother liquor outlet of the inorganic salt separation device (8) is connected to the mother liquor recovery unit (8b); the mother liquor return port of the mother liquor recovery unit (8b) is connected to the feed end of the pretreatment and blending device and the wastewater neutralization unit (10).

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