Method for preparing magnesium fluoride from municipal solid waste incineration fly ash

Abstract

The application discloses a method for preparing magnesium fluoride from household garbage incineration fly ash. The household garbage fly ash is subjected to acid washing / water washing to obtain aqueous fly ash slurry. The slurry is subjected to solid-liquid separation to obtain calcium and magnesium-containing crude salt water. The solid is subjected to a subsequent process. The calcium and magnesium-containing crude salt water is subjected to a heavy component removal process, and is subjected to static sedimentation. The clear liquid is subjected to a denitrification process, an oxidation process and a neutralization process, and is subjected to solid-liquid separation to obtain calcium and magnesium-containing refined salt water. The calcium and magnesium-containing refined salt water is subjected to a calcium removal process, and is subjected to solid-liquid separation to obtain refined magnesium water. The refined magnesium-containing refined salt water is subjected to a magnesium removal process by adding alkali to obtain magnesium hydroxide. The magnesium hydroxide is configured into slurry with a certain concentration, is slowly added into pretreated waste hydrofluoric acid solution, is fully stirred, is subjected to solid-liquid separation, and is subjected to three-stage countercurrent oxidation washing and then thermal desorption to obtain magnesium fluoride products.

Description

Technical Field

[0001] This invention belongs to the field of waste resource utilization, specifically relating to a method for preparing magnesium fluoride from fly ash from municipal solid waste incineration. Background Technology

[0002] Currently, the annual production of fly ash from municipal solid waste incineration in China is approximately 9 million tons. With the increase in municipal solid waste collection and the ratio of waste to electricity generation through incineration, the peak production of fly ash is estimated at around 15 million tons. Fly ash primarily contains large amounts of soluble salts, as well as substances such as calcium, silicon, aluminum, activated carbon, lead, zinc, copper, and dioxins. It also contains 2%-5% magnesium. Fly ash resource recovery technologies mainly target the removal or recovery of dioxins, heavy metals, and soluble salts, but generally do not involve the removal or recovery of magnesium.

[0003] Etching wastewater mainly originates from the cleaning and etching processes. The principle of glass etching solution is to react hydrofluoric acid with glass (SiO2), generating fluorosilicic acid (H2SiF6) and glass slag and silica (water glass) during the reaction. As hydrofluoric acid is gradually consumed and the concentration of aging substances continuously increases, the glass etching bath becomes aged, leading to a decrease in etching rate and an increase in etching defect rate. The traditional method to solve these problems is to treat the aged bath solution as waste acid and discharge it, then rebuild the bath and prepare a new etching solution. However, waste acid often still contains a high concentration of hydrofluoric acid. Taking the fluoride-containing wastewater generated from the etching and cleaning processes in the chip manufacturing process of an electronics factory as an example, the ions contained therein, besides F... - In addition, it contains a high concentration of SO4. 2- The fluoride-containing waste acid contains organic matter and trace amounts of metal impurities, including ions. Currently, the common method for treating fluoride-containing waste acid is to add alkaline metal oxides to neutralize it and achieve neutral sludge production, but this does not realize the resource utilization of the etching solution.

[0004] This invention creatively proposes a technical solution for preparing magnesium fluoride from fly ash of municipal solid waste incineration, realizing the resource utilization of magnesium ions while solving the problem of treating waste hydrofluoric acid containing etching waste liquid. It has great significance in the field of hazardous solid waste treatment and high-quality resource utilization. Summary of the Invention

[0005] To address the limitations of existing technologies for treating municipal solid waste incineration fly ash, this invention discloses a method for preparing magnesium fluoride from municipal solid waste incineration fly ash. The fly ash is first acid-washed / water-washed to obtain an aqueous fly ash slurry. The slurry undergoes solid-liquid separation to obtain a high-calcium-magnesium crude brine, with the solids proceeding to the next stage. The high-calcium-magnesium crude brine undergoes a degravation process, followed by settling. The clarified liquid then undergoes denitrification, oxidation, and neutralization reactions, followed by solid-liquid separation to obtain a calcium-magnesium refined brine. This calcium-magnesium refined brine undergoes a calcium removal process, followed by solid-liquid separation to obtain a magnesium refined brine. The magnesium refined brine undergoes an alkali removal process to obtain magnesium hydroxide. The magnesium hydroxide is prepared into a slurry of a certain concentration and slowly added to pretreated waste hydrofluoric acid. After thorough stirring and solid-liquid separation, the resulting solids undergo three-stage countercurrent oxidation washing, followed by thermal desorption and drying to obtain a high-purity magnesium fluoride product.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A method for preparing magnesium fluoride from fly ash of municipal solid waste incineration, comprising the following specific steps:

[0008] (1) The fly ash from municipal solid waste is acid-washed and water-washed to obtain water-based fly ash slurry. The water-based fly ash slurry is then separated into solid and liquid to obtain crude brine with high calcium and magnesium content.

[0009] (2) The high calcium and magnesium crude brine obtained in step (1) is subjected to a weight removal process, and after settling, the clear liquid is subjected to denitrification, oxidation, neutralization reaction, and then solid-liquid separation to obtain calcium and magnesium refined brine.

[0010] (3) The calcium- and magnesium-containing refined brine obtained in step (2) is subjected to a calcium removal process and solid-liquid separation to obtain magnesium refined brine;

[0011] (4) The magnesium refined brine obtained in step (3) is subjected to an alkali removal process to obtain magnesium hydroxide. The magnesium hydroxide is prepared into a magnesium hydroxide slurry of a certain concentration and slowly added to the pretreated low-acid waste liquid. After thorough stirring, solid-liquid separation is performed to obtain crude magnesium fluoride.

[0012] (5) The crude magnesium fluoride obtained in step (4) is first washed by three-stage countercurrent oxidation and then dried by thermal desorption to obtain a high-purity magnesium fluoride product.

[0013] As an improvement of the present invention, in step (1): the washing time is 15-60 min, the mass ratio of acid to water added to the fly ash from municipal solid waste incineration and the acid and water added for pickling and washing is 1:2-3:2-3, the concentration of acid is 6-10 mol / L, the acid is hydrochloric acid, sulfuric acid, nitric acid, etc., the acid used for pickling includes commercial acid, by-product acid, and waste acid, the pollutants in the high calcium and magnesium crude brine are mainly organic pollutants (containing trace amounts of dioxins), inorganic pollutants, and heavy metal pollutants, and the solid-liquid separation device includes plate and frame diaphragm filter, belt filter press, horizontal vacuum filter, filter centrifuge, horizontal centrifuge, etc.

[0014] As an improvement of the present invention, the specific steps of step (2) are as follows: black heavy metal mud is obtained by adding a de-gravity agent and performing de-gravity sedimentation separation. The obtained filtrate is denitrified by adding a denitrification agent, oxidizing by adding an oxidant for multiphase catalytic oxidation, and neutralizing to obtain yellow heavy metal mud. The filtrate obtained by solid-liquid separation is calcium-magnesium refined brine.

[0015] As an improvement of the present invention, in step (2): the weight removal time is 0.5-1h, the denitrification and multiphase catalytic oxidation time is 1-2h, the weight removal agent is a soluble sulfide, potassium ferrocyanide, etc., the soluble sulfide includes sodium sulfide, potassium sulfide, etc., the amount of the weight removal agent used is 0.3-0.8% of the total mass of the solution, the denitrification agent is sodium hypochlorite, sodium chlorate, etc., the amount of the denitrification agent used is 0.5-1% of the solution volume, the oxidant is hydrogen peroxide, ozone, permanganate, persulfate, etc., the permanganate includes potassium permanganate, sodium permanganate, etc., the persulfate includes sodium persulfate, potassium persulfate, etc., the oxidant used is 1-5% of the solution volume, and the calcium-magnesium refined brine mainly contains sodium chloride, potassium chloride, calcium chloride and magnesium chloride, etc.

[0016] As an improvement of the present invention, the specific steps of step (3) are as follows: a calcium removal agent is added to the calcium-magnesium refined brine obtained in step (2), and the magnesium refined brine is obtained by solid-liquid separation through stirring reaction, sedimentation, filtration and other processes; the calcium removal agent includes commercial sulfate, carbonate and sulfuric acid, as well as waste sulfate, waste carbonate and waste sulfuric acid generated in the production process, the carbonate includes sodium carbonate, potassium carbonate, etc., the amount of calcium removal agent added is 0.3 to 30% of the total mass of the solution, the calcium removal pH is 5 to 7, and the calcium ion concentration of the magnesium refined brine can be reduced to less than 2 g / L.

[0017] As an improvement to the present invention, step (4) specifically involves the following steps:

[0018] S1. A magnesium removal agent is added to magnesium-refined brine, and solid-liquid separation is performed to obtain magnesium-removed water and magnesium salts. The magnesium salts are prepared into a slurry with a mass concentration of 10-20% for later use. The magnesium removal agent is a soluble hydroxide, carbonate, etc. The soluble hydroxide includes sodium hydroxide, potassium hydroxide, etc., and the carbonate includes sodium carbonate, potassium carbonate, etc. The dosage of the magnesium removal agent is 0.1-5% of the total mass of the solution. The magnesium removal pH is 11-12. The magnesium ion concentration in the magnesium removal water can be reduced to below 10 ppm. The magnesium salts include insoluble solids such as magnesium hydroxide and magnesium carbonate.

[0019] S2. Add a pH adjuster to waste hydrofluoric acid to convert the waste hydrofluoric acid into a milder low-acid waste liquid. The pH adjuster is a soluble hydroxide, carbonate, etc., and also includes waste alkali and waste carbonate generated in the industrial production process. The amount of pH adjuster added is 4-40% of the total mass of the solution, and the pH of the adjusted low-acid waste liquid is 1-3.

[0020] S3. Add a heavy metal removal agent to the low-acid waste liquid adjusted in step S2 to remove heavy metals under acidic conditions. After solid-liquid separation, heavy water is obtained. The heavy metal removal agent is a sulfide, potassium ferrocyanide, etc. The sulfide includes sodium sulfide, potassium sulfide, etc. The amount of heavy metal removal agent used is 0.3 to 0.8% of the total mass of the solution, and the pH of the heavy metal removal reaction is 1 to 3.

[0021] S4. Slowly add magnesium hydroxide or magnesium carbonate slurry prepared in S1 to the low-acid wastewater obtained in S3 at a molar ratio of Mg:F = 0.5-1:1 and stir to react. Avoid adding all at once, which may cause slurry agglomeration and affect the magnesium fluoride content. The addition time is 1-2 hours, and the reaction continues for 0.5-1.5 hours. After solid-liquid separation, crude magnesium oxide is obtained. The pH value at the reaction endpoint is controlled at 4-5.

[0022] The high-purity magnesium fluoride product obtained by this invention can meet the requirements of "protecting human health and ecological safety" in the "Law of the People's Republic of China on the Prevention and Control of Environmental Pollution by Solid Waste" (revised in 2020). The quality indicators of high-purity magnesium fluoride products can meet the technical requirements of MF-1 products specified in Table 1 of "Magnesium Fluoride" (YS / T 691-2009); the toxic and harmful pollutant indicators in the leachate prepared by high-purity magnesium fluoride products according to the "Solid Waste Leaching Toxicity Leaching Method Horizontal Oscillation Method" (HJ557) meet the Class III water standard limits specified in Table 1 of "Surface Water Environmental Quality Standard" (GB3838), and specific items can meet the centralized drinking water standard limits specified in Table 3; if acid washing is carried out using waste acid as raw material, and the waste acid involves pesticide pollutants, pharmaceutical and veterinary drug pollutants, the specific item limits in the product can also meet the relevant standard limits specified in "National Food Safety Standard Maximum Residue Limits for Pesticides in Food" (GB2763) and "National Food Safety Standard Maximum Residue Limits for 41 Veterinary Drugs in Food" (GB31650), and the specific item limits for the toxicity equivalent of dioxins and their analogues should meet the limit requirement of 8.0 pg / g wet weight.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. Removal of calcium and magnesium basic ions from fly ash by acid washing and water washing: This invention washes fly ash under strong acid to maximize the removal of basic ions from the fly ash into the solution, thereby providing the possibility of magnesium salt resource utilization and achieving efficient and deep desalination of fly ash.

[0025] 2. Simultaneous resource utilization of fly ash and waste hydrofluoric acid liquid: This invention utilizes magnesium elements in fly ash and fluoride ions in waste hydrofluoric acid liquid. Through a strict impurity removal process, high-purity magnesium fluoride products are extracted, which solves the problem of difficult disposal of fly ash and waste hydrofluoric acid liquid and realizes high-value resource utilization.

[0026] 3. Preparation and separation of high-purity magnesium fluoride products. This patent removes impurities and organic matter from the raw materials beforehand, avoiding the introduction of other impurities into the magnesium fluoride preparation process. Simultaneously, the hydrofluoric acid waste liquid is neutralized and deweighted to obtain clean and safe deweighted water, improving the safety and operability of the reaction. After three-stage oxidation countercurrent washing and thermal desorption, the purity of the magnesium fluoride is further guaranteed.

[0027] 4. Preparation of large-particle magnesium fluoride for industrial separation. Magnesium fluoride particles are fine and easily gel, making them extremely difficult to separate in industrial production. This patent first extracts magnesium hydroxide or magnesium carbonate solids from a magnesium-containing solution after impurity removal, then prepares a slurry of a certain concentration, and slowly and evenly adds it to hydrofluoric acid-de-heavy water. By strictly controlling the reaction time and addition rate, magnesium fluoride is promoted to grow layer by layer with the slurry particles as the core, ultimately producing large-particle magnesium fluoride. This improves purity and also makes filtration and washing possible. Detailed Implementation

[0028] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0029] Example 1

[0030] A method for preparing magnesium fluoride from fly ash of municipal solid waste incineration, comprising the following specific steps:

[0031] (1) The fly ash from municipal solid waste is acid-washed and washed with water for 15-60 minutes to obtain water-based fly ash slurry. The water-based fly ash slurry is then separated into solid and liquid to obtain crude brine with high calcium and magnesium content.

[0032] The mass ratio of acid to water added during the acid washing and water washing processes of municipal solid waste incineration fly ash is 1:2-3:2-3. The acid concentration is 6-10 mol / L, and the acid is hydrochloric acid, sulfuric acid, nitric acid, etc. The acid used for acid washing includes commercial acid, by-product acid, and waste acid. The pollutants in the high calcium and magnesium content crude brine are mainly organic pollutants (containing trace amounts of dioxins), inorganic pollutants, and heavy metal pollutants. The solid-liquid separation device includes plate and frame diaphragm filters, belt filter presses, horizontal vacuum filters, filter centrifuges, horizontal centrifuges, etc.

[0033] (2) The high calcium and magnesium crude brine obtained in step (1) is separated by adding a de-gravity agent and then undergoing sedimentation to obtain black heavy metal mud. The obtained filtrate is denitrified by adding a denitrification agent, undergoing multiphase catalytic oxidation by adding an oxidant, and then undergoing a neutralization reaction to obtain yellow heavy metal mud. The filtrate obtained by solid-liquid separation is calcium and magnesium refined brine.

[0034] The degrafting time is 0.5–1 h, the denitrification and heterogeneous catalytic oxidation time is 1–2 h, the degrafting agent is a soluble sulfide, potassium ferrocyanide, etc., the amount of the degrafting agent used is 0.3–0.8% of the total mass of the solution, the amount of the denitrification agent used is 0.5–1% of the solution volume, the oxidant is hydrogen peroxide, ozone, permanganate, persulfate, etc., the amount of the oxidant used is 1–5% of the solution volume, and the calcium-magnesium refined brine mainly contains sodium chloride, potassium chloride, calcium chloride and magnesium chloride, etc.

[0035] (3) Add calcium removal agent to the calcium-magnesium refined brine obtained in step (2), and obtain magnesium refined brine by solid-liquid separation through stirring reaction, sedimentation and filtration. The calcium removal agent includes commercial sulfate, carbonate and sulfuric acid, as well as waste sulfate, waste carbonate and waste sulfuric acid generated in the production process. The amount of calcium removal agent added is 0.3 to 30% of the total mass of the solution. The calcium removal pH is 5 to 7. The calcium ion concentration of the magnesium refined brine can be reduced to less than 2 g / L.

[0036] (4) The magnesium refined brine obtained in step (3) is subjected to an alkali removal process to obtain magnesium hydroxide. The magnesium hydroxide is prepared into a magnesium carbonate slurry of a certain concentration and slowly added to the pretreated hydrofluoric acid waste glass etching solution. After thorough stirring, solid and liquid separation is performed to obtain crude magnesium fluoride.

[0037] Step (4) consists of the following steps:

[0038] S1. A magnesium removal agent is added to magnesium-refined brine, and magnesium removal water and magnesium salt are obtained by solid-liquid separation. The magnesium salt is prepared into a magnesium carbonate slurry with a mass concentration of 10-20% for later use. The magnesium removal agent is a soluble hydroxide, carbonate, etc. The dosage of the magnesium removal agent is 0.1-5% of the total mass of the solution. The magnesium removal pH is 11-12. The magnesium ion concentration in the magnesium removal water can be reduced to below 10 ppm. The magnesium salt includes insoluble solids such as magnesium hydroxide and magnesium carbonate.

[0039] S2. Add a pH adjuster to the waste hydrofluoric acid in the glass etching waste liquid to convert the waste hydrofluoric acid in the glass etching waste liquid into a milder waste hydrofluoric acid etching waste liquid. The pH adjuster is a soluble hydroxide, carbonate, etc., and also includes waste alkali and waste carbonate generated in the industrial production process. The amount of pH adjuster added is 4-40% of the total mass of the solution, and the pH of the adjusted waste hydrofluoric acid etching waste liquid is 1-3.

[0040] S3. Add a heavy metal removal agent to the hydrofluoric acid etching waste liquid after adjustment in step S2, remove heavy metals under acidic conditions, and obtain the etching waste liquid with heavy metal removal after solid-liquid separation. The heavy metal removal agent is sulfide, potassium ferrocyanide, etc., and the amount of heavy metal removal agent used is 0.3 to 0.8% of the total mass of the solution. The pH of the heavy metal removal reaction is 1 to 3.

[0041] S4. Slowly add magnesium hydroxide or magnesium carbonate slurry prepared in S1 to the etching waste liquid obtained in S3 (excluding heavy water) at a molar ratio of Mg:F = 0.5-1:1 and stir to react. Avoid adding all at once, which may cause slurry agglomeration and affect the magnesium fluoride content. The addition time is 1-2 hours, and the reaction continues for 0.5-1.5 hours. After solid-liquid separation, crude magnesium oxide is obtained. The pH value at the reaction endpoint is controlled at 4-5.

[0042] (5) The crude magnesium fluoride obtained in step (4) is first washed by three-stage countercurrent oxidation and then dried by thermal desorption to obtain a high-purity magnesium fluoride product.

[0043] Example 2

[0044] The fly ash from the incineration of municipal solid waste comes from a thermal power company in Jiangsu Province and is a typical type of fly ash from municipal solid waste incineration; the etching waste liquid comes from a semiconductor company in Jiangsu Province.

[0045] According to the steps of Example 1, the fly ash is acid-washed / water-washed to remove impurities and then reacted with etching waste liquid to prepare magnesium fluoride products. The specific experimental steps and results are shown in Table 1, and the magnesium fluoride products are shown in Table 2.

[0046] Table 1 Experimental steps and results

[0047]

[0048]

[0049] As shown in Table 1, after dissolving, adjusting the pH value, stirring, and rinsing multiple times, most of the magnesium in the fly ash dissolves into the solution. After impurity removal, most of the heavy metal ions are removed, resulting in clean magnesium hydroxide. Etching waste liquid, after neutralization and weight removal, can remove most of the heavy metal ions. Slowly adding magnesium hydroxide slurry and then performing three-stage countercurrent washing can produce large-particle, high-purity magnesium fluoride. Magnesium fluoride products are shown in Table 2.

[0050] Table 2 Magnesium Fluoride Product Indicators

[0051]

[0052]

[0053] Table 2 shows that the refined magnesium fluoride meets the MF-1 product standard limit specified in Table 1 of "Magnesium Fluoride" (YS / T 691-2009), and the specific item limit of the toxicity equivalent of dioxins and their analogues should meet the limit requirement of 8.0 pg / g wet weight.

[0054] Example 3

[0055] The fly ash from the incineration of municipal solid waste comes from a thermal power company in Shanghai and is a typical type of fly ash from municipal solid waste incineration; the etching waste liquid comes from a chip processing company in Jiangsu.

[0056] According to the steps of Example 1, the fly ash is acid-washed / water-washed to remove impurities and then reacted with etching waste liquid to prepare high-purity magnesium fluoride products. The specific experimental steps and results are shown in Table 3, and the magnesium fluoride products are shown in Table 4.

[0057] Table 3 Experimental steps and results

[0058]

[0059]

[0060] As shown in Table 3, after dissolving, adjusting the pH value, stirring, and rinsing multiple times, most of the magnesium in the fly ash dissolves into the solution. After impurity removal, most of the heavy metal ions are removed, resulting in clean magnesium hydroxide. Etching waste liquid, after neutralization and weight removal, can remove most of the heavy metal ions. Slowly adding magnesium hydroxide slurry and then performing three-stage countercurrent washing can produce large-particle, high-purity magnesium fluoride. Magnesium fluoride products are shown in Table 4.

[0061] Table 4 Magnesium Fluoride Product Indicators

[0062] Serial Number name unit Magnesium fluoride product data MF-1 standard limits 1 F ％ 60.3 60 2 Mg ％ 38.5 38 3 Ca ％ 0.08 0.3 4 SiO2 ％ 0.01 0.2 5 Fe2O3 ％ 0.1 0.3 6 <![CDATA[SO4 2- ]]> ％ 0.38 0.6 7 H2O ％ 0.16 0.2 8 Dioxins pg / g 1.2 8

[0063] Table 4 shows that the refined magnesium fluoride meets the MF-1 product standard limit specified in Table 1 of "Magnesium Fluoride" (YS / T 691-2009), and the specific item limit of the toxicity equivalent of dioxins and their analogues should meet the limit requirement of 8.0 pg / g wet weight.

[0064] It should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. For those skilled in the art, several improvements and modifications can be made on the basis of the above embodiments without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing magnesium fluoride from fly ash of municipal solid waste incineration, characterized in that, The specific steps are as follows: (1) The fly ash from municipal solid waste is acid-washed and water-washed to obtain water-based fly ash slurry. The water-based fly ash slurry is then separated into solid and liquid to obtain crude brine with high calcium and magnesium content. (2) The high calcium and magnesium crude brine obtained in step (1) is subjected to a weight removal process, and after settling, the clear liquid is subjected to denitrification, oxidation, and neutralization reactions, and then solid-liquid separation to obtain calcium and magnesium refined brine. (3) The calcium- and magnesium-containing refined brine obtained in step (2) is subjected to a calcium removal process and solid-liquid separation to obtain magnesium refined brine; (4) The specific steps of step (4) are as follows: S1. Add magnesium removal agent to the magnesium refined brine obtained in step (3), and separate the solid and liquid to obtain magnesium removal water and magnesium salt. Prepare the magnesium salt into a slurry with a mass concentration of 10~20% for later use. The magnesium removal agent is any one of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. The amount of magnesium removal agent added is 0.1~5% of the total mass of the solution, and the magnesium removal pH is 11~12. S2. Add a pH adjuster to waste hydrofluoric acid to convert the waste hydrofluoric acid into a milder, low-acid waste liquid. The pH adjuster is a soluble hydroxide or carbonate. The amount of pH adjuster added is 4-40% of the total mass of the solution. The pH of the adjusted low-acid waste liquid is 1-3. S3. Add a heavy metal removal agent to the low-acid waste liquid after adjustment in step S2 to remove heavy metals under acidic conditions. After solid-liquid separation, heavy metal removal wastewater is obtained. The heavy metal removal agent is sodium sulfide, potassium sulfide, or potassium ferrocyanide. The amount of heavy metal removal agent used is 0.3-0.8% of the total mass of the solution, and the pH of the heavy metal removal reaction is 1-3. S4. Add magnesium hydroxide or magnesium carbonate slurry prepared in S1 slowly dropwise to the low-acid hydrofluoric acid wastewater obtained in S3 (excluding heavy water) at a molar ratio of Mg:F = 0.5-1:1 and stir to react. The addition time is 1-2 hours, and the reaction continues for 0.5-1.5 hours. After solid-liquid separation, crude magnesium oxide is obtained. The pH value at the reaction endpoint is controlled at 4-5. (5) The crude magnesium fluoride obtained in step (4) is first washed by three-stage countercurrent oxidation and then dried by thermal desorption to obtain the magnesium fluoride product.

2. The method for preparing magnesium fluoride from fly ash of municipal solid waste incineration according to claim 1, characterized in that, In step (1): the washing time is 15~60min, the mass ratio of acid to water added to the fly ash from municipal solid waste incineration, acid washing, and water washing is 1:2~3:2~3, the acid concentration is 6~10mol / L, and the acid is any one or more of hydrochloric acid, sulfuric acid, and nitric acid.

3. The method for preparing magnesium fluoride from fly ash of municipal solid waste incineration according to claim 1, characterized in that, The specific steps of step (2) are as follows: black heavy metal mud is obtained by adding a denitrification agent and performing sedimentation separation. The obtained filtrate is denitrified by adding a denitrification agent, oxidized by adding an oxidant for multiphase catalytic oxidation, and neutralized to obtain yellow heavy metal mud. The filtrate obtained by solid-liquid separation is calcium-magnesium refined brine.

4. The method for preparing magnesium fluoride from fly ash of municipal solid waste incineration according to claim 3, characterized in that: The weight removal time is 0.5-1 h, the denitrification and heterogeneous catalytic oxidation time is 1-2 h, the weight removal agent is sodium sulfide, potassium sulfide, or potassium ferrocyanide, and the amount of the weight removal agent used is 0.3-0.8% of the total mass of the solution. The denitrification agent is sodium hypochlorite or sodium chlorate, and the amount of the denitrification agent used is 0.5-1% of the solution volume. The oxidant is any one of hydrogen peroxide, ozone, potassium permanganate, sodium permanganate, sodium persulfate, or potassium persulfate, and the amount of the oxidant used is 1-5% of the solution volume.

5. The method for preparing magnesium fluoride from municipal solid waste incineration fly ash according to claim 1, characterized in that, The specific steps of step (3) are as follows: add a calcium removal agent to the calcium-magnesium refined brine obtained in step (2), and obtain magnesium refined brine by stirring, settling, and filtering to separate solid and liquid. The calcium removal agent is sodium carbonate, potassium carbonate or sulfuric acid, and the amount of calcium removal agent added is 0.3~30% of the total mass of the solution. The calcium removal pH is 5~7.

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