A method for reducing dioxin content in fly ash

Abstract

The application provides a method for reducing the content of dioxin in fly ash. The method for reducing the content of dioxin in fly ash comprises the following steps: adding an appropriate amount of water into fly ash to obtain a first slurry, then adding a strong acid, stirring and filtering to obtain a first filtrate and a first filter cake; adding a sulfide salt into the first filtrate, stirring and filtering to obtain a second filtrate. Fly ash is pretreated by slurry, and metal ions inducing the generation of dioxin in fly ash are removed through the steps of leaching and directional precipitation, so that the generation of dioxin in the subsequent treatment process is inhibited, and the content of dioxin is effectively reduced. The method is mild, simple to operate, low in energy consumption and pollution-free, and high in removal efficiency, so that the cost of the process of removing dioxin from fly ash is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of fly ash and flue gas treatment technology, and particularly relates to a method for reducing the dioxin content in fly ash. Background Technology

[0002] The incineration of municipal solid waste produces fly ash rich in metals such as calcium, potassium, and sodium, often referred to as "misplaced resources." However, excessive dioxins in fly ash pose a significant risk of carcinogenesis, teratogenesis, and mutagenesis in humans, severely limiting its resource recovery and harmless utilization. Therefore, removing dioxins from fly ash is crucial for its comprehensive utilization.

[0003] Currently, common dioxin degradation technologies mainly include mechanochemical degradation, photocatalytic degradation, hydrothermal degradation, and high-temperature thermal decomposition.

[0004] Mechanochemical degradation effectively alters the structure and properties of pollutants through grinding operations such as extrusion, impact, friction, and shearing during mechanochemical processing. Patent CN115970820A, using ball milling as an example, shows that after 1 hour of ball milling, the dioxin toxicity equivalent (TEQ) reduction rate reached 59.0% and 60.4%, respectively, and the total dioxin concentration decreased with increasing ball milling time. However, mechanochemical degradation is currently in the pilot-scale stage, and the dioxin removal rate is not high.

[0005] Photocatalytic degradation, as described in patent CN220677391U, utilizes sunlight and ultraviolet radiation to oxidize and decompose dioxins. Although the reaction conditions for photocatalytic degradation of dioxins are mild, most reported catalysts use precious metals as raw materials, resulting in high costs and complex preparation processes.

[0006] Hydrothermal degradation, as disclosed in patent CN113521637B, involves adding alkaline substances or auxiliary reagents such as hydrazine carbonate and ferrous sulfate to decompose dioxins in the supercritical state of water. The auxiliary groups can effectively promote dioxin dechlorination, thereby reducing the dioxin toxicity equivalent. Although hydrothermal methods have made progress in recent years, they generate large amounts of wastewater during the treatment process.

[0007] High-temperature pyrolysis is currently the most widely used technology. For example, patent CN109603457B describes equipment used for the thermal treatment of waste fly ash, including plasma furnaces, rotary kilns, natural gas furnaces, and coke bed smelting furnaces. Among these, plasma technology has seen rapid development due to its high dioxin degradation rate. However, this method is energy-intensive and costly.

[0008] In conclusion, achieving low-energy, low-cost, and environmentally friendly removal of dioxins from fly ash remains a technological bottleneck for this technology.

[0009] In view of this, the present invention is hereby proposed. Summary of the Invention

[0010] The purpose of this invention is to provide a method for reducing the dioxin content in fly ash, so as to solve the above-mentioned problems.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A method for reducing the dioxin content in fly ash, the method comprising the following steps:

[0013] S1: Add an appropriate amount of water to the fly ash, slurry it to obtain the first slurry, then add strong acid, stir and filter to obtain the first filtrate and the first filter cake; the fly ash is subjected to slurry pretreatment to remove some of the elements in the fly ash that induce dioxin production and inhibit the formation of dioxins in subsequent treatment steps.

[0014] The mass ratio of fly ash to water is 1:1 to 4.

[0015] The pulping process includes stirring at room temperature for 30–60 minutes;

[0016] The strong acid includes any one of sulfuric acid, hydrochloric acid, and nitric acid; the mass ratio of the strong acid to the first slurry is 1 to 5:75; the function of the strong acid is to leach some insoluble metals and chlorine from the solid phase to the liquid phase, so as to facilitate subsequent removal.

[0017] The stirring is carried out at room temperature for 60–90 minutes.

[0018] The pH value of the first filtrate is 1 to 4.

[0019] S2: Add sulfide salt to the first filtrate, stir and filter to obtain the second filtrate; sulfide salt can precipitate the Cu, Zn, Cr, Cd and other metals leached in the above steps, making them easier to remove in subsequent filtration. The interaction between Cu and one of the metals is the cause of dioxin production during fly ash treatment.

[0020] The mass ratio of the sulfide salt to the first filtrate is 1-50:150;

[0021] The stirring is carried out at room temperature for 60–90 minutes.

[0022] The sulfide salt is either sodium sulfide or potassium sulfide;

[0023] The pH value of the second filtrate is 7-9.

[0024] Optionally, the method includes S3: preparing a second slurry using the second filtrate and the first filter cake, then adding chloride salt, preferably hypochlorous acid simultaneously with the addition of chloride salt, and sequentially stirring, settling, and filtering to obtain a second filter cake and a third filtrate; chloride salt can make it easier to reduce the toxic equivalent of dioxins in fly ash by promoting the breaking of C-Cl and CC bonds of dioxins, thereby reducing the toxic equivalent; the synergistic effect of hypochlorous acid and metal ions in chloride salt promotes the ring-opening of dioxins, thereby promoting the decomposition of dioxins and reducing the toxic equivalent. At this point, the third filtrate is tested and the dioxin toxic equivalent meets the standard, and the dioxins are mainly present in the second filter cake;

[0025] The mass ratio of the chloride salt to the second slurry is 1-100:200; the mass ratio of the hypochlorous acid to the chloride salt is 1-20:1.

[0026] The chloride salt includes any one or more of ferric chloride, ferrous chloride, and manganese chloride; preferably ferric chloride; compared with ferric ions, ferrous ions have a lower valence state, poorer oxidizing power, and are more difficult to break C-Cl bonds, so their effect on reducing toxicity equivalent is slightly worse than that of ferric ions.

[0027] The stirring includes stirring at room temperature for 30 to 60 minutes;

[0028] The settling period includes standing at room temperature for 30 to 60 minutes.

[0029] Preferably, the method further includes: ball milling the second filter cake;

[0030] The ball milling process includes: adding the second filter cake into a ball mill, using an alumina liner as the grinding medium, adding an appropriate amount of hydrogen peroxide, providing a specified atmosphere, and performing ball milling. The ash obtained from ball milling is dried and then tested for dioxin content.

[0031] The mass ratio of hydrogen peroxide to the second filter cake is 1 to 10:50;

[0032] The specified atmosphere includes any one of nitrogen, argon, and oxygen;

[0033] The volume ratio of the grinding media to the second filter cake is 1 to 20:1;

[0034] The ball milling rate is 300–400 r / min;

[0035] The ball milling time is 60–90 minutes.

[0036] This invention first aims to reduce dioxin content by inhibiting dioxin formation. This is achieved through pre-treatment of fly ash by pulping, incorporating leaching and directional precipitation steps to remove metal ions that induce dioxin formation, thus inhibiting dioxin formation during subsequent treatment. Simultaneously, existing dioxins are chemically and physically decomposed by introducing specific ions and additives into the fly ash. The metal ions in chloride salts promote the breaking of C-Cl and C-C bonds in dioxins, reducing their toxicity equivalent. Hypochlorous acid synergistically interacts with the metal ions in chloride salts to promote dioxin ring-opening, leading to dioxin decomposition. Furthermore, ball milling further decomposes the dioxins in the filter cake, achieving a highly efficient, inexpensive, and gentle method for removing dioxins from fly ash.

[0037] The beneficial effects of this invention are:

[0038] This invention removes excessive dioxins from fly ash using a gentle and simple method. By introducing special additives, it reduces dioxin formation and promotes dioxin decomposition. The process is energy-efficient, pollution-free, and simple, significantly reducing the cost of dioxin removal from fly ash. Compared with traditional high-temperature thermal decomposition dioxin processes, this invention greatly reduces energy consumption. Compared with existing mechanical degradation dioxin processes, this invention avoids dioxin formation during ball milling, significantly improving the dioxin removal rate. Attached Figure Description

[0039] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the process flow provided in Example 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] It should be noted that the dioxin content detection mentioned in the embodiments of the present invention mainly includes polychlorinated dibenzo-p-dioxins (pCDDs) and polychlorinated dibenzofurans (PCDFs).

[0043] Example 1

[0044] This embodiment provides a method for reducing the dioxin content in fly ash, specifically including the following steps:

[0045] S1: Add 1500g of water to 500g of grate furnace fly ash, stir and slurry at room temperature for 30min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.200μg TEQ / kg, then add 40g of concentrated hydrochloric acid to the obtained slurry, stir at room temperature for 60min, and filter to obtain the first filtrate and the first filter cake. The pH of the obtained first filtrate is 1.1.

[0046] S2: Continue to add 20g of sodium sulfide to the first filtrate to precipitate heavy metals such as Cu, Zn, Cr, and Cd. Filter to obtain the second filtrate, and the pH of the obtained filtrate is 7.8.

[0047] S3: Add the second filtrate to the first filter cake, re-slurry, add 20g of ferric chloride to the slurry, stir at room temperature for 30min, let stand for 30min, filter to obtain the third filtrate and the second filter cake, the third filtrate meets the standard and is used for metal ion recovery.

[0048] S4: Add the second filter cake to the ball mill, add an equal volume of grinding media alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the resulting ash and test the dioxin content to be 0.064μg TEQ / kg.

[0049] The process flow diagram of the method for reducing dioxin content in fly ash provided in this embodiment is shown below. Figure 1 As shown.

[0050] Example 2

[0051] This embodiment provides a method for reducing the dioxin content in fly ash, specifically including the following steps:

[0052] S1: Add 1500g of water to 500g of grate furnace fly ash, stir and slurry at room temperature for 30min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2μg TEQ / kg; add 40g of concentrated sulfuric acid to the slurry, stir at room temperature for 60min, and then filter to obtain the first filtrate and the first filter cake. The pH of the obtained filtrate is 1.3.

[0053] S2: Continue to add 20g of sodium sulfide to the first filtrate to precipitate heavy metals such as Cu, Zn, Cr, and Cd. Filter to obtain the second filtrate, and the pH of the obtained filtrate is 8.1.

[0054] S3: Add the second filtrate to the first filter cake, re-slurry, add 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stir at room temperature for 30min, let stand for 30min, filter to obtain the third filtrate and the second filter cake, the third filtrate meets the standard and is used for metal ion recovery.

[0055] S4: Add the second filter cake to the ball mill, add an equal volume of grinding media alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the ash material and test the dioxin content to be 0.058μg TEQ / kg.

[0056] Example 3

[0057] This embodiment provides a method for reducing the dioxin content in fly ash, specifically including the following steps:

[0058] S1: Add 1500g of water to 500g of grate furnace fly ash, stir and slurry at room temperature for 30min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.200μg TEQ / kg, then add 40g of concentrated hydrochloric acid to the obtained slurry, stir at room temperature for 60min, and filter to obtain the first filtrate and the first filter cake. The pH of the obtained first filtrate is 1.1.

[0059] S2: Continue to add 20g of sodium sulfide to the first filtrate to precipitate heavy metals such as Cu, Zn, Cr, and Cd. Filter to obtain the second filtrate, and the pH of the obtained filtrate is 7.8.

[0060] S3: Add the second filtrate to the first filter cake, re-slurry, add 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stir at room temperature for 30min, let stand for 30min, filter to obtain the third filtrate and the second filter cake, the third filtrate meets the standard and is used for metal ion recovery.

[0061] S4: Add the second filter cake to the ball mill, add an equal volume of grinding media (alumina liner) and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the resulting ash and test the dioxin content, which is 0.013μg TEQ / kg. The national standard for fly ash is: TEQ<0.05μg TEQ / kg.

[0062] Example 4

[0063] This embodiment provides a method for reducing the dioxin content in fly ash, specifically including the following steps:

[0064] S1: Add 1500g of water to 500g of grate furnace fly ash, stir and slurry at room temperature for 30min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2μg TEQ / kg; add 40g of concentrated sulfuric acid to the slurry, stir at room temperature for 60min, and then filter to obtain the first filtrate and the first filter cake. The pH of the obtained filtrate is 1.3.

[0065] S2: Continue to add 20g of sodium sulfide to the first filtrate to precipitate heavy metals such as Cu, Zn, Cr, and Cd. Filter to obtain the second filtrate, and the pH of the obtained filtrate is 8.1.

[0066] S3: Add the second filtrate to the first filter cake, re-slurry, add 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stir at room temperature for 30min, let stand for 30min, filter to obtain the third filtrate and the second filter cake, the third filtrate meets the standard and is used for metal ion recovery.

[0067] S4: Add the second filter cake to the ball mill, add an equal volume of grinding media alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the ash material and test the dioxin content to be 0.008μg TEQ / kg.

[0068] Comparative Example 1

[0069] The difference between this comparative example and Example 4 is that ferric chloride was not added in S3. Instead, the second filtrate was added to the first filter cake, re-pulverized, and after standing for 30 minutes, filtered to obtain the third filtrate and the second filter cake.

[0070] The dioxin content detected in step S4 was 0.832 μg TEQ / kg.

[0071] Comparative Example 2

[0072] The difference between this comparative example and Example 3 is that in S4, the second filter cake was added to the ball mill, and an equal volume of alumina liner was added. The mill was then ball-milled at room temperature at a speed of 300 r / min for 60 min under a nitrogen atmosphere. The resulting ash was dried, and the dioxin content was found to be 0.687 μg TEQ / kg.

[0073] Comparative Example 3

[0074] The difference between this comparative example and Example 3 is that the chloride salt used in S3 was replaced with ferrous chloride; and the dioxin content measured in S4 was 0.305 μg TEQ / kg.

[0075] Comparative Example 4

[0076] The difference between this comparative example and Example 1 is that sodium sulfide is not used, specifically:

[0077] S1: Add 1500g of water to 500g of grate furnace fly ash and stir and slurry at room temperature for 30 minutes. The initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2μg TEQ / kg. Add 40g of concentrated hydrochloric acid to the slurry and stir at room temperature for 60 minutes. Then add 20g of ferric chloride solution and stir at room temperature for 30 minutes. After standing for 30 minutes, filter to obtain the first filtrate and the first filter cake. The first filtrate meets the standards and can be used for metal ion recovery.

[0078] S2: Add the first filter cake to the ball mill, add an equal volume of grinding media alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the ash and test the dioxin content to be 0.441μg TEQ / kg.

[0079] Comparative Example 5

[0080] The difference from Example 4 is that, in this comparative example:

[0081] S1: Add 1500g of water to 500g of grate furnace fly ash, stir and slurry at room temperature for 30min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2μg TEQ / kg; add 40g of concentrated sulfuric acid to the slurry, stir at room temperature for 60min, and then filter to obtain the first filtrate and the first filter cake. The pH of the obtained filtrate is 1.3.

[0082] S2: Continue to add 20g of sodium sulfide and 100g of hypochlorous acid to the first filtrate to precipitate heavy metals such as Cu, Zn, Cr, and Cd. Filter to obtain the second filtrate, and the pH of the obtained filtrate is 2.0.

[0083] S3: Add the second filtrate to the first filter cake, re-slurry, add 20g of ferric chloride to the slurry, stir the solution at room temperature for 30min, let it stand for 30min, and then filter to obtain the third filtrate and the second filter cake. The third filtrate meets the standards and can be used for metal ion recovery.

[0084] S4: Add the second filter cake to the ball mill, add an equal volume of grinding media alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min under a nitrogen atmosphere. After ball milling, dry the ash material and test the dioxin content to be 0.288μg TEQ / kg.

[0085] Therefore, the timing of the addition of hypochlorous acid is very important. In this comparative example, hypochlorous acid was added in step S2. At this time, hypochlorous acid reacted with sodium sulfide in the system, which prevented hypochlorous acid from working synergistically with chloride in step S3, resulting in poor removal effect.

[0086] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for reducing the dioxin content in fly ash, characterized in that, The method includes the following steps: S1: Add an appropriate amount of water to the fly ash, slurry it to obtain the first slurry, then add strong acid, stir and filter to obtain the first filtrate and the first filter cake; S2: Add sulfide salt to the first filtrate, stir and filter to obtain the second filtrate; S3: The second slurry is prepared by using the second filtrate and the first filter cake, and then chloride salt is added. After stirring, settling and filtering in sequence, the second filter cake and the third filtrate are obtained; hypochlorous acid is added at the same time as the chloride salt is added.

2. The method according to claim 1, characterized in that, Step S1 satisfies one or more of the following conditions: a. The mass ratio of fly ash to water is 1:1~4; b. The pulping process includes stirring at room temperature for 30-60 minutes; c. The strong acid includes any one of sulfuric acid, hydrochloric acid, and nitric acid; d. The mass ratio of the strong acid to the first slurry is 1~5:75; e. The stirring is carried out at room temperature for 60-90 minutes; f. The pH value of the first filtrate is 1 to 4.

3. The method according to claim 1, characterized in that, Step S2 satisfies one or more of the following conditions: g. The mass ratio of the sulfide salt to the first filtrate is 1~50:150; h. The stirring is carried out at room temperature for 60-90 minutes; i. The sulfide salt is either sodium sulfide or potassium sulfide; j. The pH value of the second filtrate is 7~9.

4. The method according to claim 1, characterized in that, Step S3 satisfies one or more of the following conditions: k. The mass ratio of the chloride salt to the second slurry is 1~100:200; l. The chloride salt includes any one or more of ferric chloride, ferrous chloride, and manganese chloride; m. The stirring includes stirring at room temperature for 30-60 minutes; n. The standing period includes standing at room temperature for 30 to 60 minutes.

5. The method according to claim 1, characterized in that, The method further includes: ball milling the second filter cake.

6. The method according to claim 5, characterized in that, The ball milling process includes: adding the second filter cake into a ball mill, using an alumina liner as the grinding medium, adding an appropriate amount of hydrogen peroxide, providing a specified atmosphere, and performing ball milling. The ash obtained from ball milling is dried and then tested for dioxin content.

7. The method according to claim 6, characterized in that, The ball milling process satisfies one or more of the following conditions: o. The mass ratio of hydrogen peroxide to the second filter cake is 1~10:50; p. The specified atmosphere includes any one of nitrogen, argon, and oxygen; q. The volume ratio of the grinding media to the second filter cake is 1~20:1; r. The ball milling rate is 300~400 r / min; s. The ball milling time is 60~90 min.

8. The method according to claim 1, characterized in that, The mass ratio of hypochlorous acid to chloride salt is 1~20:1.

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