A method for selectively removing heavy metals in fly ash based on process stabilization

Abstract

The application discloses a method for selectively removing heavy metals in fly ash based on process stabilization, and belongs to the technical field of solid waste treatment and resource utilization. The application proposes a whole-chain treatment method of hierarchical pH selective extraction-stepwise precipitation directional separation-depth reduction stabilization-furnace high-temperature solidification for solving the problems of multiple types of heavy metals, complex chemical forms and difficult simultaneous and efficient removal in waste incineration fly ash. The water washing process is divided into an acid section, a near-neutral section and a weak alkaline section, so that the selective dissolution of hexavalent chromium, lead, cadmium, copper, zinc, arsenic and selenium is realized; then, the concentrated sludge enriched with different heavy metals is obtained through stepwise precipitation; especially for the highly toxic hexavalent chromium, the deep detoxification is realized through reduction-coprecipitation under weak acid conditions; finally, the heavy metal-removed fly ash is molded and high-temperature treated in a furnace, and the residual heavy metals are permanently solidified in the slag lattice. The application realizes the selective separation and stabilization of heavy metals in fly ash, and the treated fly ash can be resourceized with the slag, the heavy metal-enriched sludge is small in quantity and easy to be classified and disposed, and the application has the outstanding advantages of high removal efficiency, good selectivity, low environmental risk, low carbon and energy saving.

Description

Technical Field

[0001] This invention relates to the field of hazardous solid waste treatment, disposal, and resource utilization technology, and in particular to a method for selectively removing heavy metals from fly ash based on process stabilization. Specifically, it relates to a comprehensive method and system for efficiently, stably, and thoroughly treating heavy metals in fly ash generated during solid waste incineration and realizing the resource utilization of some components. Background Technology

[0002] With the rapid development of my country's municipal solid waste incineration power generation industry, the amount of fly ash generated from waste incineration has been increasing year by year. As fine particulate matter captured by the waste incineration flue gas purification system, fly ash is enriched with high concentrations of heavy metals (such as Pb, Cd, Cr, Cu, Zn, As, Hg, etc.), persistent organic pollutants such as dioxins, and a large amount of soluble chloride salts (content can reach 15%~25%), and is listed as HW18 hazardous waste in the "National Hazardous Waste List".

[0003] Currently, the mainstream technologies for fly ash treatment mainly include:

[0004] (1) Chelating agent stabilization + landfill: Heavy metals are converted into an insoluble state by adding heavy metal chelating agents and then landfilled in the municipal solid waste landfill in designated areas. This technology has obvious drawbacks: the long-term chemical stability of the chelating agent is insufficient, and heavy metals may re-dissolve in an acidic environment; at the same time, soluble chloride salts are not removed, and the presence of chloride ions will accelerate the leaching and migration of heavy metals; and the fly ash mass is not reduced, and landfill occupies a large amount of storage space.

[0005] (2) Acid washing / water washing dechlorination + heat treatment: Soluble chloride salts in fly ash are removed by water washing, and then the dechlorinated fly ash is subjected to high-temperature sintering or melting treatment. However, conventional water washing processes lack selective control of heavy metals, resulting in mixed heavy metal ions in the water washing waste liquid. This makes subsequent water treatment difficult, and the generated mixed salts and mixed heavy metal sludge are large in quantity and complex in composition, making them difficult to utilize as resources. Instead, it increases the burden of hazardous waste disposal.

[0006] (3) High-temperature melting / sintering: Fly ash is melted at a high temperature above 1200℃ to form a glassy slag, thereby solidifying heavy metals and completely degrading dioxins. However, this method has extremely high energy consumption and high operating costs. Furthermore, some volatile heavy metals will escape with the flue gas at high temperatures, requiring a complex and expensive flue gas purification system.

[0007] The aforementioned existing technologies also have the following problems:

[0008] There is a lack of ability to selectively separate, classify, and stabilize multiple heavy metals in fly ash. The chemical properties of different heavy metals vary significantly, and a uniform treatment strategy cannot take into account the optimal removal conditions for each type of heavy metal, resulting in low removal efficiency, mixed products, and a high risk of secondary pollution.

[0009] Therefore, developing a fly ash treatment method that can selectively separate and remove heavy metals based on their different chemical behaviors and deeply stabilize highly toxic heavy metals has significant engineering application value and environmental significance. Summary of the Invention

[0010] This invention addresses the problems of poor selectivity in heavy metal removal, difficulty in deep detoxification of hexavalent chromium, mixed treatment products, and high risk of secondary pollution in existing fly ash treatment technologies. It provides a method for selectively removing heavy metals from fly ash based on process stabilization.

[0011] This invention provides a method for selectively removing heavy metals from fly ash based on process stabilization, comprising the following steps:

[0012] S1. Staged water washing dechlorination and selective extraction: The fly ash from waste incineration is mixed with process water at a liquid-solid ratio of 3:1 to 8:1 and subjected to multi-stage countercurrent water washing at a temperature of 20 to 60°C. The pH value of each stage of the water washing unit is adjusted in stages to achieve selective leaching of different heavy metals.

[0013] S2. Stepwise sedimentation and directional separation: The heavy metal-containing washing liquid produced from each stage of water washing is introduced into the corresponding sedimentation reactor. By adding selective precipitants and / or adjusting the pH value, the target heavy metals are precipitated in the form of hydroxides, sulfides or complex precipitates, respectively, and precipitated sludge enriched with different heavy metals is obtained.

[0014] S3. Deep reduction and stabilization of heavy metals: The enriched precipitate containing hexavalent chromium obtained in step S2 is reduced to trivalent chromium by adding a reducing agent at pH 4-7, and a stabilizing agent is added to form chromium-iron coprecipitate or chromate mineral phase, thereby achieving deep detoxification and stabilization of hexavalent chromium.

[0015] S4. Shaping and heat treatment of fly ash containing heavy metals: After dehydration and granulation, the water-containing fly ash from step S1 is sent to the waste incinerator for high-temperature heat treatment, so that the residual heavy metals are solidified in the slag lattice.

[0016] S5. Co-control of heavy metals in flue gas: The waste gas generated by each treatment unit in steps S1 to S3 and the flue gas generated in step S4 are collected and then subjected to secondary combustion and adsorption. The secondary capture of heavy metals in the flue gas is achieved through high-temperature oxidation and activated carbon adsorption.

[0017] Furthermore, in step S1, the pH adjustment of each stage of the water washing unit is selected from one or more of sulfuric acid, hydrochloric acid, sodium hydroxide, and calcium hydroxide; the number of stages of the multi-stage countercurrent water washing is 3 to 6, and the residence time of each stage is 15 to 60 minutes.

[0018] Furthermore, in step S1, the countercurrent water washing stage is three stages. The pH of the first stage water washing is controlled at 4.0~6.0 to selectively dissolve hexavalent chromium, copper, and zinc; the pH of the second stage water washing is controlled at 6.5~8.5 to selectively dissolve lead and cadmium; and the pH of the third stage water washing is controlled at 8.5~10.5 to selectively dissolve arsenic and selenium.

[0019] Furthermore, in step S2, the selective precipitant includes: sodium sulfide or sodium hydrosulfide for precipitating lead and cadmium; sodium hydroxide or sodium carbonate for precipitating copper and zinc; and polyferric sulfate or polyaluminum chloride for precipitating arsenic and selenium.

[0020] Furthermore, step S2 includes the following steps:

[0021] S210. Adjust the pH of the primary washing solution to 4-7, add a reducing agent to remove Cr. 6 ⁺ is reduced to Cr³⁺; then the pH is adjusted to 7.5~9.0, and sodium hydroxide or sodium carbonate is added to precipitate Cu²⁺ and Zn²⁺ in the form of hydroxides or carbonates, thus obtaining copper- and zinc-enriched precipitated sludge.

[0022] S220. Adjust the pH of the secondary washing liquid to 8.0~10.0, add sodium sulfide or sodium hydrosulfide to cause Pb²⁺ and Cd²⁺ to precipitate out in the form of sulfide precipitates, and obtain lead- and cadmium-enriched precipitated sludge.

[0023] S230. Adjust the pH of the tertiary washing liquid to 5.0~7.0, add polyferric sulfate or polyaluminum chloride, and through the coagulation and co-precipitation of iron salts / aluminum salts, As and Se form insoluble arsenates, arsenites and selenates with Fe³⁺ / Al³⁺, thereby obtaining precipitated sludge enriched with arsenic and selenium.

[0024] Furthermore, in step S3, the reducing agent is selected from one or more of ferrous sulfate, sodium sulfite, sodium metabisulfite, and zero-valent iron powder; the stabilizing agent is selected from one or more of sodium hydroxide, calcium hydroxide, and phosphate; the reduction reaction time is 10-60 min, and the reaction temperature is 25-80℃.

[0025] Furthermore, in step S4, the high-temperature heat treatment temperature is 900~1100℃, and the residence time is 30~60min.

[0026] Furthermore, in step S5, the secondary combustion temperature is ≥1100℃ and the residence time is ≥2s.

[0027] Compared with the prior art, the present invention has the following beneficial technical effects:

[0028] 1. By precisely controlling the pH value of the washing water through graded classification, the different pH solubility differences of various heavy metals are fully utilized to achieve selective group leaching of chromium, lead, cadmium, copper, zinc, arsenic, and selenium, avoiding the subsequent processing difficulties caused by mixed extraction. An independent deep stabilization process was designed for highly toxic hexavalent chromium. Under the optimal pH window, through Fe²⁺ reduction and chromium-iron co-precipitation mechanisms, hexavalent chromium is completely reduced to trivalent chromium and stabilized in a chromite-like form, solving the technical problems of incomplete reduction or easy re-oxidation of hexavalent chromium in conventional treatment methods.

[0029] 2. Through stepwise precipitation, Cu / Zn sludge, Pb / Cd sludge, As / Se sludge, and ferrochromium co-precipitates are obtained separately. The composition of each type of sludge is relatively simple, facilitating the separate recovery of valuable metals or targeted safe disposal, significantly reducing the cost and difficulty of mixed hazardous waste disposal. After high-temperature remelting, the residual heavy metals in the fly ash after heavy metal removal are melted and solidified in the slag lattice, forming a chemically extremely stable mineral phase. This eliminates the long-term acid leaching risk associated with chelating agent stabilization methods, achieving true permanent harmlessness.

[0030] 3. Fully utilizes the waste heat resources of existing incinerators in the incineration plant, eliminating the need for external fuel supply; the entire process is integrated within the plant area, requiring no additional land for construction and not relying on off-site hazardous waste disposal facilities, ensuring long-term processing capacity and operational stability. Process wastewater is fully recycled, achieving zero emissions. Waste gas from each unit is collected and sent to the incinerator for co-treatment, preventing direct discharge into the environment. Final products can be utilized as resources; heavy metal-enriched sludge is classified and outsourced for disposal, achieving the goal of near-zero fly ash landfill. Detailed Implementation

[0031] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0032] Example 1

[0033] A method for selectively removing heavy metals from fly ash based on process stabilization includes the following steps:

[0034] S1. Staged water washing dechlorination and selective extraction: The fly ash from waste incineration is mixed with process water at a liquid-solid ratio of 3:1 to 8:1 and subjected to multi-stage countercurrent water washing at a temperature of 20 to 60°C. The pH value of each stage of the water washing unit is adjusted in stages to achieve selective leaching of different heavy metals.

[0035] Specifically, the fly ash from waste incineration processed in this embodiment originates from the solid waste fly ash products treated by the applicant. The collected fly ash is transported and mixed with water at a solid-liquid ratio of 5:1. After mixing, the solution is sent to a multi-stage countercurrent water washing device. In this embodiment, a three-stage countercurrent water washing process is used, with the washing temperature set at 35°C and the residence time for each stage at 30 minutes.

[0036] The first stage of water washing is an acid washing section, in which the pH of the washing solution is adjusted to 5.0 with dilute hydrochloric acid. Within this pH range, hexavalent chromium in the fly ash preferentially dissolves into the liquid phase due to reduced electrostatic adsorption, while Cu... 2+ Zn 2+ It has high solubility under weakly acidic conditions and is also selectively extracted. The dissolution rate of hexavalent chromium can reach 90% at this stage, as well as Cu. 2+ Dissolution rate 85%, Zn 2+ Dissolution rate: 84%.

[0037] The secondary water wash is a neutral stage, where the pH is adjusted to 7.5 using NaOH solution. Within this pH range, Pb... 2+ Cd 2+ Hydroxide precipitates begin to form under alkaline conditions, but their soluble form still dominates under near-neutral conditions, allowing for effective dissolution. At this stage, the dissolution efficiency for lead can reach 87%, and for cadmium, it can reach 91%.

[0038] The third stage of water washing is an alkaline washing stage. Within this pH range, the solubility of the oxyacid anions of the amphoteric metals arsenic and selenium increases under alkaline conditions, leading to selective extraction. The leaching rate of As is 80%, and the leaching rate of Se is 79%.

[0039] S2. Stepwise sedimentation and directional separation: The heavy metal-containing washing liquid produced from each stage of water washing is introduced into the corresponding sedimentation reactor. By adding selective precipitants and / or adjusting the pH value, the target heavy metals are precipitated in the form of hydroxides, sulfides or complex precipitates, respectively, and precipitated sludge enriched with different heavy metals is obtained.

[0040] Specifically, the washing solutions generated in step S1 are respectively sent to their corresponding sedimentation tanks for treatment. The primary washing solution is processed according to the reaction of the reducing agent and Cr. 6+ Add ferrous sulfate reducing agent at a molar ratio of 5:1. Adjust the pH to 5.5 and react at 60℃ for 30 minutes to allow Cr to... 6+ Reduced to Cr 3+ Then, the pH was adjusted to 8.5 with NaOH solution to precipitate Cu. 2+ and Zn 2+ Centrifugation yielded a Cu / Zn / Cr mixed sludge.

[0041] Adjust the pH of the secondary washing solution to 9, add Na2S solution at a molar ratio of S²⁻ / (Pb²⁺+Cd²⁺) of 1.2:1, and react at room temperature for 20 min to obtain Pb / Cd sulfide sludge.

[0042] Adjust the pH of the tertiary washing solution to 6.5, add polyferric sulfate at a Fe³⁺ / (As+Se) molar ratio of 3:1, stir for 30 min to obtain As / Se iron salt coprecipitated sludge.

[0043] S3. Deep reduction and stabilization of heavy metals: The chromium-enriched precipitate obtained in step S2 is reduced to trivalent chromium by adding a reducing agent at pH 4-7, and a stabilizing agent is added to form chromium-iron coprecipitate or chromate mineral phase, thereby achieving deep detoxification and stabilization of hexavalent chromium.

[0044] Specifically, the Cu / Zn / Cr mixed sludge from step S2 undergoes advanced treatment. Under pH 5.5 conditions, the ratio of reducing agent to Cr... 6+ Zero-valent iron powder was added at a molar ratio of 4:1, and the mixture was stirred at 70°C for 45 minutes. Then, Ca(OH)2 was added to adjust the pH to 8.0 and the reaction was continued for 15 minutes to obtain mixed sludge and waste liquid containing Cu / Zn / Cr.

[0045] Cr in the treated mixed sludge 6+ The content was reduced from the original 8.5 mg / kg to 0.12 mg / kg, the total chromium leaching concentration (TCLP method) was 0.42 mg / L, the hexavalent chromium leaching concentration was 0.04 mg / L (the standard limit of HJ 1134-2020 is 0.5 mg / L), and the removal rate reached over 99.5%.

[0046] S4. Shaping and heat treatment of fly ash containing heavy metals: After dehydration and granulation, the water-containing fly ash from step S1 is sent to the waste incinerator for high-temperature heat treatment, so that the residual heavy metals are solidified in the slag lattice.

[0047] Specifically, the water-containing fly ash in step S1 is dewatered by plate and frame filter press or centrifugal dewatering, and then plastically granulated by a granulator with a particle diameter controlled at 5~20mm to improve its fluidization characteristics and residence time distribution in the incinerator.

[0048] The shaped fly ash particles are fed into a waste incinerator, preferably a mechanical grate incinerator or a circulating fluidized bed incinerator, where they undergo heat treatment in a high-temperature zone of 900-1100°C for 30-60 minutes. The proportion of shaped fly ash recycled into the incinerator does not exceed 5% of the incineration line's processing capacity to ensure that it does not significantly affect the normal operation of the incinerator or flue gas emission indicators.

[0049] During the high-temperature heat treatment process, the trace heavy metals remaining in the fly ash after heavy metal removal undergo a solid-phase reaction with components such as SiO2, Al2O3, and Fe2O3 in the slag, becoming locked in the silicate glass phase or spinel mineral lattice, forming chemically extremely stable mineral inclusions. Simultaneously, the small amount of dioxin-like organic matter remaining in the fly ash is completely decomposed at temperatures above 850℃.

[0050] The slag produced after heat treatment has a soluble chlorine content of ≤1%, a dioxin content of ≤50 ng-TEQ / kg, and a heavy metal leaching concentration that meets the requirements of HJ 1134-2020 standard. It can be used in subsequent resource utilization pathways, such as making building material aggregates or landfill cover soil.

[0051] S5. Co-control of heavy metals in flue gas: The waste gas generated by each treatment unit in steps S1 to S3 and the flue gas generated in step S4 are collected and then subjected to secondary combustion and adsorption. The secondary capture of heavy metals in the flue gas is achieved through high-temperature oxidation and activated carbon adsorption.

[0052] Specifically, to prevent trace amounts of volatile heavy metals from entering the flue gas and causing secondary pollution during high-temperature heat treatment, this invention collects the volatile organic compound-containing waste gas generated from each water washing and sedimentation treatment unit in steps S1-3 and the heat treatment flue gas in step S4, and then sends it through a closed pipeline into the secondary combustion chamber of the waste incinerator. At a temperature ≥1100℃ and a residence time ≥2s, high-temperature oxidation treatment is performed, allowing the volatile heavy metal chlorides to be further oxidized at high temperature and partially condensed on the surface of fly ash particles, where they are captured by the subsequent dust removal system. The dust removal system can be a flue gas purification system, equipped with an activated carbon injection device, with an activated carbon injection rate of 5-20 mg / Nm³. 3 The activated carbon traps residual gaseous heavy metals, especially mercury and its compounds, in flue gas through porous adsorption. The trapped heavy metal-containing activated carbon is collected by a dust collector and returned to step S1 for reprocessing, forming a closed-loop cycle to prevent heavy metal emissions. Simultaneously, the supernatant from the washing liquid generated during the water washing process in step S1, after precipitation treatment in step S2 and reduction treatment in step S3, as well as the flue gas condensate generated during the heat treatment in step S4, are all collected and returned to step S1 as process water for recycling. Water lost due to evaporation and fly ash is replenished by the condensate from the waste heat steam of the incinerator, achieving zero wastewater discharge for the entire process system.

[0053] The above measures ensure that the concentration of heavy metal emissions in flue gas meets the requirements of the "Standard for Pollution Control of Municipal Solid Waste Incineration" (GB 18485-2014) and stricter local emission standards.

[0054] While several embodiments of the present invention have been provided herein, those skilled in the art should understand that modifications can be made to these embodiments without departing from the spirit of the invention. The above embodiments are merely exemplary and should not be construed as limiting the scope of the invention.

Claims

1. A method for selectively removing heavy metals from fly ash based on process stabilization, characterized in that, Includes the following steps: S1. Staged water washing dechlorination and selective extraction: The fly ash from waste incineration is mixed with process water at a liquid-solid ratio of 3:1 to 8:1 and subjected to multi-stage countercurrent water washing at a temperature of 20 to 60°C. The pH value of each stage of the water washing unit is adjusted in stages to achieve selective leaching of different heavy metals. S2. Stepwise sedimentation and directional separation: The heavy metal-containing washing liquid produced from each stage of water washing is introduced into the corresponding sedimentation reactor. By adding selective precipitants and / or adjusting the pH value, the target heavy metals are precipitated in the form of hydroxides, sulfides or complex precipitates, respectively, and precipitated sludge enriched with different heavy metals is obtained. S3. Deep reduction and stabilization of heavy metals: The chromium-enriched precipitate obtained in step S2 is reduced to trivalent chromium by adding a reducing agent at pH 4-7, and a stabilizing agent is added to form chromium-iron coprecipitate or chromate mineral phase, thereby achieving deep detoxification and stabilization of hexavalent chromium. S4. Shaping and heat treatment of fly ash containing heavy metals: After dehydration and granulation, the water-containing fly ash from step S1 is sent to the waste incinerator for high-temperature heat treatment, so that the residual heavy metals are solidified in the slag lattice. S5. Co-control of heavy metals in flue gas: The waste gas generated by each treatment unit in steps S1 to S3 and the flue gas generated in step S4 are collected and then subjected to secondary combustion and adsorption. The secondary capture of heavy metals in the flue gas is achieved through high-temperature oxidation and activated carbon adsorption.

2. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S1, the pH adjustment of each stage of the water washing unit is selected from one or more of sulfuric acid, hydrochloric acid, sodium hydroxide, and calcium hydroxide; the number of stages of multi-stage countercurrent water washing is 3 to 6, and the residence time of each stage is 15 to 60 minutes.

3. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S1, the countercurrent water washing stage is three stages. The pH of the first stage water washing is controlled at 4.0~6.0 to selectively dissolve hexavalent chromium, copper, and zinc. The pH of the second stage water washing is controlled at 6.5~8.5 to selectively dissolve lead and cadmium. The pH of the third stage water washing is controlled at 8.5~10.5 to selectively dissolve arsenic and selenium.

4. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S2, the selective precipitant includes: sodium sulfide or sodium hydrosulfide for precipitating lead and cadmium; sodium hydroxide or sodium carbonate for precipitating copper and zinc; and polyferric sulfate or polyaluminum chloride for precipitating arsenic and selenium.

5. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 3, characterized in that, Step S2 includes the following steps: S210. Adjust the pH of the primary washing solution to 4-7, add a reducing agent to remove Cr. 6+ Reduced to Cr 3+ Then adjust the pH to 7.5-9.0, and add sodium hydroxide or sodium carbonate to make Cu... 2+ Zn 2+ The copper and zinc are precipitated out as hydroxides or carbonates to obtain copper- and zinc-enriched precipitated sludge. S220. Adjust the pH of the secondary washing solution to 8.0-10.0, and add sodium sulfide or sodium hydrosulfide to reduce Pb content. 2+ Cd 2+ Lead and cadmium-enriched precipitated sludge is obtained by precipitating sulfides. S230. Adjust the pH of the tertiary washing solution to 5.0-7.0, add polyferric sulfate or polyaluminum chloride, and through the co-precipitation of iron / aluminum salts, allow As, Se and Fe to react. 3+ / Al 3+ Insoluble arsenates, arsenites, and selenates are formed, resulting in precipitated sludge enriched with arsenic and selenium.

6. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S3, the reducing agent is selected from one or more of ferrous sulfate, sodium sulfite, sodium metabisulfite, and zero-valent iron powder; the stabilizing agent is selected from one or more of sodium hydroxide, calcium hydroxide, and phosphate; the reduction reaction time is 10-60 min, and the reaction temperature is 25-80℃.

7. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S4, the high-temperature heat treatment temperature is 900~1100℃, and the residence time is 30~60min.

8. The method for selectively removing heavy metals from fly ash based on process stabilization according to claim 1, characterized in that, In step S5, the secondary combustion temperature is ≥1100℃ and the residence time is ≥2s.