Garbage incineration fly ash harmless treatment method based on water washing-sulfide synergistic circulation

Abstract

The invention relates to the technical field of solid waste treatment and resource utilization, and particularly discloses a waste incineration fly ash harmless treatment method based on water washing-sulfide cooperative circulation. The method comprises the following steps: firstly, carrying out crushing, drying and sieving pretreatment on the fly ash; secondly, carrying out water washing desalination and detoxification on the fly ash by adopting deionized water under the conditions of optimized liquid-solid ratio, water washing temperature, time and stirring rate, and carrying out solid-liquid separation to obtain water-washed fly ash and water washing liquid; then adding a sodium sulfide solution with a certain concentration and proportion into the water washing liquid, and removing heavy metal ions through reaction, centrifugation and membrane filtration to obtain purified water washing liquid; and finally, the residual sulfur ions in the purified water washing liquid are partially recycled to the untreated water washing liquid, so that closed circulation of the sulfur ions and medicament saving are realized. By means of the method, the leaching concentration of Cl and heavy metal in the fly ash can be remarkably reduced, the fly ash can meet the hazardous waste landfill pollution control standard (GB18598-2019), and the treated washing liquid can reach the first-level emission limit value of the integrated wastewater emission standard (GB8978-1996). Compared with a traditional solidification / stabilization method, the method has the advantages of being high in desalination rate, high in heavy metal removal efficiency, low in energy consumption, low in operation cost, near-zero in emission and the like, and an efficient, economical and environment-friendly solution is provided for harmless treatment and resource utilization of the waste incineration fly ash.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment and resource utilization technology. Specifically, it relates to methods for the harmless treatment and resource utilization of fly ash from municipal solid waste incineration. Background Technology

[0002] According to the "China Statistical Yearbook 2023," the estimated urban domestic waste collection volume in 2025 (based on 2023 estimates and projected at an average annual growth rate of 2%-3%) is approximately 260-265 million tons. Of this, approximately 258-262 million tons will be treated harmlessly, maintaining a proportion of over 98%, while the total volume treated by incineration is projected to be approximately 222-225 million tons. Due to its advantages in volume reduction, resource recovery, and harmlessness, the proportion of incineration treatment is increasing year by year, and is expected to exceed 85% in the next few years. Fly ash is a residue produced during the incineration of domestic waste. It is an ash compound formed by inorganic particles in the incineration flue gas, combined with added chemical agents and reaction products, through the waste heat utilization system and flue gas purification system. It is mainly intercepted and collected in the desulfurization tower and dust collector. Fly ash production typically accounts for 3%-5% of the waste incineration volume, or approximately 7-11 million tons per year. The National Hazardous Waste List (2021 Edition) explicitly classifies fly ash as HW18 hazardous waste. Therefore, the collection, storage, transportation, and disposal of fly ash may have adverse effects on the environment and human health, and must be handled properly.

[0003] The harmless treatment of fly ash from municipal solid waste incineration refers to the process of removing one or more substances such as heavy metals, dioxins, and chlorides from fly ash to a certain extent, or inhibiting their leaching, through physical or chemical reactions, so that the treated fly ash meets the requirements for subsequent utilization or disposal. Currently, commonly used harmless treatment processes for municipal solid waste incineration fly ash both domestically and internationally mainly include solidification / stabilization, melt solidification, and hydrothermal stabilization. While these processes can partially solve the fly ash pollution problem, they still have drawbacks such as high cost, technological limitations, and difficulty in resource utilization, and urgently require further optimization and innovation.

[0004] Fly ash washing is a common hazardous waste treatment technology, mainly used to treat fly ash generated from waste incineration and coal-fired power plants. Its core principle is to remove soluble salts, heavy metals, and organic pollutants from fly ash through washing, thereby reducing its environmental risks and demonstrating broad market application potential. Research by Chang Wei et al. indicates that chlorine in grate furnace fly ash washing solutions mainly exists as soluble...

[0005] The fly ash contains substances in the forms of NaCl, KCl, CaCl2, and CaClOH, accounting for over 97%. Huang et al. used single-factor experiments to study the effects of liquid-to-solid ratio, washing time, and temperature on the dechlorination efficiency of fly ash washing. The results showed that the highest dechlorination efficiency of 88.72% was achieved when the liquid-to-solid ratio was 8:1, the washing time was 5 min, and the washing temperature was 70℃. Yang et al. analyzed the heavy metals in the fly ash washing solutions obtained with different washing times and a liquid-to-solid ratio of 10 mL / g. The results showed that the metal concentrations of the washing solutions obtained with the two different washing times were similar. Pb and Al are two abundant amphoteric elements in fly ash and are more easily released by water rinsing. Zhang Ling et al. used TCLP and a horizontal oscillation program to conduct a comparative leaching experiment on fly ash before and after washing. The results showed that after TCLP treatment, only Cd in the fly ash exceeded the standard limit before and after water washing; after horizontal oscillation treatment, Zn, Cu, Cr and Cd in the fly ash before and after water washing were all below the limit, only Pb concentration was high, exceeding the maximum concentration limit of the leachate.

[0006] However, some heavy metals dissolve during the washing process, making it impossible to solidify all heavy metals in the fly ash. This can even lead to some heavy metals in the washing solution exceeding the limits set by the "Integrated Wastewater Discharge Standard" (GB 8978-1996). Therefore, pretreatment with water washing is necessary to maximize the fixation of heavy metals in the fly ash and ensure that the heavy metal content in the solution meets discharge standards. Chemical precipitation is commonly used to recover heavy metals from aqueous solutions. Commonly used reagents include inorganic salts (Na₂S, Ca₃(PO₄)₂, FeSO₄, Na₂CO₃, and CO₂). Among these, Na₂S, Ca₃(PO₄)₂, and FeSO₄ are chemical stabilizers. 2- PO4 2- strong affinity and Fe 2+ Its strong redox properties make it readily react with heavy metal ions to form insoluble sulfides, phosphates, and iron oxides, effectively stabilizing heavy metals and reducing their leaching concentration. Furthermore, compared to other reagents, sulfur-containing organic reagents exhibit superior stability and require smaller quantities. The sulfur-containing moiety plays a crucial role in stabilizing heavy metals through organic agents because sulfur atoms with empty electron orbitals readily form covalent bonds with heavy metals, leading to chelate formation.

[0007] This invention, through a synergistic process of washing fly ash with water and treating the washing liquid with sulfides, not only significantly reduces the leaching toxicity and environmental risks of fly ash, but also enables wastewater reuse and resource recovery. It provides an efficient, economical, and environmentally friendly innovative solution for the harmless and resource-based treatment of fly ash, and has significant social, environmental, and economic value. Summary of the Invention

[0008] This invention provides a method for the harmless treatment of waste incineration fly ash based on a water washing-sulfide synergistic cycle. This method can efficiently remove soluble salts and heavy metals from fly ash, significantly reduce reagent consumption, and achieve near-zero wastewater discharge. The treated fly ash and the purified washing solution both meet relevant standards, exhibiting the characteristics of high efficiency, low consumption, and environmental friendliness.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: a method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling, comprising the following steps:

[0010] S1. Crush the fly ash from municipal solid waste incineration, dry it at 105℃ to constant weight, and pass it through a 100-mesh sieve for later use;

[0011] S2. Optimize the liquid-to-solid ratio, water washing temperature, and stirring rate of the fly ash from S1.

[0012] S3. The fly ash from S2 is separated from the washing liquid by filtration; the fly ash is then sent for testing.

[0013] S4. Prepare a sodium sulfide solution of 0.05 mol / L to 2 mol / L. Add the sodium sulfide solution to the water washing liquid filtered in S3 at a ratio of 3:1 to 20:1 and stir for 20 to 60 min.

[0014] S5. The solution obtained in S4 is centrifuged at 5000 rpm for 5 min and filtered through an aqueous membrane with a pore size of 0.25 μm to obtain purified washing solution.

[0015] S6. 20% to 50% of the washing solution after S5 treatment is reused in the untreated washing solution;

[0016] Furthermore, in S2, the liquid-to-solid ratio is preferably 15:1, the water washing temperature is 70°C, the water washing time is 90 min, and the stirring speed is 400 rpm.

[0017] Furthermore, in S4, the concentration of the sodium sulfide solution is 0.05 mol / L to 2 mol / L, preferably 0.2 mol / L;

[0018] Furthermore, in S4, the sodium sulfide addition ratio is 5:1 to 10:1 of the water washing liquid volume, preferably 8:1;

[0019] Furthermore, the washing liquid purified by S5 meets the limits of the Integrated Wastewater Discharge Standard (GB 8978-1996);

[0020] Furthermore, the Cl- removal rate in the water-washed fly ash after S3 treatment is greater than 90%, and the concentrations of heavy metals such as Pb, Cd, and Cr are lower than those specified in the "Standard for Pollution Control of Hazardous Waste Landfill".

[0021] (GB18598-2019) Limits.

[0022] The beneficial effects of this invention are:

[0023] 1. This invention optimizes the liquid-to-solid ratio, water washing temperature, and stirring rate to achieve a soluble salt removal rate of over 90% in fly ash and significantly reduces the leaching concentration of heavy metals, enabling fly ash to meet the "Standard for Pollution Control of Hazardous Waste Landfill" and improving the resource utilization value of fly ash.

[0024] 2. This invention employs a synergistic process combining sodium sulfide precipitation with centrifugation and membrane filtration, achieving a heavy metal removal rate of over 99%. The treated washing liquid meets the Class I discharge limit of the "Integrated Wastewater Discharge Standard," ensuring the compliant discharge and recycling of wastewater.

[0025] 3. This invention achieves a closed-loop circulation of sulfur ions by reusing 20% ​​to 50% of the residual sulfur ions in the purified washing solution, which significantly reduces the amount of sodium sulfide added and the consumption of fresh water, lowers operating costs, and effectively reduces secondary pollution, thus having good economic and environmental benefits. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the process flow of the present invention.

[0027] Figure 2 The fly ash washing liquid-solid ratio of this invention

[0028] Figure 3 The ratio of sodium sulfide to washing solution in this invention is as follows. Detailed Implementation

[0029] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0030] Example 1: Conventional water washing desalination + sulfide precipitation

[0031] Materials and Equipment:

[0032] 100g of fly ash from municipal solid waste incineration; constant temperature water bath, stirrer, centrifuge, and 0.25μm membrane filter.

[0033] Experimental steps:

[0034] (1) Dry the fly ash at 105℃ for 12 hours and grind it through a 100-mesh sieve.

[0035] (2) Add deionized water (1000 mL) at a liquid-to-solid ratio of 10:1, and stir and wash at 80°C for 60 min (300 rpm).

[0036] (3) Solid-liquid separation to obtain water-washed fly ash and water washing liquid.

[0037] (4) Prepare a 0.1 mol / L Na2S solution and add it to the washing solution at a ratio of 5:1 (volume ratio). React for 30 min.

[0038] (5) Centrifuge at 5000 rpm for 5 min, then filter through a 0.25 μm membrane.

[0039] According to this method, the Cl- removal rate of fly ash after water washing was 92.3%; the Pb, Cd, and Cr in the fly ash leachate were 0.18, 0.09, and 3.2 mg / L, respectively, which are lower than the "Standard for Pollution Control of Hazardous Waste Landfill"; the Pb, Cd, and Cr in the treated water washing solution were 0.7, 0.06, and 1.2 mg / L, respectively, which meet the Class I discharge limit of the "Integrated Wastewater Discharge Standard".

[0040] Example 2: Optimization of water washing at higher temperatures

[0041] Materials and equipment: Same as in Example 1.

[0042] Experimental steps:

[0043] (1) Take 100g of dried and sieved fly ash;

[0044] (2) Liquid-to-solid ratio 15:1, stir at 70℃ for 90 min (400 rpm);

[0045] (3) Prepare a 0.2 mol / L Na2S solution, add it at a ratio of 8:1, and react for 40 min;

[0046] (4) Centrifugation + membrane separation is the same as above.

[0047] The dechlorination rate obtained by this method was 95.8%; the Pb, Cd, and Cr concentrations in the fly ash leachate were 0.14, 0.07, and 2.8 mg / L, respectively; and the Pb, Cd, and Cr concentrations in the purified washing solution were 0.45, 0.04, and 0.9 mg / L, respectively. The purification effect was superior to that of Example 1.

[0048] Example 3: Process testing at low liquid-to-solid ratio

[0049] Experimental steps:

[0050] (1) 100g fly ash; liquid-solid ratio 5:1, stir at room temperature for 30min (200rpm);

[0051] (2) Prepare a 0.1 mol / L Na2S solution with an addition ratio of 10:1 and react for 60 min.

[0052] The dechlorination rate obtained by this method was 81.4%, significantly lower than that under high liquid-to-solid ratio conditions; the Pb, Cd, and Cr concentrations in the fly ash leachate were 0.22, 0.11, and 4.1 mg / L, respectively (still meeting the standards); the Pb, Cd, and Cr concentrations in the purified water washing solution were 0.9, 0.08, and 1.4 mg / L, close to the emission limits.

[0053] Example 4: Sulfide ion recycling cycle test

[0054] Experimental steps:

[0055] (1) The optimal washing conditions of Example 2 were adopted;

[0056] (2) After sodium sulfide precipitation, about 30% of the qualified washing solution is retained and directly reused in a new batch of washing solution;

[0057] (3) The remaining waste liquid was tested again before being discharged.

[0058] According to this method, the consumption of fresh Na2S was reduced by about 28%; the levels of Pb, Cd, and Cr in the reuse system were stably controlled below 0.5, 0.05, and 1.1 mg / L, respectively; and after 5 cycles, the treatment effect remained stable with no significant attenuation.

[0059] Example 5: Comparative Experiment

[0060] Traditional method: using cement-fly ash to solidify fly ash (addition amount 30%); Method of the present invention: using the conditions of Example 2.

[0061]

Claims

1. A method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling, characterized in that, Includes the following steps: S1. Crush the fly ash from municipal solid waste incineration, dry it at 105℃ to constant weight, and pass it through a 100-mesh sieve for later use; S2. Optimize the liquid-to-solid ratio, water washing temperature, and stirring rate of the fly ash from S1. S3. The fly ash washed in S2 is separated from the washing liquid by filtration; Water-washed fly ash was sent for testing; S4. Prepare a sodium sulfide solution of 0.05 mol / L to 2 mol / L. Add the sodium sulfide solution to the water washing liquid filtered in S3 at a ratio of 5:1 to 20:1 and stir for 20 to 60 minutes. S5. The solution obtained in S4 is centrifuged at 5000 rpm for 5 min and filtered through an aqueous membrane with a pore size of 0.25 μm to obtain purified washing solution. S6. 20% to 50% of the washing solution treated in S5 is reused in the untreated washing solution.

2. The method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling as described in claim 1, characterized in that: In the S2 process, the liquid-to-solid ratio is preferably 15:1, the water washing temperature is 70°C, the water washing time is 90 min, and the stirring speed is 400 rpm.

3. The method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling according to claim 1, characterized in that: In S4, the concentration of sodium sulfide solution is 0.05 mol / L to 2 mol / L, preferably 0.2 mol / L.

4. The method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling according to claim 1, characterized in that: In step S4, the sodium sulfide addition ratio is 3:1 to 20:1 of the water washing liquid volume, preferably 8:

1.

5. The method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling according to claim 1, characterized in that: The washing liquid purified by S5 meets the limits of the Integrated Wastewater Discharge Standard (GB 8978-1996).

6. The method for harmless treatment of waste incineration fly ash based on water washing-sulfide synergistic recycling according to claim 1, characterized in that: The removal rate of Cl⁻ in the water-washed fly ash after S3 treatment is greater than 90%, and the concentrations of heavy metals such as Pb, Cd, and Cr are lower than the limits of the "Standard for Pollution Control of Hazardous Waste Landfill" (GB18598-2019).

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