Method for recovering heavy metal and soluble chlorine salt from fly ash generated by incineration of domestic waste

Abstract

A method for recovering heavy metals and soluble chlorine salts from fly ash generated by incineration of domestic waste. The method comprises the following steps: first, mixing fly ash with water and an acid, performing a leaching reaction when a pH value is controlled, improving the leaching efficiency of lead and zinc by means of air agitation, and measuring dissolved oxygen or redox potential to determine a leaching reaction endpoint; second, using a vulcanizing agent to recover leached heavy metals, and adjusting and monitoring the pH value of a leachate so as to control the adding amount of the vulcanizing agent; third, adding an alkali to adjust the pH value of the leachate so as to precipitate impurities such as aluminum and iron, and using an excessive amount of the vulcanizing agent to further remove heavy metals; and finally, adding an oxidizing agent and an iron salt coagulant into the leachate from which the heavy metals and impurities are removed, so as to remove divalent sulfur and arsenic, wherein the finally purified leachate can be used for recovering soluble chlorine salts. The method has a simple and efficient process, can significantly increase the recovery rate of heavy metals, and recovers heavy metals in the form of metal sulfides, thus facilitating subsequent separation treatment. The treated fly ash can be used as a raw material of cement, thus achieving efficient utilization of resources.

Description

A method for recovering heavy metals and soluble chloride salts from fly ash from incineration of domestic waste Technical Field

[0001] The invention belongs to the technical field of chemical industry and environmental protection, and particularly relates to a method for recovering heavy metals and soluble chloride salts from fly ash from domestic waste incineration. Background Art

[0002] Municipal solid waste incineration technology has become a widely adopted method for treating municipal solid waste in many countries worldwide due to its advantages, such as its small footprint, effective waste volume reduction, and ability to generate cogeneration of heat and power. However, municipal solid waste incineration produces fly ash (hereinafter referred to as fly ash), equivalent to 2-5% of the waste weight. Fly ash contains heavy metals such as lead, cadmium, chromium, zinc, and copper, as well as dioxins, and is therefore classified as hazardous waste. However, heavy metals such as zinc, copper, and lead, as well as soluble chloride salts such as calcium chloride, potassium chloride, and sodium chloride, contained in fly ash have high recovery value.

[0003] Currently, the most widely used fly ash treatment method is solidification and stabilization followed by landfill. However, this method has a large volume expansion ratio, requires a large amount of landfill space, and cannot recover heavy metals and soluble chloride salts in the fly ash. Thermal treatment methods (such as sintering or melting) are also commonly used fly ash treatment methods. These methods are effective, but energy consumption and processing costs are high. After water washing, fly ash is used as a raw material for cement production in countries such as China and Japan. This method can realize fly ash resource utilization and recover soluble chloride salts, but it cannot recover heavy metals. Acid leaching is one of the main treatment methods in Switzerland. This method can recover heavy metals, but it currently suffers from low leaching efficiency for lead and copper, and the heavy metal hydroxide sludge produced is difficult to utilize in existing mineral processing technologies, resulting in low recovery value. Therefore, there is a need to develop a simple method that can be compatible with existing mineral processing technologies and can simultaneously recover heavy metals and soluble chloride salts from fly ash.

[0004] Acid leaching effectively recovers heavy metals from fly ash, and the purified leachate can be used as a raw material for recovering soluble chloride salts. Furthermore, after acid leaching and water washing, the heavy metal and chlorine content in fly ash is reduced, making it suitable for use as a raw material in cement production. The acid required for fly ash leaching can be recovered from the flue gas treatment system of municipal waste incineration plants. Deacidified water (primarily containing hydrochloric acid) from incineration plants using wet flue gas treatment systems can be used as the acid for fly ash leaching. Furthermore, hydrochloric acid, a byproduct of the production of potassium sulfate and chlorine-containing organic chemical raw materials, can also be used for fly ash leaching, achieving synergistic treatment of waste hydrochloric acid and fly ash. Technical issues

[0005] The current fly ash acid leaching method still has the following technical difficulties:

[0006] 1. Because fly ash contains a large amount of zero-valent aluminum, it undergoes a replacement reaction with lead and copper during the leaching process to produce zero-valent lead and zero-valent copper, resulting in very low lead and copper leaching rates. Swiss processes add hydrogen peroxide to the leaching process to improve lead and copper leaching efficiency, but this significantly increases processing costs and makes it difficult to accurately control the addition amount. Therefore, a low-cost and efficient method to improve lead and copper leaching rates is needed.

[0007] 2. In the currently used fly ash acid leaching process, heavy metals are precipitated as hydroxide sludge. However, the separation of heavy metal hydroxides is difficult, making this heavy metal hydroxide sludge difficult to utilize by existing mineral processing companies.

[0008] 3. It is necessary to take into account both the recovery of heavy metals and the purification of leachate to achieve the recovery of heavy metals and soluble chloride salts. Technical Solutions

[0009] The purpose of the present invention is to develop a method for simultaneously recovering heavy metals and soluble chloride salts from fly ash.

[0010] The technical solutions provided by the present invention are as follows:

[0011] A. Add fly ash, water, and hydrochloric acid (or deacidified water from the flue gas treatment system of a municipal waste incineration plant) to the leaching reactor to ensure good fluidity of the slurry. Monitor and control the pH of the slurry to keep it below 3.0. Monitor the slurry temperature to keep it above 40°C. Stir the slurry using air agitation or combined mechanical-air agitation. The completion of the leaching reaction can be determined by monitoring the dissolved oxygen concentration, redox potential, or both in the slurry. A significant increase in the dissolved oxygen concentration or redox potential indicates that the leaching reaction is complete. The slurry is then subjected to solid-liquid separation, resulting in leachate 1 as the liquid phase and the leached residue as the solid phase.

[0012] B. This step is optional. Depending on the subsequent treatment or resource utilization of the leached residue from Step A, you can choose whether to wash the leached residue, as well as determine the solid-to-liquid ratio, frequency, and method of the washing process. If washing is performed, the washing liquid obtained after solid-liquid separation will be returned to Step A to improve heavy metal recovery.

[0013] C. Adjust the pH of leachate 1 obtained in step A to a value within the range of 2.5-3.0 (determined based on actual operational results). Subsequently, add a sulfiding agent (the sulfiding agent herein may include, but is not limited to, sodium sulfide solution or potassium sulfide solution) to the leachate and stir to precipitate the heavy metals in leachate 1. While adding the sulfiding agent, monitor the pH of leachate 1. Stop adding the sulfiding agent when the pH of leachate 1 significantly rises and exceeds a value between pH 3.0 and pH 3.3 (determined based on actual operational results). Solid-liquid separation is performed to obtain leachate 2 and a heavy metal precipitate.

[0014] D. Adding an alkali (herein, the alkali may be, but is not limited to, lime slurry or sodium hydroxide solution) to the leachate 2 obtained in step C to adjust the pH of the leachate 2 to 9 to 10 to precipitate impurities such as aluminum, silicon, iron, and magnesium in the leachate 2 and further remove heavy metals. Solid-liquid separation is performed to obtain a leachate 3 and an impurity precipitate.

[0015] E. To the leachate 3 obtained in step D, an oxidant (the oxidant herein may include, but is not limited to, sodium hypochlorite, calcium hypochlorite, hydrogen peroxide, chlorine, ozone, chlorine dioxide, etc.) and an iron salt coagulant (the iron salt coagulant herein may include, but is not limited to, ferric chloride or polyferric chloride, etc.) are sequentially or simultaneously added. The pH of the leachate 3 is adjusted based on the type of oxidant added to remove divalent sulfur and arsenic from the leachate 3. Different oxidants have their own optimal pH values. The optimal pH for arsenic removal with an iron salt coagulant is 5 to 7. Therefore, oxidation followed by coagulation or simultaneous oxidation and coagulation can be selected. Solid-liquid separation is performed to obtain leachate 4 and desulfurization and dearsenicization slag. Leachate 4 can be used as a raw material for recovering soluble chloride salts.

[0016] The basis of the present invention is:

[0017] 1. Municipal solid waste incineration fly ash contains significant amounts of zero-valent aluminum. During the acid leaching process, zero-valent aluminum undergoes a replacement reaction with lead and copper to produce zero-valent lead and zero-valent copper. Zero-valent lead reacts very slowly with hydrochloric acid, while zero-valent copper does not react with hydrochloric acid. However, the presence of oxygen promotes oxygen-absorbing corrosion of zero-valent lead and zero-valent copper, leading to their leaching. During this process, zero-valent lead and zero-valent copper react with oxygen to form their corresponding oxides, which then react with hydrochloric acid. Due to the high concentration of chloride ions in the slurry, ferric and copper ions can form complexes with the chloride ions and dissolve in the liquid phase. Therefore, the iron ions in the reaction system act as a catalyst, with ferric ions undergoing a redox reaction with zero-valent lead and zero-valent copper to produce ferrous ions, lead ions, and copper ions. The ferrous ions then react with oxygen and hydrogen ions to produce ferric ions and water. The resulting copper ions also act as a catalyst. Divalent copper ions first react with zero-valent copper to form monovalent copper ions, which then react with oxygen and hydrogen ions to form divalent copper ions. Therefore, in step A, air agitation or combined mechanical-air agitation is used to provide oxygen to the reaction system, thereby increasing the leaching rate of lead and copper.

[0018] 2. Experimental results show that increasing the reaction temperature can accelerate the leaching of lead and copper, and increase the leaching rate of zinc.

[0019] 3. The experimental results show that when the pH value of the mortar exceeds 3.3,

[0020] 4. Experimental results show that dissolved oxygen in the slurry is consumed rapidly, remaining at a low level until zero-valent lead and zero-valent copper are fully leached. Furthermore, the leaching of the three major heavy metals in fly ash (zinc, lead, and copper) exhibits a distinct sequential order: zinc leaches first, followed by lead, and finally copper. This is related to the order of activity of the three metals. Therefore, after copper leaching is complete, the dissolved oxygen in the slurry begins to rise, indicating that zinc and lead leaching has also been completed. This increase in dissolved oxygen also causes an increase in the redox potential. Therefore, the endpoint of the leaching reaction can be determined by monitoring the dissolved oxygen concentration or the redox potential in the slurry, or both.

[0021] 5. In step C, the endpoint of the heavy metal precipitation reaction is determined by monitoring the pH change of the leachate. This is because the solubility product constant of heavy metal sulfides is very low. When insufficient sulfiding agent is added, the concentration of divalent sulfur in the leachate is very low, so the hydrolysis of divalent sulfide ions does not cause a change in the leachate pH. Therefore, when insufficient sulfiding agent is added, the pH of the leachate remains stable. However, when excessive sulfiding agent is added, the hydrolysis of the excess divalent sulfide ions in the leachate produces hydrosulfide ions and hydrogen sulfide, which reduces the hydrogen ion concentration in the leachate and significantly increases the pH. When the pH rises to 3.3, the aluminum ions in the leachate begin to hydrolyze to form aluminum hydroxide precipitate, which stabilizes the leachate pH again. Therefore, the amount of sulfiding agent added can be determined by the change in the leachate pH, and the separation of heavy metals from impurities such as aluminum, iron, and magnesium can be achieved.

[0022] 6. Heavy metal sulfides can be easily separated to obtain the corresponding concentrates by flotation, which is a widely used and mature technology in the ore processing industry. Therefore, the present invention uses a sulfiding agent to recover heavy metals.

[0023] 7. The leachate 2 contains excessive divalent sulfur, a small amount of heavy metal ions, and impurities such as aluminum, silicon, iron, and magnesium. In step D, increasing the pH value of the leachate can convert hydrosulfide ions and hydrogen sulfide into divalent sulfur ions, thereby further precipitating the residual heavy metals. At the same time, increasing the pH value can also precipitate impurities such as aluminum, silicon, iron, and magnesium. The hydroxide precipitate formed by aluminum and iron can be used as a coagulant to remove other impurities. Under alkaline conditions, divalent iron ions and divalent sulfur form ferrous sulfide precipitates, thereby removing excess sulfur in the leachate. However, whether the divalent sulfur in the leachate can be completely removed depends on the content of iron ions in the leachate.

[0024] 8. The main pollutants in the leachate 3 are divalent sulfur and arsenic. It is worth noting that in step C, divalent sulfur can reduce pentavalent arsenic to trivalent arsenic. The toxicity of trivalent arsenic is 60 times that of pentavalent arsenic, and its removal is more difficult. In step E, the addition of an oxidant can oxidize trivalent arsenic to pentavalent arsenic, and the subsequent ferric arsenate precipitate generated by pentavalent arsenic and trivalent iron has very low solubility. In addition, iron hydroxide flocs can also effectively adsorb and remove pentavalent arsenic. The oxidant can also oxidize divalent sulfur, converting it into elemental sulfur or further oxidizing it into sulfite and sulfate. The newly generated elemental sulfur appears in the form of fine particles, while sulfate and sulfite will form precipitates with calcium ions in the leachate. Iron hydroxide flocs can effectively remove these precipitates. Beneficial effects

[0025] The present invention has the following beneficial effects:

[0026] 1. The invention can significantly improve the leaching efficiency of lead and copper, which not only increases the recovery rate of heavy metals but also reduces the impact of fly ash on the environment after treatment.

[0027] 2. The end point of the leaching reaction can be accurately determined, thereby ensuring that heavy metals are fully leached, while saving reaction time and energy consumption.

[0028] 3. Heavy metals are recovered in the form of sulfides, which makes subsequent mineral processing more convenient and can be combined with existing ore processing technology.

[0029] 4. By monitoring the pH changes during the precipitation reaction, the amount of sulfiding agent added can be controlled to a slight excess. This allows for full recovery of heavy metals while conserving reagents. Excess sulfiding agent can further remove heavy metals from the leachate during subsequent treatment.

[0030] 5. The fly ash after treatment is significantly reduced in volume, the heavy metal content is greatly reduced, and it is more stable. The chlorine content is reduced to below 1%, making it suitable as a raw material for cement.

[0031] 6. The use of sulfiding agent as a heavy metal remover makes the heavy metal content in the leachate very low. The heavy metal content of the salt obtained after evaporation and drying is lower than the edible salt standard.

[0032] 7. The impurity precipitates produced contain low levels of heavy metals and are in the form of heavy metal sulfides, which are not easily leached. Therefore, these precipitates can be directly sent to landfills for disposal, which is relatively low in cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 provides further illustration of this process, but is not intended to be limiting. In this process, the leached residue undergoes a secondary countercurrent water wash to remove chlorine. The wash solution is then recycled to the acid leaching step to improve heavy metal recovery. The treated fly ash can be used as a cement raw material. Best Mode for Carrying Out the Invention

[0034] The technical method described in this patent is employed in municipal waste incineration plants. First, hydrochloric acid is recovered from the incinerator's flue gas treatment system and used to leach fly ash. Second, waste heat from the incinerator is used to heat the leaching reactor and evaporate the leachate, reducing energy consumption. Finally, the treated fly ash is incinerated again to decompose dioxins and evaporate any remaining heavy metals. This ensures the complete and harmless treatment of fly ash, converting it into bottom ash.

[0035] Fly ash, water, and deacidified water from a municipal waste incineration plant are added to the leaching reactor, maintaining a solid-to-liquid ratio between 1:3 and 1:5. The slurry is agitated using air or combined mechanical-air agitation. The reaction temperature is maintained at approximately 60°C. High temperatures help accelerate the leaching rate of lead and copper and improve the leaching rate of zinc. Since the deacidified water temperature is also around 60°C, and the reaction of hydrochloric acid with the alkali in the fly ash releases heat, maintaining the reaction temperature at 60°C does not significantly increase energy consumption. The pH of the slurry is monitored and controlled at approximately 2.5 using hydrochloric acid. This pH value achieves high heavy metal leaching efficiency. The dissolved oxygen or redox potential in the slurry is monitored; a significant increase indicates the end of the leaching reaction.

[0036] The slurry is transported to the target location using a hose pump. A vacuum belt filter is then used to separate the solid and liquid phases of the slurry. The resulting solid phase is called leachate, while the liquid phase is called leachate.

[0037] The leached residue and water are added to a washing tank at a solid-liquid ratio of 1:5. After washing, the residue is pumped to a designated location using a hose pump and separated using a belt vacuum filter. After separation, the liquid phase is returned to the leaching reactor, while the solid phase is sent to an incinerator.

[0038] Use a hose pump to transfer leachate 1 to the heavy metal recovery reactor. Since this reaction produces hydrogen sulfide, the reactor should be sealed and equipped with a hydrogen sulfide absorber. While leachate 1 is being transferred to the reactor, it should be stirred using a mechanical stirring paddle. Subsequently, adjust the pH of leachate 1 to 2.8 using sodium hydroxide solution, and gradually add sodium sulfide solution. Simultaneously, continuously monitor the pH of leachate 1. Stop adding sodium sulfide when the pH of leachate 1 reaches 3.2.

[0039] Leachate 1 is delivered to the target location using a hose pump, and then subjected to solid-liquid separation using a vacuum belt filter. After separation, the solid phase obtained is the heavy metal precipitate, while the liquid phase is called leachate 2.

[0040] Leachate 2 was pumped to an impurity removal reactor using a hose pump. In the reactor, leachate 2 was stirred using a mechanical agitator and its pH was adjusted to 9.5 using lime slurry. The mixture was then subjected to solid-liquid separation using a horizontal spiral centrifuge, yielding an impurity precipitate and leachate 3.

[0041] Leachate 3 is transferred to a desulfurization and dearsenicization reactor. Appropriate amounts of sodium hypochlorite solution and ferric chloride solution are added to the reactor, and the pH is adjusted to between 5 and 6. After adding the reagents, the mixture is slowly stirred for a period of time. Subsequently, the mixture is subjected to solid-liquid separation by precipitation and filtration to obtain leachate 4 and dearsenicization and desulfurization slag. Leachate 4 can be used as a raw material for recovering soluble chloride salts. Modes for Carrying Out the Invention

[0042] According to the present invention, by-product hydrochloric acid and waste hydrochloric acid can be used as raw materials to leach fly ash, thereby realizing the coordinated treatment of waste acid and fly ash. Since the method described in this patent can remove heavy metals, sulfur and arsenic, and the fly ash leachate contains a large amount of calcium ions, it can form calcium fluoride precipitate with fluoride ions. Therefore, by-product hydrochloric acid and waste hydrochloric acid containing heavy metals, sulfur, arsenic and fluorine can also be used as raw materials to leach fly ash. The treated fly ash can be used as a cement raw material, or be landfilled after solidification and stabilization. According to the present invention, a waste acid and fly ash coordinated treatment plant can be established to treat waste acid and fly ash at the same time.

[0043] Fly ash, water, and hydrochloric acid are added to the leaching reactor, maintaining a solid-to-liquid ratio between 1:3 and 1:5. The slurry is agitated using air or combined mechanical-air agitation. The reaction temperature is maintained at approximately 60°C; high temperatures help accelerate the leaching rate of lead and copper and increase the leaching rate of zinc. Because the reaction of hydrochloric acid with the alkali in the fly ash releases heat, maintaining the reaction temperature at 60°C does not significantly increase energy consumption. The pH of the slurry is monitored and controlled at approximately 2.5 using hydrochloric acid. This pH achieves high heavy metal leaching efficiency and prevents the formation of aluminum hydroxide. Due to the high water content of the aluminum hydroxide precipitate, it may remove chlorine, making subsequent water washing and dechlorination difficult. Monitor the dissolved oxygen or redox potential in the slurry and stop stirring when it increases significantly.

[0044] The slurry is transported to the target location using a hose pump. A vacuum belt filter is then used to separate the solid and liquid phases of the slurry. The resulting solid phase is called leachate, while the liquid phase is called leachate.

[0045] The leached residue can be washed multiple times in countercurrent, with the washed fly ash used as cement raw material. Alternatively, the once-washed or unwashed leached residue can be solidified and landfilled. The separated washing liquid is returned to the leaching reactor.

[0046] Use a hose pump to transfer leachate 1 to the heavy metal recovery reactor. Since this reaction produces hydrogen sulfide, the reactor should be sealed and equipped with a hydrogen sulfide absorber. While leachate 1 is being transferred to the reactor, it should be stirred using a mechanical stirring paddle. Subsequently, adjust the pH of leachate 1 to 2.8 using sodium hydroxide solution, and gradually add sodium sulfide solution. Simultaneously, continuously monitor the pH of leachate 1. Stop adding sodium sulfide when the pH of leachate 1 reaches 3.2.

[0047] Leachate 1 is delivered to the target location using a hose pump, and then subjected to solid-liquid separation using a vacuum belt filter. After separation, the solid phase obtained is the heavy metal precipitate, while the liquid phase is called leachate 2.

[0048] Leachate 2 was pumped to an impurity removal reactor using a hose pump. In the reactor, leachate 2 was stirred using a mechanical agitator and its pH was adjusted to 9.5 using lime slurry. The mixture was then subjected to solid-liquid separation using a horizontal spiral centrifuge, yielding an impurity precipitate and leachate 3.

[0049] Leachate 3 is transferred to a desulfurization and dearsenicization reactor. Appropriate amounts of sodium hypochlorite solution and ferric chloride solution are added to the reactor, and the pH is adjusted to between 5 and 6. After adding the reagents, the mixture is slowly stirred for a period of time. Subsequently, the mixture is subjected to solid-liquid separation by precipitation and filtration to obtain leachate 4 and dearsenicization and desulfurization slag. Leachate 4 can be used as a raw material for recovering soluble chloride salts. Industrial Applicability

[0050] Acid leaching has been used in fly ash treatment, especially in Switzerland, where it is the main fly ash treatment method. Compared to existing acid leaching methods, the method described in the present invention has higher leaching efficiency and lower processing costs. The recovered heavy metals can be used by existing ore processing companies. In addition, since this method can purify the leachate while recovering heavy metals, by-product hydrochloric acid and waste hydrochloric acid containing impurities such as heavy metals, arsenic, fluorine, sulfur, and iron can be used as leaching agents to achieve coordinated treatment of fly ash and waste acid. Therefore, this method has extremely high industrial applicability. Examples

[0051] The following is a further description of the present invention, but not a limitation of the present invention.

[0052] A method for recovering heavy metals and soluble chloride salts from fly ash from the incineration of domestic waste is provided. The process flow chart is shown in FIG1 and comprises the following steps:

[0053] A. Add 140g of fly ash to a 1L glass bottle, then add 700ml of deionized water and a magnetic stirrer, stirring at 400r / min. Then add 80ml of 31% hydrochloric acid. Use an air pump to introduce air into the slurry at a rate of 8L / min. Use an automatic titrator to control the pH value of the slurry at 2±0.01 and measure the dissolved oxygen concentration. Set the reaction time to 5h. After the reaction is completed, centrifuge the slurry at 3000r / min for 5min to separate the solid and liquid. Filter the separated liquid phase with filter paper and then dilute it to 1L to obtain leachate 1. The separated solid phase is called leachate residue, washed twice with 400ml of deionized water, and then dried at 105℃ for 12h.

[0054] B. Adjust the pH of leachate 1 to 2.8 using lime slurry. Then, add 1 mol / L sodium sulfide solution dropwise to the leachate. Stop adding sodium sulfide when the pH of the leachate rises to 3.2 and remains stable. After the reaction is complete, centrifuge leachate 1 at 3000 rpm for 5 minutes to separate the solid and liquid. The resulting liquid phase is leachate 2, and the solid phase is the heavy metal precipitate, which is dried at 105°C for 24 hours.

[0055] C. Adjust the pH of leachate 2 to 10 using lime slurry. After the reaction, centrifuge leachate 2 at 3000 rpm for 5 minutes to separate the solid and liquid. The resulting liquid phase is leachate 3, and the solid phase is the impurity precipitate, which is then dried at 105°C for 48 hours.

[0056] The chemical compositions of fly ash, leached residue, heavy metal precipitate, and impurity precipitate were determined by XRF. The results are shown in Table 1.

[0057] Table 1 Heavy metal contents of fly ash, leached residue, heavy metal precipitate and impurity precipitate obtained under the conditions of aeration rate of 8 L / min and reaction time of 5 h.

[0058]

[0059] The mass of the leached residue accounts for 22.01% of the original fly ash mass, and the chlorine content in the leached residue is 0.52%.

Claims

1. A method for leaching heavy metals from fly ash from the incineration of domestic waste, characterized in that: a. Use hydrochloric acid or deacidified water from the flue gas treatment system of a domestic waste incineration plant as the leaching agent; b. Use air mixing or mechanical-air combined mixing to mix and oxygenate the mortar to ensure adequate mixing and oxygen supply; c. Determine the dissolved oxygen concentration or redox potential of the slurry, or both, and determine the end point of the leaching reaction by the change in dissolved oxygen concentration or redox potential; d. Monitor and adjust the reaction pH.

2. The method according to claim 1, wherein in feature d, the reaction pH should be controlled below 3.0 during the leaching reaction to increase the leaching rate of heavy metals.

3. A method for recovering and removing heavy metals from fly ash acid leaching solution from incineration of domestic waste, characterized in that: The following steps are involved: a. adjusting the pH value of the leaching solution to a specific value A; b. Add a sulfiding agent to the leachate and stir it, while detecting the pH change of the leachate. When the pH of the leachate rises significantly and exceeds a specific value B, stop adding the sulfiding agent; c. performing solid-liquid separation, the solid phase obtained is a heavy metal precipitate, and the liquid phase is a leachate 2; d. adding alkali to the leachate 2 to increase its pH to alkaline to remove impurities and further remove heavy metals; e. performing solid-liquid separation again to obtain an impurity precipitate and a leachate 3.

4. The method according to claim 3, wherein the specific value A in step a is between 2.5 and 3.0, and the specific value B in step b is between 3.0 and 3.

3.

5. A method for recovering heavy metals and soluble chloride salts from fly ash from incineration of domestic waste, characterized in that: The following steps are involved: a. Using hydrochloric acid or deacidified water from the flue gas treatment system of a municipal solid waste incineration plant as a leaching agent, heavy metals in the fly ash of the municipal solid waste incineration are leached out; after the leaching is completed, solid-liquid separation is performed to obtain a leachate 1 and a leached residue. b. Depending on the final treatment method of the leached residue, you can choose whether to wash the leached residue; If the leached residue is washed with water, the liquid phase after solid-liquid separation is returned to step a for preparing slurry. c. adding a sulfiding agent to the leaching solution 1 to precipitate the heavy metals, and determining the amount of the sulfiding agent added by adjusting the pH and detecting the pH change; After the addition of the sulfiding agent is completed, solid-liquid separation is performed to obtain heavy metal precipitates and leachate 2. d. Alkali is added to leachate 2 to adjust the pH value of leachate 2 to alkaline to remove impurities and further remove heavy metals; after the pH is adjusted, solid-liquid separation is performed to obtain impurity precipitates and leachate 3. e. An oxidant and an iron salt coagulant are added to leachate 3 in sequence or simultaneously, and the pH is adjusted to adapt to the optimal pH range of the oxidant and the iron salt coagulant to remove sulfur and arsenic; after the reaction is completed, solid-liquid separation is performed to obtain desulfurization and dearsenicification slag and leachate 4, and leachate 4 is the raw material for further recovery of soluble salts.

6. The method according to claim 5, wherein in step a, the mortar is stirred by air stirring or mechanical-air combined stirring.

7. The method according to claim 5, wherein in step a, the dissolved oxygen concentration or the redox potential of the slurry is measured, or both are measured simultaneously, and the endpoint of the leaching reaction is determined by the change in the dissolved oxygen concentration or the redox potential.

8. The method according to claim 5, wherein the specific method for determining the amount of sulfiding agent added in step c is as follows: first, the pH value of the leachate is adjusted to a specific value A between 2.5 and 3.0; then, the sulfiding agent is gradually added to the leachate and stirred, while monitoring the pH change of the leachate; when the pH of the leachate rises significantly and exceeds a specific value B between 3.0 and 3.3, the addition of the sulfiding agent is stopped.

Images