Method for reducing content of dioxin in fly ash

Abstract

The invention provides a method for reducing the content of dioxin in fly ash. The method for reducing the dioxin content in the fly ash comprises the following steps: adding a proper amount of water into the fly ash, slurrying to obtain first slurry, then adding strong acid, stirring and filtering to obtain first filtrate and a first filter cake; adding sulfide salt into the first filtrate, stirring and filtering to obtain second filtrate. According to the method, the fly ash is subjected to pulping pretreatment, metal ions for inducing generation of dioxin in the fly ash are removed through the steps of leaching and directional precipitation, generation of dioxin in the follow-up treatment process is inhibited, and the content of dioxin is effectively reduced. The method is mild in means, easy to operate, low in energy consumption in the process, free of pollution and high in removal efficiency, and the cost of the process of removing dioxin from fly ash is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of fly ash and flue gas treatment, and in particular relates to a method for reducing the content of dioxins in fly ash. Background Art

[0002] During the incineration process of municipal solid waste, fly ash rich in metals such as Ca, K, and Na is produced, which is known as a "misplaced resource". However, the excessive dioxins contained in fly ash have a strong risk of carcinogenicity, teratogenicity, and mutagenicity to humans, which seriously limits the resource utilization and harmless utilization of fly ash. Therefore, removing dioxins from fly ash is critical to its comprehensive utilization.

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

[0004] Mechanochemical degradation is to effectively change the structure and properties of pollutants through grinding operations such as extrusion, impact, friction, and shearing during mechanochemical processing. Patent CN115970820A takes ball milling as an example. After 1h of ball milling, the dioxin toxic equivalent TEQ reduction rate reached 59.0% and 60.4% respectively, and the total dioxin concentration decreased with the increase of ball milling time. However, the mechanochemical degradation method is still in the pilot stage, and the dioxin removal rate is not high.

[0005] Photocatalytic degradation, patent CN220677391U uses sunlight and ultraviolet radiation to oxidatively decompose dioxins. Although the reaction conditions for photodegradation of dioxins are mild, most of the reported catalyst raw materials are precious metals, which are costly and have complex preparation processes.

[0006] Hydrothermal degradation, the method disclosed in patent CN113521637B is to add alkaline substances or auxiliary reagents such as hydrazine carbonate and ferrous sulfate to decompose dioxins in the supercritical state of water. The auxiliary group can effectively promote the dechlorination of dioxins and reduce the toxic equivalent of dioxins. Although the hydrothermal method has made progress in recent years, a large amount of wastewater is generated during the treatment process.

[0007] High temperature thermal decomposition is the most widely used technology at present. For example, patent CN109603457B, the equipment used for heat treatment of garbage fly ash includes plasma furnace, rotary kiln, natural gas furnace and coke bed smelting furnace, among which plasma technology has been rapidly developed due to its high degradation rate of dioxins. However, this method consumes a lot of energy and is costly.

[0008] In summary, low-energy, low-cost and environmentally friendly removal of dioxins from fly ash remains the technical bottleneck of this technology.

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

[0010] The object of the present invention is to provide a method for reducing the dioxin content in fly ash to solve the above problems.

[0011] In order to achieve the above objectives, the present invention particularly adopts the following technical solutions:

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

[0013] S1: adding an appropriate amount of water to the fly ash, slurrying to obtain a first slurry, then adding a strong acid, stirring and filtering to obtain a first filtrate and a first filter cake; slurrying the fly ash for pretreatment to remove some elements in the fly ash that induce dioxin production, thereby inhibiting the generation of dioxins in subsequent treatment steps;

[0014] Wherein, the mass ratio of fly ash to water is 1:1-4;

[0015] The slurrying comprises: stirring at room temperature for 30 to 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 part of the insoluble metals and chlorine from the solid phase to the liquid phase, so as to facilitate the subsequent removal;

[0017] The stirring is carried out at room temperature for 60 to 90 minutes;

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

[0019] S2: adding sulfide salt to the first filtrate, stirring and filtering to obtain a second filtrate; the sulfide salt can partially precipitate the metals such as Cu, Zn, Cr, Cd, etc. leached in the above step to facilitate 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 to 50:150;

[0021] The stirring is carried out at room temperature for 60 to 90 minutes;

[0022] The sulfide salt is any one of sodium sulfide and potassium sulfide;

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

[0024] Optionally, the method includes S3: using the second filtrate and the first filter cake to prepare a second slurry, then adding chloride salt, preferably, adding hypochlorous acid at the same time as the chloride salt, stirring, standing and filtering in sequence to obtain a second filter cake and a third filtrate; the chloride salt can make it easier to reduce the toxic equivalent of dioxins in fly ash, and reduce the toxic equivalent by promoting the breaking of the C-Cl bond and CC bond of dioxins; the synergistic effect of hypochlorous acid and metal ions in the chloride salt promotes the ring opening of dioxins, thereby promoting the decomposition of dioxins and reducing the toxic equivalent. At this time, the third filtrate is tested to have a dioxin toxic equivalent that 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 to 100:200; the mass ratio of the hypochlorous acid to the chloride salt is 1 to 20:1;

[0026] The chloride salt includes any one or more of ferric chloride, ferrous chloride, and manganese chloride; preferably ferric chloride; relative to ferric ions, ferrous ions have a low valence, poor oxidizability, and are difficult to break C-Cl bonds, and the effect of reducing toxic equivalents is slightly worse than that of ferric ions;

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

[0028] The standing still comprises standing still at room temperature for 30 to 60 minutes.

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

[0030] The ball milling treatment comprises: adding the second filter cake into a ball mill, using an alumina liner as a grinding medium, adding an appropriate amount of hydrogen peroxide, providing a specified atmosphere, and performing ball milling treatment, and drying the ash obtained by ball milling to detect the content of dioxins;

[0031] The mass ratio of the 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 medium 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 to 90 minutes.

[0036] The present invention firstly achieves the purpose of reducing the dioxin content by inhibiting the generation of dioxins. The fly ash is pre-treated by beating, and the leaching and directional precipitation steps are set to remove the metal ions in the fly ash that induce the generation of dioxins, thereby inhibiting the generation of dioxins in the subsequent treatment process. At the same time, the generated dioxins are chemically and physically decomposed, and specific auxiliary ions and additives are introduced into the fly ash. The metal ions in the chloride salt promote the breaking of the C-Cl bond and the CC bond of the dioxins, thereby reducing the toxic equivalent of the dioxins. The hypochlorous acid promotes the ring opening of the dioxins by synergistic action with the metal ions in the chloride salt, thereby causing the decomposition of the dioxins. The dioxins in the filter cake are further decomposed by ball milling, thereby achieving the purpose of removing the dioxins in the fly ash efficiently, cheaply and gently.

[0037] Beneficial effects of the present invention:

[0038] The present invention removes excessive dioxins in fly ash by gentle and simple means. By introducing special additives, the generation of dioxins is reduced on the one hand, and the decomposition of dioxins is promoted on the other hand. The process has low energy consumption, no pollution, and a simple process, which greatly reduces the cost of fly ash dioxin removal. Compared with the traditional high-temperature thermal decomposition of dioxin process, the present invention greatly reduces the energy consumption of the treatment process; compared with the existing mechanical degradation of dioxin process, the present invention avoids the generation of dioxins during ball milling and significantly improves the removal rate of dioxins. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

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

[0041] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work 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, which specifically includes the following steps:

[0045] S1: adding 1500 g of water to 500 g of grate furnace fly ash, stirring and slurrying for 30 min at room temperature, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.200 μg TEQ / kg, then adding 40 g of concentrated hydrochloric acid to the obtained slurry, stirring at room temperature for 60 min, and then filtering to obtain a first filtrate and a first filter cake, wherein 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, and filter to obtain a second filtrate, the pH of which is 7.8;

[0047] S3: adding the second filtrate to the first filter cake, re-slurrying, adding 20g of ferric chloride to the slurry, stirring at room temperature for 30min, standing for 30min, filtering to obtain a third filtrate and a second filter cake, and the third filtrate meets the standards for metal ion recovery;

[0048] S4: The second filter cake was added into a ball mill, and an equal volume of a grinding medium alumina liner and 50 g of hydrogen peroxide were added. The ball milling was performed at a speed of 300 r / min at room temperature for 60 min under a nitrogen atmosphere. The ash obtained after ball milling was dried, and the dioxin content was detected to be 0.064 μg TEQ / kg.

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

[0050] Example 2

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

[0052] S1: adding 1500 g of water to 500 g of grate furnace fly ash, stirring at room temperature for 30 min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2 μg TEQ / kg; adding 40 g of concentrated sulfuric acid to the slurry, stirring at room temperature for 60 min, and filtering to obtain a first filtrate and a first filter cake, wherein 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, and filter to obtain a second filtrate, the pH of which is 8.1;

[0054] S3: adding the second filtrate to the first filter cake, re-slurrying, adding 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stirring at room temperature for 30min, standing for 30min, filtering to obtain a third filtrate and a second filter cake, and the third filtrate meets the standards for metal ion recovery;

[0055] S4: The second filter cake was added into a ball mill, and an equal volume of alumina liner and 50 g of hydrogen peroxide were added. The ball mill was performed at a speed of 300 r / min for 60 min at room temperature under a nitrogen atmosphere. The ash obtained after the ball milling was dried, and the dioxin content was detected to be 0.058 μg TEQ / kg.

[0056] Example 3

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

[0058] S1: adding 1500 g of water to 500 g of grate furnace fly ash, stirring and slurrying for 30 min at room temperature, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.200 μg TEQ / kg, then adding 40 g of concentrated hydrochloric acid to the obtained slurry, stirring at room temperature for 60 min, and then filtering to obtain a first filtrate and a first filter cake, wherein 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, and filter to obtain a second filtrate, the pH of which is 7.8;

[0060] S3: adding the second filtrate to the first filter cake, re-slurrying, adding 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stirring at room temperature for 30min, standing for 30min, filtering to obtain a third filtrate and a second filter cake, and the third filtrate meets the standards for metal ion recovery;

[0061] S4: Add the second filter cake into a ball mill, add an equal volume of grinding medium alumina liner and 50g of hydrogen peroxide, and ball mill at room temperature at a speed of 300r / min for 60min in a nitrogen atmosphere. Dry the ash obtained after ball milling. The dioxin content is detected to be 0.013μg TEQ / kg. The qualified range of fly ash in the national standard is: TEQ<0.05μg TEQ / kg.

[0062] Example 4

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

[0064] S1: adding 1500 g of water to 500 g of grate furnace fly ash, stirring at room temperature for 30 min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2 μg TEQ / kg; adding 40 g of concentrated sulfuric acid to the slurry, stirring at room temperature for 60 min, and filtering to obtain a first filtrate and a first filter cake, wherein 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, and filter to obtain a second filtrate, the pH of which is 8.1;

[0066] S3: adding the second filtrate to the first filter cake, re-slurrying, adding 20g of ferric chloride and 100g of hypochlorous acid to the slurry, stirring at room temperature for 30min, standing for 30min, filtering to obtain a third filtrate and a second filter cake, and the third filtrate meets the standards for metal ion recovery;

[0067] S4: The second filter cake was added into a ball mill, and an equal volume of alumina liner and 50 g of hydrogen peroxide were added. The ball mill was performed at a speed of 300 r / min for 60 min at room temperature under a nitrogen atmosphere. The ash obtained after the ball milling was dried, and the dioxin content was detected to be 0.008 μg TEQ / kg.

[0068] Comparative Example 1

[0069] The difference between this comparative example and Example 4 is that no ferric chloride is added to S3, but the second filtrate is added to the first filter cake, and the slurry is re-beaten, and after standing for 30 minutes, the third filtrate and the second filter cake are obtained by filtering.

[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 is added to the ball mill, and an equal volume of alumina liner is added as a grinding medium. The ball milling is carried out at a speed of 300 r / min at room temperature for 60 min under a nitrogen atmosphere. The ash obtained after ball milling is dried, and the dioxin content is detected 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 is replaced by ferrous chloride; the dioxin content measured in S4 is 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: adding 1500g of water to 500g of grate furnace fly ash, stirring at room temperature for 30min, wherein the initial toxic equivalent (TEQ) of the grate furnace fly ash is 1.2μg TEQ / kg, adding 40g of concentrated hydrochloric acid to the slurry, stirring at room temperature for 60min, then adding 20g of ferric chloride solution and stirring at room temperature for 30min, after standing for 30min, filtering to obtain a first filtrate and a first filter cake, the first filtrate meets the standards and is used for metal ion recovery;

[0078] S2: The first filter cake was added into a ball mill, and an equal volume of a grinding medium alumina liner and 50 g of hydrogen peroxide were added. The ball milling was performed at room temperature at a speed of 300 r / min for 60 min under a nitrogen atmosphere. The ash after ball milling was dried, and the dioxin content was detected to be 0.441 μg TEQ / kg.

[0079] Comparative Example 5

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

[0081] S1: adding 1500 g of water to 500 g of grate furnace fly ash, stirring at room temperature for 30 min, wherein the initial toxicity equivalent (TEQ) of the grate furnace fly ash is 1.2 μg TEQ / kg; adding 40 g of concentrated sulfuric acid to the slurry, stirring at room temperature for 60 min, and filtering to obtain a first filtrate and a first filter cake, wherein 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, and filter to obtain a second filtrate, the pH of the obtained filtrate is 2.0;

[0083] S3: adding the second filtrate to the first filter cake, re-slurrying, adding 20g of ferric chloride to the slurry, stirring the solution at room temperature for 30min, standing for 30min, filtering to obtain a third filtrate and a second filter cake, and the third filtrate meets the standards for metal ion recovery;

[0084] S4: The second filter cake was added into a ball mill, and an equal volume of alumina liner and 50 g of hydrogen peroxide were added. The ball mill was performed at a speed of 300 r / min for 60 min at room temperature under a nitrogen atmosphere. The ash obtained after the ball milling was dried, and the dioxin content was detected to be 0.288 μg TEQ / kg.

[0085] It can be seen that the timing of adding hypochlorous acid is very important. In this comparative example, hypochlorous acid is added in step S2. At this time, hypochlorous acid undergoes an oxidation reaction with sodium sulfide in the system, resulting in the inability of hypochlorous acid to work synergistically with the chloride salt in S3, which leads to poor removal effect.

[0086] Please note that the technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above embodiments only express several implementation methods of the present application, and their descriptions are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, without departing from the concept of the present application, several variations and improvements can be made, which all belong to the scope of protection of the present application. Therefore, the scope of protection of the patent in this application shall be based on the attached claims.

Claims

1. A method for reducing the content of dioxins in fly ash, characterized in that: The method comprises the following steps: S1: adding an appropriate amount of water to the 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; S2: adding sulfide salt to the first filtrate, stirring and filtering to obtain a second filtrate.

2. The method according to claim 1, characterized in that The step S1 satisfies one or more of the following conditions: a. The mass ratio of fly ash to water is 1:1 to 4; b. The slurrying comprises: stirring at room temperature for 30 to 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 to 5:75; e. The stirring is carried out at room temperature for 60 to 90 minutes; f. The pH value of the first filtrate is 1-4.

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

4. The method according to claim 1, characterized in that: The method further comprises step S3: preparing a second slurry using the second filtrate and the first filter cake, then adding chloride salt, stirring, standing and filtering in sequence to obtain a second filter cake and a third filtrate.

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

6. The method according to claim 1 or 4, characterized in that: The method further comprises: subjecting the second filter cake to ball milling.

7. The method according to claim 6, characterized in that The ball milling treatment includes: adding the second filter cake into a ball mill, using an alumina liner as a grinding medium, adding an appropriate amount of hydrogen peroxide, providing a specified atmosphere, and performing ball milling treatment, and testing the dioxin content of the ash obtained by ball milling after drying.

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

9. The method according to claim 4 or 7, characterized in that: The hypochlorous acid is added simultaneously with the addition of the chloride salt.

10. The method according to claim 9, characterized in that The mass ratio of the hypochlorous acid to the chlorinated salt is 1 to 20:1.

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