Pickling fly ash pretreatment method

Abstract

The invention relates to the technical field of solid waste treatment, in particular to a pickling fly ash pretreatment method, which comprises the steps of storage and transportation, constant volume extraction, water washing, water washing filtrate treatment, drying and dioxin detoxification, takes enhanced pickling pretreatment as a core, and integrates the technologies of efficient solid-liquid separation, multi-stage water washing, MVR wastewater zero discharge treatment and reuse, dioxin high-temperature detoxification and the like. The method aims at solving the problems that in traditional fly ash treatment, harmful substances such as chlorides, heavy metals and dioxin are not thoroughly removed, stabilization is insufficient, water consumption is large, high-salinity wastewater discharge pressure is high, and resource utilization is limited. By adopting the technical scheme, harmful substances such as chlorides, heavy metals and dioxin in the fly ash can be thoroughly removed and stabilized, the problems of high water consumption and high-salinity wastewater are solved, and the resource utilization value of the fly ash and the environmental safety are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of solid waste treatment technology, and more specifically, relates to a method for pretreatment of acid-washed fly ash. Background Technology

[0002] With the acceleration of global urbanization and the continuous development of the industrial economy, the amount of solid waste generated is increasing dramatically. Among these, municipal solid waste incineration has become one of the mainstream treatment methods due to its significant advantages in volume reduction and harmless treatment. However, fly ash produced by waste incineration, as a hazardous waste enriched with chlorides, dioxins, and various heavy metals, presents a major challenge to environmental protection and the circular economy in terms of proper, efficient, and environmentally friendly disposal and resource utilization. How to effectively pretreat fly ash to reduce its toxicity, improve its stability, and safely apply it to resource utilization pathways such as building materials and soil remediation is the core focus of technological research in this field.

[0003] In the existing technology, several technical solutions have been proposed and applied for the pretreatment and resource utilization of fly ash. For example, patent document with publication number CN113145606B discloses a method for preparing a paste slurry for the resource utilization of fly ash from municipal solid waste incineration. This method aims to pretreat fly ash through a combination of mechanochemical stabilization and mineral stabilization to prepare a fly ash slurry with low solubility, low migration, and low toxicity.

[0004] The patent document with publication number CN118527462B proposes a method and system for efficient detoxification, quality improvement and reuse of fly ash from waste incineration. This technical solution significantly improves the dechlorination efficiency and heavy metal stability of fly ash by using a wet mechanical-chemical method coupled with carbonation and multi-stage water washing processes, and further realizes resource utilization through the preparation of non-fired bricks.

[0005] The above methods have the following problems: the removal effect on soluble salts, especially chlorides, contained in fly ash is relatively limited. Furthermore, relying solely on mechanochemical stabilization and mineral stabilizers may not achieve thorough and long-term removal of certain highly toxic heavy metals that exist in multiple complex forms. Summary of the Invention

[0006] To achieve the complete removal and stabilization of harmful substances such as chlorides, various heavy metals, and dioxins in municipal solid waste incineration fly ash, this invention provides an acid washing fly ash pretreatment method. Through acid washing pretreatment, multi-stage water washing, MVR, and high-temperature dioxin detoxification, a systematic solution integrating harmful substance removal, water resource recycling, and high-quality assurance of resource products is constructed, thereby effectively improving the value and environmental friendliness of fly ash resource utilization.

[0007] The specific technical solution of this invention is as follows: A method for pretreating pickling fly ash includes the following steps: Storage and transportation: receiving fly ash collected from waste incineration plants and transporting it to a dedicated fly ash silo through a closed negative pressure conveying system. The fly ash silo is equipped with a dust removal device to ensure that particulate matter emissions meet standards during the storage and transportation of fly ash. Volumetric leaching: The fly ash in the fly ash silo is transported to the acid-resistant leaching tank through a precise metering device, and the fly ash is mixed and stirred with hydrochloric acid to form a slurry; Water washing: The slurry is fed into the solid-liquid separation equipment to separate the obtained filter residue and slurry filtrate. The filter residue is washed by a multi-stage countercurrent washing system to obtain fly ash filter cake. Filtrate treatment: The slurry filtrate is subjected to deep treatment. First, suspended solids are removed by flocculation and sedimentation, then calcium and magnesium ions are removed by chemical co-precipitation, and finally the filtrate is introduced into the MVR evaporation and crystallization system to obtain a clear liquid that meets the reuse standards. Drying: The fly ash filter cake is conveyed to a drying device for dehydration and drying, so that its moisture content is reduced to the standard range; Dioxin detoxification: The dried fly ash filter cake is fed into a high-temperature thermal desorption device and heated in an oxygen-free or oxygen-deficient environment, causing dioxin-like substances in the fly ash to undergo thermal desorption, transferring from the solid phase to the gas phase. The dioxin-containing gas phase substances are then rapidly cooled and adsorbed to ensure that the exhaust gas meets emission standards, thereby ensuring that the treated fly ash residue meets the requirements for dioxin toxicity equivalent limits for resource utilization.

[0008] Furthermore, the closed negative pressure conveying system in the storage and transportation step consists of a vacuum pump, a suction hose, a cyclone separator, and a bag filter connected in series. The vacuum pump provides a negative pressure suction of 0.06 to 0.08 MPa. The dust removal device is a pulse-jet bag filter with a filtration accuracy of 0.5 μm and a collection efficiency of ≥99.99%. The fly ash silo has a conical bottom design and is equipped with a vibration-assisted unloading device.

[0009] Furthermore, in the constant volume extraction step, the solid-liquid ratio of fly ash to hydrochloric acid in the extraction tank is 1:3 to 1:5, the mass ratio of hydrochloric acid to fly ash is 0.15 to 0.3:1, the residence time in the extraction tank is 60 to 120 minutes, and the temperature in the extraction tank is 50 to 70°C.

[0010] Furthermore, in the washing step, the solid-liquid separation equipment is a filter press or a centrifuge; the multi-stage countercurrent washing system is a four-stage washing system, each stage of washing includes a stirring and mixing unit and a solid-liquid separation unit. In the stirring and mixing unit, the mass ratio of washing water to filter cake is 1:1 to 2:1, and the stirring time is 15 to 30 minutes. The solid-liquid separation unit is a belt vacuum filter or a diaphragm filter press. The filtrate generated by the next stage solid-liquid separation unit in the multi-stage countercurrent washing system is used countercurrently as washing water for the previous stage. The fly ash filter cake has a moisture content ≤30% and a total chlorine content ≤0.1%.

[0011] Furthermore, in the filtrate treatment, the flocculant in the flocculation precipitation is a 10% polyaluminum chloride solution, with an addition amount of 100–150 mg / L; in the chemical precipitation, the pH value is 9.5–10, and the precipitant is sodium carbonate or calcium hydroxide.

[0012] Furthermore, in the drying step, the drying temperature is 200-220℃, the drying time is 1-2 hours, and the moisture content of the dried fly ash filter cake is ≤4%.

[0013] Furthermore, the dioxin detoxification includes the following steps: the dried fly ash filter cake is fed into a high-temperature thermal desorption device, the oxygen content in the high-temperature thermal desorption device is set to ≤0.5%, the temperature is 390~410℃, and after staying for 30~1h, desorbed flue gas and fly ash residue are obtained. The fly ash residue is discharged after cooling. The desorbed flue gas enters a quenching device to be cooled to below 180℃ and then enters an adsorption device. After adsorption, it is discharged after dust removal by a bag filter.

[0014] This invention, through the integrated application of technologies such as enhanced acid washing, multi-stage countercurrent washing, MVR zero-emission, and dioxin thermal desorption, demonstrates significant advantages over traditional treatment methods in terms of hazardous substance removal efficiency, water resource recycling, and the resource value of the final products. Especially in the complete removal of chlorides and dioxins, and the efficient reduction of heavy metals, this invention provides solutions that meet higher environmental standards and resource utilization requirements. Detailed Implementation

[0015] This invention provides a method for pretreatment of pickling fly ash, comprising the following steps: Storage and transportation: receiving fly ash collected from waste incineration plants and transporting it to a dedicated fly ash silo through a closed negative pressure conveying system. The fly ash silo is equipped with a dust removal device to ensure that particulate matter emissions meet standards during the storage and transportation of fly ash. Specifically, in this invention, the receiving, storage, and conveying processes of fly ash are all carried out in a strictly sealed negative pressure environment to completely eliminate secondary pollution caused by dust escape. The sealed negative pressure conveying system in this invention consists of a vacuum pump, a suction hose, a cyclone separator, and a bag filter connected in series. The vacuum pump provides a negative pressure suction of 0.06–0.08 MPa to ensure high-speed, leak-free transmission of fly ash within the suction hose. The cyclone separator is used for preliminary separation of large fly ash particles to reduce the load on the downstream filter. The bag filter is equipped with polytetrafluoroethylene (PTFE) membrane filter media, achieving a filtration accuracy of up to 0.5 μm and a total collection efficiency exceeding 99.99%. After suction, the fly ash enters a dedicated fly ash silo, which is equipped with a pulse-jet bag filter at the top. The bottom of the silo features a 60-degree conical design and is equipped with a high-frequency vibration motor to ensure smooth discharge of fly ash under gravity and vibration assistance, effectively avoiding clogging and bridging.

[0016] Volumetric leaching: The fly ash in the fly ash silo is transported to the acid-resistant leaching tank through a precise metering device, and the fly ash is mixed and stirred with hydrochloric acid to form a slurry; Specifically, in the constant volume leaching step, the solid-liquid ratio of fly ash to hydrochloric acid in the leaching tank is 1:3 to 1:5, the mass ratio of hydrochloric acid to fly ash is 0.15 to 0.3:1, the residence time in the leaching tank is 60 to 120 minutes, and the temperature in the leaching tank is 50 to 70℃.

[0017] Water washing: The slurry is fed into the solid-liquid separation equipment to separate the obtained filter residue and slurry filtrate. The filter residue is washed by a multi-stage countercurrent washing system to obtain fly ash filter cake. Specifically, the aim is to thoroughly remove residual soluble chloride ions and other salts from the wet fly ash residue after acid leaching, ensuring that the final product meets the requirement of a chloride content of less than 0.1% for building material applications. The leached wet fly ash residue is then transported to a four-stage countercurrent washing system. In this invention, each stage of the four-stage countercurrent washing system consists of a stirred mixing tank and a solid-liquid separation device connected in series. The stirred mixing tank has an effective volume of 10 m³. 3 The impeller-driven mixing tank has a mixing power of 5.5 kW and a mixing speed of 60 rpm to ensure thorough mixing of the slurry within a 15-minute residence time. In the first-stage washing, wet fly ash slag is mixed with wash water at a mass ratio of 1:1.5. The mixed slurry then enters a continuous belt vacuum filter; in this embodiment, the vacuum filter has a filtration area of ​​15 m². 2The filter cloth is made of polypropylene, and the vacuum degree is -0.08 MPa. The filter cake thickness after filtration is 20 mm, and the filtration efficiency reaches over 98%. The separated filter cake has a moisture content of approximately 35% and a chlorine content of approximately 0.8%. The process flow for the second to fourth stages of washing is similar to that of the first stage, but the washing water used in each stage is the filtrate produced in the next stage of washing, thus forming a highly efficient countercurrent washing mode. After the fourth stage of washing and filtration, the fly ash filter cake has a moisture content controlled below 30% and a total chlorine content significantly reduced to 0.08%. It is then transported to the drying section via a belt conveyor for further processing. The high-concentration fly ash washing filtrate is uniformly transported to the washing filtrate treatment section for deep treatment and resource utilization. The design of the four-stage countercurrent washing system in this invention maximizes washing efficiency and minimizes water consumption by using the filtrate produced in each stage of washing in a countercurrent manner for the previous stage of washing.

[0018] Filtrate treatment: The slurry filtrate is subjected to deep treatment. First, suspended solids are removed by flocculation and sedimentation, then calcium and magnesium ions are removed by chemical co-precipitation, and finally the filtrate is introduced into the MVR evaporation and crystallization system to obtain a clear liquid that meets the reuse standards. Specifically, high-concentration fly ash washing filtrate undergoes advanced treatment to achieve zero discharge of water resources and recovery of valuable substances. In addition to high levels of chloride, potassium, and sodium ions, the filtrate contains approximately 200 mg / L of fine suspended solids, 250 mg / L of calcium ions, and 80 mg / L of magnesium ions. First, the filtrate enters a pretreatment unit. This unit includes a coagulant dosing system (dosing 10% polyaluminum chloride (PAC) solution at a dosage of 100 mg / L) and a flocculant dosing system (dosing 0.2% nonionic polyacrylamide (PAM) solution at a dosage of 5 mg / L). The filtrate first enters a rapid mixing tank (retention time 30 seconds, G value 300 s⁻¹), followed by a slow mixing tank (retention time 20 minutes, G value 30 s⁻¹) to promote floc formation. The mixed slurry then flows into an inclined tube sedimentation tank (surface loading rate 1.0 m / (h·m²), inclination angle 60 degrees, plate spacing 80 mm). Suspended solids form dense flocs and settle. The clarified liquid overflows into subsequent treatment stages, with an effluent suspended solids concentration below 10 mg / L. The sludge discharged from the bottom of the sedimentation tank is dewatered by a plate and frame filter press, and the sludge cake can be safely disposed of according to its composition. Subsequently, the clarified liquid enters a chemical co-precipitation unit, where the pH is precisely adjusted to 9.5±0.1, and a 15% sodium carbonate solution is added to promote the precipitation of calcium ions as calcium carbonate. The dosage is calculated based on the stoichiometry of calcium ion concentration. Simultaneously, magnesium ions precipitate as magnesium hydroxide at a higher pH. After dewatering by the plate and frame filter press, the moisture content of the calcium-magnesium sludge can be reduced to 65%. It is then returned to the upstream constant volume extraction section via a dedicated screw conveyor system, where unreacted calcium and magnesium components react further with hydrochloric acid, achieving resource recycling and expected to reduce acid consumption by approximately 5%. The supernatant after chemical co-precipitation, with calcium and magnesium ion concentrations reduced to below 50 mg / L and 30 mg / L respectively, further enters the clear water tank. In the clear water tank, the clarified liquid first passes through a multi-media filter (1.5 m filter bed height, 8 m / h filtration rate, composed of quartz sand, anthracite, and magnetite) to remove residual suspended solids larger than 10 micrometers, resulting in an effluent turbidity below 2 NTU. Subsequently, the clarified liquid enters an ultrafiltration membrane system (using a PVDF hollow fiber membrane, 0.02 micrometer pore size, membrane flux of 50 L / (m²·h), and transmembrane pressure difference of 0.05 MPa) for further fine filtration, ensuring an effluent suspended solids concentration below 0.5 mg / L and an effluent SDI15 value below 3, providing high-quality feed water for the subsequent mechanical compression evaporation crystallization (MVR) process.

[0019] Drying: The fly ash filter cake is conveyed to a drying device for dehydration and drying, so that its moisture content is reduced to the standard range; Specifically, the fly ash filter cake after four stages of washing has a moisture content of approximately 28%, is a viscous slurry, and easily forms clumps. To meet the requirements of subsequent processing and resource utilization, it needs to be dehydrated and dried. This invention uses a rotary paddle dryer to dry the fly ash filter cake. The rotary paddle dryer is a twin-shaft nested dryer with multiple layers of hollow paddles inside. High-temperature heat transfer oil generated from a heat transfer oil boiler is introduced into the paddles and the jacket, with an inlet temperature of 250℃ and an outlet temperature of 200℃. The material is indirectly heated through the rotation of the paddles and heat conduction, while the stirring action breaks up clumps and prevents adhesion. The dryer cylinder is 10 meters long and 1.5 meters in diameter, equipped with a 20 kW drive motor with an adjustable speed range of 2-8 rpm. The hot air during the drying process is provided by an external natural gas hot air furnace, with the hot air temperature controlled between 200℃ and 220℃ and a hot air volume of 10,000 Nm³ / h. The hot air comes into countercurrent contact with the wet fly ash, carrying away the water vapor generated by evaporation. The residence time of fly ash in the dryer is approximately 60 minutes. Some of the dried fly ash residue has a moisture content reduced to below 4%, meeting the requirements for calorific value and composition for co-processing in cement kilns, such as achieving acceptable heavy metal leaching concentrations and loss on ignition. It can be directly fed into the kiln for co-processing via a pneumatic conveying system (conveyor velocity 25 m / s, conveying distance 200 meters) or a sealed belt conveyor, serving as a raw material substitute for cement production and achieving resource utilization. Another portion of the fly ash residue, after drying, will be sent to the dioxin detoxification section. Water vapor generated during the drying process enters a multi-stage cyclone separator and bag filter combination device along with the hot air to remove fine particulate matter, and then enters a water vapor collection and condensation device. The condensation device uses a shell-and-tube heat exchanger with cooling water as the medium to condense the water vapor into liquid water. The condensed water can be reused or discharged in compliance with standards, depending on its quality (conductivity, pH value). The precise design of this drying section ensures that the fly ash meets the physical requirements for subsequent co-processing in cement kilns or utilization in building materials, and that energy consumption is less than 60 kWh / ton of dry ash.

[0020] Dioxin detoxification: The dried fly ash filter cake is fed into a high-temperature thermal desorption device and heated in an oxygen-free or oxygen-deficient environment, causing dioxin-like substances in the fly ash to undergo thermal desorption, transferring from the solid phase to the gas phase. The dioxin-containing gas phase substances are then rapidly cooled and adsorbed to ensure that the exhaust gas meets emission standards, thereby ensuring that the treated fly ash residue meets the requirements for dioxin toxicity equivalent limits for resource utilization.

[0021] Specifically, due to their poor water solubility, low volatility, and chemical stability, dioxins have not been effectively removed in previous acid washing, water washing, and drying processes. This invention employs high-temperature thermal desorption technology to detoxify dioxins from dried fly ash. Specifically, the dried fly ash is fed into an indirect-heating rotary kiln pyrolysis furnace via a closed conveying system. Heat is provided by an external electric heater, and an atmosphere-isolated heating chamber is located outside the furnace. Inside the rotary kiln, the fly ash is heated to 400℃±10℃ in a nitrogen-protected, oxygen-free or extremely oxygen-deficient environment for 30 minutes. At this temperature, dioxin-like substances in the fly ash undergo thermal desorption, efficiently transferring from the solid phase to the gas phase. This process effectively prevents the de novo synthesis of dioxins. The fly ash residue after thermal desorption (after dioxin detoxification), after being cooled to below 100°C, can be safely fed into subsequent building material manufacturing processes for resource utilization as a harmless byproduct. For example, it can be used as a cement admixture, concrete admixture, or ceramsite raw material, with its activity index and strength indicators meeting relevant building material standards. The dioxin-containing gaseous substances discharged from the pyrolysis furnace are first subjected to rapid cooling. A spray quenching tower injects atomized water into the flue gas, rapidly reducing the flue gas temperature to below 180°C within one second to maximally inhibit dioxin resynthesis. Subsequently, the rapidly cooled flue gas enters an activated carbon adsorption tower. This activated carbon adsorption tower is a two-stage series fixed-bed adsorption tower filled with high-efficiency granular activated carbon with an iodine value of not less than 1100 mg / g and a specific surface area greater than 1200 m² / g. The activated carbon, through its large specific surface area and porous structure, efficiently physical adsorbs dioxin-like substances in the gas phase. The exhaust gas, after being adsorbed by activated carbon, is filtered through a bag filter with a filtration precision of less than 1 μm. After further purification, it is discharged at high altitude through a chimney, ensuring that the final exhaust gas emissions meet the national emission standards for dioxins and that the particulate matter concentration is below 5 mg / Nm³. The saturated activated carbon is replaced periodically (every 3 months) and sent to a qualified hazardous waste incinerator for high-temperature incineration to completely destroy the dioxins.

[0022] This invention, through the integrated application of technologies such as enhanced acid washing, multi-stage countercurrent washing, MVR zero-emission, and dioxin thermal desorption, demonstrates significant advantages over traditional treatment methods in terms of hazardous substance removal efficiency, water resource recycling, and the resource value of the final products. Especially in the complete removal of chlorides and dioxins, and the efficient reduction of heavy metals, this invention provides solutions that meet higher environmental standards and resource utilization requirements.

Claims

1. A method for pretreatment of pickling fly ash, characterized in that, Includes the following steps: Storage and transportation: receiving fly ash collected from waste incineration plants and transporting it to a dedicated fly ash silo through a closed negative pressure conveying system. The fly ash silo is equipped with a dust removal device to ensure that particulate matter emissions meet standards during the storage and transportation of fly ash. Volumetric leaching: The fly ash in the fly ash silo is transported to the acid-resistant leaching tank through a precise metering device, and the fly ash is mixed and stirred with hydrochloric acid to form a slurry; Water washing: The slurry is fed into the solid-liquid separation equipment to separate the obtained filter residue and slurry filtrate. The filter residue is washed by a multi-stage countercurrent washing system to obtain fly ash filter cake. Filtrate treatment: The slurry filtrate is subjected to deep treatment. First, suspended solids are removed by flocculation and sedimentation, then calcium and magnesium ions are removed by chemical co-precipitation, and finally the filtrate is introduced into the MVR evaporation and crystallization system to obtain a clear liquid that meets the reuse standards. Drying: The fly ash filter cake is conveyed to a drying device for dehydration and drying, so that its moisture content is reduced to the standard range; Dioxin detoxification: The dried fly ash filter cake is fed into a high-temperature thermal desorption device and heated in an oxygen-free or oxygen-deficient environment, causing dioxin-like substances in the fly ash to undergo thermal desorption, transferring from the solid phase to the gas phase. The dioxin-containing gas phase substances are then rapidly cooled and adsorbed to ensure that the exhaust gas meets emission standards, thereby ensuring that the treated fly ash residue meets the requirements for dioxin toxicity equivalent limits for resource utilization.

2. The pretreatment method for pickling fly ash according to claim 1, characterized in that: The closed negative pressure conveying system in the storage and transportation process consists of a vacuum pump, a suction hose, a cyclone separator, and a bag filter connected in series. The vacuum pump provides a negative pressure suction of 0.06 to 0.08 MPa. The dust removal device is a pulse-jet bag filter with a filtration accuracy of 0.5 μm and a collection efficiency of ≥99.99%. The fly ash silo has a conical bottom design and is equipped with a vibration-assisted unloading device.

3. The pretreatment method for pickling fly ash according to claim 1, characterized in that: In the constant volume extraction step, the solid-liquid ratio of fly ash to hydrochloric acid in the extraction tank is 1:3 to 1:5, the mass ratio of hydrochloric acid to fly ash is 0.15 to 0.3:1, the residence time in the extraction tank is 60 to 120 minutes, and the temperature in the extraction tank is 50 to 70°C.

4. The pretreatment method for pickling fly ash as described in claim 1, characterized in that: In the washing step, the solid-liquid separation equipment is a filter press or a centrifuge; the multi-stage countercurrent washing system is a four-stage washing system, each stage of washing includes a stirring and mixing unit and a solid-liquid separation unit. In the stirring and mixing unit, the mass ratio of washing water to filter cake is 1:1 to 2:1, and the stirring time is 15 to 30 minutes. The solid-liquid separation unit is a belt vacuum filter or a diaphragm filter press. The filtrate generated by the next stage solid-liquid separation unit in the multi-stage countercurrent washing system is used countercurrently as washing water for the previous stage. The fly ash filter cake has a moisture content ≤30% and a total chlorine content ≤0.1%.

5. The pretreatment method for pickling fly ash according to claim 1, characterized in that: In the filtrate treatment, the flocculant in the flocculation and precipitation process is a 10% polyaluminum chloride solution, with a dosage of 100–150 mg / L; in the chemical precipitation process, the pH value is 9.5–10, and the precipitant is sodium carbonate or calcium hydroxide.

6. The pretreatment method for pickling fly ash according to claim 1, characterized in that: In the drying step, the drying temperature is 200-220℃, the drying time is 1-2 hours, and the moisture content of the dried fly ash filter cake is ≤4%.

7. The pretreatment method for pickling fly ash according to claim 1, characterized in that: The dioxin detoxification includes the following steps: the dried fly ash filter cake is fed into a high-temperature thermal desorption device, the oxygen content in the high-temperature thermal desorption device is set to ≤0.5%, the temperature is 390~410℃, and after staying for 30~1h, desorbed flue gas and fly ash residue are obtained, and the fly ash residue is discharged after cooling. After the desorbed flue gas is cooled to below 180°C by the quenching device, it enters the adsorption device, and after adsorption, it is discharged after dust removal by the bag filter.