A recovery process for preparing high-purity calcium oxalate from waste incineration fly ash

Abstract

This invention discloses a recovery process for preparing high-purity calcium oxalate from municipal solid waste incineration fly ash, comprising the following steps: treating the municipal solid waste incineration fly ash with dioxins to reduce the dioxin content in the treated fly ash to less than a predetermined threshold; performing multi-stage countercurrent rinsing on the detoxified fly ash to dissolve calcium ions from the fly ash into the liquid phase; treating the liquid phase obtained in step b with heavy metal removal; adding oxalate to the heavy metal-removed liquid phase to generate calcium oxalate precipitate, followed by solid-liquid separation to obtain crude calcium oxalate; purifying the crude calcium oxalate by acid washing, adjusting the pH to a first predetermined range, and performing solid-liquid separation; neutralizing the acid-washed and purified calcium oxalate by alkali washing, adjusting the pH to a second predetermined range, and performing solid-liquid separation to obtain high-purity calcium oxalate; and drying the high-purity calcium oxalate to obtain high-purity anhydrous calcium oxalate. This invention uses waste incineration fly ash to prepare widely used high-purity anhydrous calcium oxalate. The waste incineration fly ash undergoes dioxin detoxification, water washing, heavy metal removal, impurity removal, rinsing, and pH adjustment to achieve the purpose of preparing high-purity calcium oxalate.

Description

Technical Field

[0001] This invention relates to a process for recovering high-purity calcium oxalate from municipal solid waste fly ash, specifically to a method for extracting and purifying the calcium oxalate contained in municipal solid waste fly ash, belonging to the field of solid waste resource utilization. Background Technology

[0002] As the mainstream of municipal solid waste treatment gradually shifts from sanitary landfill to incineration, fly ash from municipal solid waste incineration has become a significant pollutant. Municipal solid waste incineration fly ash refers to the residue collected in the flue gas purification system of municipal solid waste incineration power plants, containing organic pollutants such as benzene compounds and dioxins, as well as trace amounts of heavy metals such as Pb and Cr. According to the "Standard for Pollution Control of Municipal Solid Waste Incineration" (GB18485-2014), "Municipal solid waste incineration fly ash should be managed as hazardous waste." Therefore, fly ash must be collected separately and must not be mixed with municipal solid waste, incineration residue, or other hazardous waste.

[0003] According to the "Technical Specification for Pollution Control of Fly Ash from Municipal Solid Waste Incineration (Trial)" (HJ 1134-2020), environmental pollution should be prevented during the collection, storage, transportation, treatment, and disposal of fly ash from municipal solid waste incineration. Fly ash treatment processes include water washing, solidification / stabilization, molding, low-temperature pyrolysis, high-temperature sintering, and high-temperature melting. Pollution prevention and control during the utilization of fly ash and its treatment products should comply with the requirements of the "Technical Guidelines for Pollution Prevention and Control of Solid Waste Recycling" (HJ 1091-2020) and the "Integrated Wastewater Discharge Standard" (GB8978-2002).

[0004] The components with higher content in fly ash include calcium chloride, calcium hydroxide, calcium sulfate, and calcium hydroxide. Traditional water washing processes often wash out soluble chloride ions and then add sodium carbonate to remove calcium ions. The decalcified solution enters an evaporator for evaporation and crystallization to obtain industrial-grade sodium chloride and potassium chloride. Sodium oxalate or calcium oxalate is often found in the mother liquor of evaporation. Calcium oxalate crystals are characterized by easy scaling and difficult cleaning. Calcium oxalate itself can be used as a ceramic glaze, in the preparation of oxalic acid, etc., and has a wide range of applications.

[0005] When fly ash with high calcium sulfate content is co-processed in a cement kiln, it will reduce the setting strength of cement clinker, which will lead to a decrease in the grade of subsequent cement products and greatly reduce the strength and stability of cement.

[0006] Chinese invention patent CN202110228506.8 describes a method for recovering lithium from lithium iron phosphate cathode materials using waste incineration fly ash. This patent utilizes the high chlorine content of waste incineration fly ash, employing the electrolysis products of chlorine in the fly ash to react with lithium iron phosphate cathode material powder, promoting the dissolution of lithium ions from the powder. A second electrolytic cell then achieves efficient separation of lithium from chlorine, phosphorus, and iron. The patent describes the recovery of over 96% of the lithium in the lithium iron phosphate cathode material powder. However, in practice, the sodium chloride / potassium chloride solution contains a TOC greater than 50 mg / kg, causing the electrolysis system to operate unstablely.

[0007] Chinese utility model patent CN202122805952.9 describes a method for the resource utilization of fly ash from waste incineration. Through a pelletizer, mixer, and smelting furnace, it achieves efficient and energy-saving treatment of fly ash from waste incineration. By melting and solidifying the fly ash, it prepares insulation cotton, thus achieving the reduction, harmlessness, and resource utilization of fly ash. However, this technology does not consider the technical specifications of the Ministry of Ecology and Environment, requiring pretreatment before fly ash can be utilized for resource recovery and harmlessness.

[0008] Chinese invention patent CN202210076178.9 describes a water washing and desalination device and method for fly ash from municipal solid waste incineration, which mainly uses a filter tank for solid-liquid separation. It does not describe high-chlorine ash, nor does it describe the anti-clogging performance of the filter media.

[0009] Chinese invention patent CN202010222818.3 discloses a high-efficiency dechlorination agent and method for waste incineration fly ash. The dechlorination agent uses a mixture of nitrates, phosphates, and organic acids for dechlorination. However, organic matter and nitrates can affect the quality of the crystalline salts and the organic matter content of the condensate. While employing electrochemical dechlorination, it lacks effective descriptions of electrode types, dechlorination effects, and the resource utilization of fly ash.

[0010] Chinese literature has explored the use of two-stage countercurrent rinsing for cleaning fly ash from grate incineration under a liquid-to-ash ratio of 6:1. The final chloride ion content can only be reduced to 1.7%, and there is a lack of recovery of effective resources other than sodium chloride and potassium chloride in the fly ash, which is a conventional treatment process. At the same time, when the liquid-to-ash ratio reaches 6:1, the water consumption is high, which increases the energy consumption of subsequent solid-liquid separation [Wang Yuting, Tang Minghui, Zong Da et al.; Characteristics of two-stage countercurrent water washing of fly ash from grate incineration [J]. Journal of Zhejiang University: Engineering Science, 2019, 53(5): 981-987]. Summary of the Invention

[0011] In view of this, the present invention provides a recycling process for preparing high-purity calcium oxalate from waste incineration fly ash to solve the problems in the background technology. This method, based on the characteristics of municipal solid waste incineration fly ash, understands the chemical properties of the chemical substances during the water washing process of waste incineration fly ash, and leverages the low content of organic soluble impurities in municipal solid waste incineration fly ash to achieve waste resource utilization. Calcium oxalate has low solubility under high chlorine concentration conditions and is sparingly soluble in water; therefore, preparing high-purity calcium oxalate using calcium hydroxide from the ash is technically feasible and innovative. This invention utilizes waste incineration fly ash to recover a broadly applicable product—calcium oxalate—which is a widely applicable solid waste resource utilization technology.

[0012] This invention provides a process for recovering high-purity calcium oxalate from waste incineration fly ash, comprising the following steps:

[0013] a. Perform dioxin detoxification treatment on fly ash from municipal solid waste incineration to ensure that the dioxin content in the treated fly ash is less than a predetermined threshold.

[0014] b. Perform multi-stage countercurrent rinsing on the detoxified fly ash to dissolve calcium ions from the fly ash into the liquid phase;

[0015] c. Perform heavy metal removal treatment on the liquid phase obtained in step b;

[0016] d. Add oxalate to the liquid phase after heavy metal removal treatment to generate calcium oxalate precipitate, and then separate the solid and liquid phases to obtain crude calcium oxalate;

[0017] e. The crude calcium oxalate is purified by acid washing, the pH is adjusted to a first predetermined range, and solid-liquid separation is performed;

[0018] f. The calcium oxalate purified by acid washing is neutralized by alkaline washing, the pH is adjusted to the second predetermined range, and high-purity calcium oxalate is obtained by solid-liquid separation;

[0019] g. Dry the high-purity calcium oxalate to obtain high-purity anhydrous calcium oxalate.

[0020] In one alternative implementation, in step a, the predetermined threshold is 50 ng-TEQ / kg.

[0021] In one optional implementation, in step b, the multi-stage countercurrent rinsing is a three-stage countercurrent rinsing, and the pH is not adjusted during the rinsing process.

[0022] In one optional embodiment, the heavy metal removal treatment includes the addition of sulfides and iron salts, wherein the concentration of the sulfides is 20-100 mg / L and the concentration of the iron salts is 50-350 mg / L.

[0023] In one alternative embodiment, the iron salt is ferrous chloride heptahydrate or ferrous sulfate heptahydrate.

[0024] In one optional embodiment, in step d, the oxalate is anhydrous sodium oxalate, and the concentration of residual calcium ions in the liquid phase at the time of addition is greater than 3000 mg / L.

[0025] In one optional embodiment, in step e, the acid washing purification uses oxalic acid to adjust the pH to 4-4.5, and the dilution ratio of crude calcium oxalate to deionized water is 1:1.

[0026] In one optional embodiment, in step f, the alkaline washing neutralization uses sodium hydroxide to adjust the pH to 7-7.5, and the dilution ratio of calcium oxalate to deionized water is 1:1.

[0027] In one optional implementation, in step g, the drying is high-temperature drying, and the resulting high-purity anhydrous calcium oxalate content is higher than 97%; if the purity does not meet the standard, steps e to f are repeated.

[0028] In one optional embodiment, the chlorine content of the fly ash after treatment is less than 1%.

[0029] The beneficial effects of the above technical solution are as follows:

[0030] The method of this invention utilizes fly ash from municipal solid waste incineration, which has a wide range of sources and stable output, to prepare high-purity calcium oxalate, achieving the significant goal of treating waste with waste and turning waste into treasure.

[0031] The process employed in this invention not only yields high-purity calcium oxalate but also reduces the amount of sodium carbonate added during the fly ash washing process in waste incineration, resulting in significant economic benefits.

[0032] This invention features continuous feeding and discharging throughout the entire process, ensuring stable operation of the entire system.

[0033] The method of this invention is simple to operate, highly adaptable, and suitable for fly ash treatment needs of different scales and different calcium contents.

[0034] This invention identifies problems related to the resource utilization of calcium ions from municipal solid waste incineration fly ash and systematically proposes solutions, making full use of the prerequisite of low TOC content in municipal solid waste incineration fly ash.

[0035] This invention employs a simple processing technique that combines the solubilizing effect under high salt content conditions with the principle of ion co-precipitation. It utilizes the refractory nature of calcium oxalate to maximize cost savings while proposing a new approach to solving the problem. Attached Figure Description

[0036] 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.

[0037] Figure 1 This is a schematic diagram of the process for preparing high-purity calcium oxalate from municipal solid waste fly ash according to the present invention. Detailed Implementation

[0038] 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, 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.

[0039] Combination Figure 1 As shown in the embodiment of the present invention, a recycling process for preparing high-purity calcium oxalate from municipal solid waste incineration fly ash is characterized by comprising the following steps:

[0040] The fly ash from waste incineration first passes through a dioxin pyrolysis unit to ensure that the dioxin content in the fly ash is less than 50 ng-TEQ / kg. It then enters a three-stage countercurrent rinsing process to dissolve sulfate and chloride ions from the fly ash as much as possible, allowing them to enter the liquid phase. The first-stage filtrate is first treated by adding trace amounts of ferrous sulfate and sodium sulfide to remove heavy metal ions from the solution. The filtrate after plate and frame filtration enters the subsequent treatment system. After adding a certain concentration of anhydrous sodium oxalate and stirring thoroughly, solid-liquid separation is performed using plate and frame filtration. The supernatant is returned to the fly ash washing process. The solids are mixed with deionized water, and the pH is adjusted to 4-4.5 with sulfuric acid. Solid-liquid separation is then performed using plate and frame filtration. The solids after plate and frame filtration are washed with deionized water, and the supernatant is adjusted to pH 7-7.5 with sodium hydroxide and then separated using a plate and frame filter press. The separated solids are high-purity calcium oxalate, which is subsequently dried to obtain high-purity anhydrous calcium oxalate.

[0041] The utilization of fly ash resources needs to meet the relevant requirements of the "Technical Specification for Pollution Control of Fly Ash from Municipal Solid Waste Incineration (Trial)" (HJ1134-2020). The main indicators for this invention include a dioxin concentration of less than 50 ng-TEQ / kg and a chlorine content of less than 1%.

[0042] Sodium sulfide is mainly used to remove amphoteric lead, zinc, nickel, and chromium ions from the solution. It can also effectively remove organic mercury. The concentration of sodium sulfide is 20-100 mg / L. After sodium sulfide is added, 300-350 mg / L of ferrous sulfate heptahydrate is added to remove excess sulfur ions.

[0043] The addition of sodium oxalate is related to the residual calcium ions. Generally, the residual calcium ion concentration should be greater than 2000 mg / L. This can not only efficiently and synergistically remove calcium ions, but also generate stable calcium oxalate crystals.

[0044] The crude calcium oxalate crystals obtained through solid-liquid separation were dissolved using deionization and diluted at a 1:1 ratio. The pH was then adjusted to 4-4.5 with 3-5 mol / L high-purity oxalic acid to ensure that some non-calcium oxalate metal oxides could dissolve in the supernatant, thus increasing the purity of the calcium oxalate.

[0045] After adjusting the pH, the resulting mixture was separated into solid and liquid components to obtain relatively high-purity calcium oxalate crystals.

[0046] The obtained calcium oxalate crystals were dissolved in deionized water at a 1:1 ratio, and then 3-5 mol / L sodium hydroxide was added to adjust the pH to between 7 and 7.5. After stirring the reaction thoroughly, solid-liquid separation was performed again by plate and frame filtration to obtain high-purity calcium oxalate containing water.

[0047] High-purity calcium oxalate is obtained by high-temperature drying to obtain high-purity anhydrous calcium oxalate with a purity higher than 97%. If the purity is less than 97%, steps e to f can be repeated until the purity is higher than 97%.

[0048] The core idea of ​​this invention is to utilize the reaction characteristics of calcium ions and oxalate ions in fly ash from municipal solid waste incineration, and to prepare high-purity calcium oxalate through steps such as dioxin detoxification, multi-stage countercurrent rinsing, heavy metal removal, calcium oxalate precipitation, and purification. The specific steps are as follows:

[0049] Dioxin detoxification treatment: The fly ash is passed through a dioxin pyrolysis unit to reduce the dioxin content to below 50 ng-TEQ / kg, meeting the requirements for harmlessness.

[0050] Multi-stage countercurrent rinsing: Three-stage countercurrent rinsing is used to maximize the dissolution of calcium ions in fly ash without adjusting the pH, allowing them to enter the liquid phase.

[0051] Heavy metal removal: Add sodium sulfide (20~100 mg / L) and iron salt (50~350 mg / L) to the liquid phase to remove heavy metal ions such as lead, zinc, nickel, and chromium. After solid-liquid separation, collect the filtrate.

[0052] Calcium oxalate precipitation: Anhydrous sodium oxalate (with calcium ion concentration >3000 mg / L) is added to the filtrate to form calcium oxalate precipitate, and crude calcium oxalate is obtained by solid-liquid separation.

[0053] Acid washing purification: Dilute crude calcium oxalate with deionized water at a ratio of 1:1, adjust the pH to 4-4.5 with 3-5 mol / L oxalic acid, dissolve non-calcium oxalate metal oxides, and separate solid and liquid.

[0054] Alkali washing and neutralization: Dilute the acid-washed calcium oxalate with deionized water at a ratio of 1:1, adjust the pH to 7-7.5 with 3-5 mol / L sodium hydroxide, and obtain high-purity calcium oxalate by solid-liquid separation.

[0055] Drying: High-temperature drying yields high-purity anhydrous calcium oxalate with a content >97%; if the purity does not meet the standard, the acid washing and alkali washing steps can be repeated.

[0056] High-efficiency resource recycling: High-purity calcium oxalate is prepared from fly ash from municipal solid waste incineration, realizing "waste treatment of waste". The purity of calcium oxalate can reach more than 95%, which can be widely used in ceramic glazing, chemical raw materials and other fields.

[0057] Cost reduction: Compared with the traditional sodium carbonate decalcification process, this process reduces the amount of sodium carbonate used by about 80%, saving 240 yuan per ton of fly ash. The economic benefits of large-scale application are significant.

[0058] The process is stable and reliable: the entire process involves continuous feeding and discharging, has strong adaptability, can process fly ash with different calcium contents, and meets the requirements of the "Technical Specification for Pollution Control of Fly Ash from Municipal Solid Waste Incineration (Trial)" (HJ 1134-2020) (dioxins <50 ng-TEQ / kg, chlorine content <1%).

[0059] This invention utilizes fly ash from municipal solid waste incineration to prepare widely applicable high-purity anhydrous calcium oxalate. The fly ash undergoes dioxin detoxification, water washing, heavy metal removal, impurity removal, rinsing, and pH adjustment to achieve the goal of producing high-purity calcium oxalate. The high-purity anhydrous calcium oxalate product prepared by this process has a calcium oxalate content of over 95%, has wide industrial applications, and is a widely available and reliable process for preparing and recycling high-purity calcium oxalate from municipal solid waste incineration fly ash.

[0060] Traditional processes rely on manual adjustment for acid washing (pH 4-4.5) and alkali washing (pH 7-7.5), which is prone to purity instability due to pH fluctuations (requiring repeated purification) and result in high reagent consumption. To address this issue, this invention introduces an online pH sensor and PLC automatic control system to monitor the pH of the reaction system in real time. Oxalic acid / sodium hydroxide solution is precisely added via a peristaltic pump, achieving a control accuracy of ±0.05 pH units. A pH adjustment model is established based on fly ash calcium content fluctuation data (e.g., automatically reducing oxalic acid dosage by 0.5 mol / L when calcium ion concentration >3500 mg / L) to avoid over-adjustment. The purity of a single purification cycle is increased to over 97%, the re-purification rate is reduced by 60%, and reagent consumption is reduced by 15%-20%.

[0061] Traditional sodium sulfide + iron salt processes are inefficient at removing heavy metals such as arsenic and cadmium (approximately 85%), and excessive sulfide ions can easily produce an unpleasant hydrogen sulfide odor. To address this issue, this invention develops a composite heavy metal removal agent that combines sodium sulfide (20-100 mg / L) with nano-zero-valent iron (nZVI, 5-10 mg / L). The strong reducing and adsorption properties of nZVI enhance the removal of arsenic and cadmium, while the Fe loaded on the nZVI surface further enhances the removal. 2+ / Fe 3+ Excess sulfur ions are catalytically decomposed. An air stripping-adsorption combined unit is added: an activated carbon adsorption tower is installed at the top of the heavy metal reaction tank to collect trace amounts of escaping hydrogen sulfide and achieve harmless treatment. The total removal rate of heavy metals is increased to over 99%, the residual sulfur ion content is <0.1 mg / L, and odor emissions are reduced by 90%.

[0062] Direct discharge or simple evaporation treatment of rinsing wastewater (containing chloride salts) is energy-intensive and does not recover resources such as sodium chloride and potassium chloride. To solve this problem, this invention uses a triple-effect falling film evaporation system to treat rinsing wastewater, utilizing waste heat from steam for staged pressure reduction evaporation, achieving energy savings of 40%~50% compared to single-effect evaporation. The evaporation condensate is deeply desalinated by ion exchange resin (conductivity <10 μS / cm) and then reused in the rinsing process, achieving a water recycling rate of >80%. The mixed salt (NaCl / KCl) obtained from evaporation crystallization is separated by a low-temperature stepwise crystallization method: KCl (potassium oxide purity >62%) is preferentially precipitated at a controlled temperature of 0~5℃, and the mother liquor is further evaporated to obtain industrial-grade NaCl (purity >97%).

[0063] Expected results: 3-5 tons of water will be saved per ton of fly ash, and approximately 2,000 tons of KCl / NaCl will be recovered annually (based on a daily processing capacity of 250 tons), resulting in an additional economic benefit of approximately 8 million yuan per year.

[0064] The coarse calcium oxalate crystals have uneven particle size (20~100 μm), which easily clogs the filter cloth during plate and frame filtration, resulting in a filtration cycle of 2~3 hours. To solve this problem, this invention introduces a crystal form regulator (such as sodium dodecyl sulfate, 0.1~0.5 mg / L) during the sodium oxalate addition stage to induce calcium oxalate to form a regular hexagonal crystal form (average particle size 50~80 μm), thereby reducing filter cake resistance.

[0065] By using a diaphragm press plate and frame filter press, combined with a three-step method of "low-pressure dehydration - high-pressure pressing - gas backflushing", the filtration cycle is shortened to 30-40 minutes, and the moisture content of the filter cake is reduced from 35% to below 28%.

[0066] Expected results: Filtration efficiency increased by 300%, drying energy consumption reduced by 15%, and filter cloth replacement frequency reduced by 50%.

[0067] High-chlorine fly ash (Cl - When fly ash containing more than 20% of calcium ions directly enters the rinsing process, chloride ions inhibit the dissolution of calcium ions, resulting in a dissolution rate of only 60% to 70%. To solve this problem, this invention adds a low-temperature baking pretreatment (200 to 250°C, inert atmosphere): this converts soluble chloride salts (CaCl2, NaCl) in fly ash into easily soluble forms, while preventing the regeneration of dioxins.

[0068] Ultrasonic-assisted countercurrent rinsing is adopted: 20 kHz ultrasound is introduced into the three-stage rinsing tank to destroy the agglomeration structure of fly ash particles through cavitation effect, and the calcium ion dissolution rate is increased to more than 90%.

[0069] Expected results: The calcium ion recovery rate of high-chlorine fly ash will increase from 70% to 92%, the calcium oxalate production will increase by 30%, and the chlorine content will meet the requirement of <1% in HJ 1134-2020.

[0070] The energy consumption for dioxin cracking (requiring 800~1000℃) and drying (requiring 200~300℃) accounts for more than 60% of the total energy consumption, resulting in high carbon emission intensity. To address this issue, this invention utilizes waste heat recovery from the cracking furnace flue gas: 0.8 MPa saturated steam is generated through a waste heat boiler and used to heat a triple-effect evaporation system, replacing purchased steam.

[0071] Heat recovery from drying exhaust gas: Heat pipe heat exchangers are used to recover the waste heat of drying exhaust gas (150~200℃) and preheat the air entering the drying system, thereby reducing natural gas consumption.

[0072] Photovoltaic-energy storage complementary power supply: A 500 kW photovoltaic power station is installed in the factory area, combined with a lithium battery energy storage system to meet the power demand of process auxiliary equipment (such as pumps and fans) and reduce grid power consumption.

[0073] Expected results: Total energy consumption will be reduced by 25% to 30%, and carbon emissions per ton of fly ash will be reduced from 300 kg CO2 to below 200 kg CO2, resulting in an annual reduction of approximately 7,500 tons of carbon emissions (based on a processing capacity of 250 tons / day).

[0074] This invention enables the process to be upgraded from "meeting emission standards" to "high-efficiency resource utilization + low-carbon operation":

[0075] Increased purity: Single purification achieves a purity of over 97%, increasing the added value of the product;

[0076] Cost reduction: The overall cost per ton of fly ash is reduced by 300 yuan (chemicals + energy consumption + water consumption), resulting in annual cost savings of approximately 27 million yuan;

[0077] Resource recycling: Simultaneous recovery of by-products such as KCl and NaCl, generating an additional annual revenue of over 15 million yuan;

[0078] Environmental upgrade: Heavy metals, odors, and carbon emissions all meet standards, satisfying the "dual carbon" policy requirements. The improved process is suitable for complex fly ash characteristics such as high chlorine and high calcium content, and can be promoted to fly ash resource utilization scenarios in municipal solid waste incineration plants nationwide.

[0079] Example 1

[0080] A low-temperature pyrolysis process for municipal solid waste fly ash includes the following steps:

[0081] The fly ash from waste incineration first passes through a dioxin pyrolysis unit to ensure the dioxin content is less than 50 ng-TEQ / kg. It then enters a three-stage countercurrent rinsing process to dissolve as much sulfate as possible from the fly ash into the liquid phase. The first-stage filtrate is treated by adding trace amounts of ferrous chloride and sodium sulfide to remove heavy metal ions. The filtrate after plate and frame filtration enters the subsequent treatment system. After adding a certain concentration of anhydrous sodium oxalate and thoroughly stirring, solid-liquid separation is performed using plate and frame filtration. The supernatant is returned to the fly ash washing process. The solids are mixed with deionized water, and the pH is adjusted to 4-4.5 with oxalic acid. Solid-liquid separation is then performed using plate and frame filtration. The solids after plate and frame filtration are washed with deionized water, and the supernatant is adjusted to pH 7-7.5 with sodium hydroxide and then separated using a plate and frame filter press. The separated solids are high-purity calcium oxalate, which is subsequently dried to obtain high-purity anhydrous calcium oxalate.

[0082] Example 2

[0083] For fly ash from a listed company's self-owned incineration plant in a certain province, the calcium ion content in the fly ash water washing was 18000 mg / L. After treatment using this process, 50 mg / L of sodium sulfide, 250 mg / L of ferrous chloride, 70 g / L of anhydrous sodium oxalate, oxalic acid concentration of 3.5 mol / L, pH adjusted to 4.35, and sodium hydroxide concentration of 3.5 mol / L adjusted to pH=7 were added. The resulting calcium oxalate crystals had a purity of 98.2%.

[0084] Example 3

[0085] For fly ash from a self-owned incineration plant in a certain province, the calcium ion content in the fly ash water washing was 19200 mg / L. After treatment using this process, 60 mg / L of sodium sulfide, 250 mg / L of ferrous chloride, 85 g / L of anhydrous sulfuric acid, 3.5 mol / L of oxalic acid, pH adjusted to 4.35, and 3.5 mol / L of sodium hydroxide, adjusted to pH=7, the purity of the obtained calcium oxalate crystals reached 98.5%.

[0086] Example 4

[0087] A process for recovering high-purity calcium oxalate from municipal solid waste incineration fly ash has been invented, with a processing capacity of 250 tons of fly ash per day. This process reduces the amount of sodium carbonate required for calcium removal from 150 kg per ton of fly ash to 30 kg per ton of fly ash through sodium oxalate co-precipitation. This saves 120 kg of calcium carbonate per ton of fly ash. With calcium carbonate priced at 2000 yuan per ton, this translates to a cost saving of 240 yuan per ton of ash, or 60,000 yuan per day. Based on 330 days per year, this translates to a cost saving of 19.8 million yuan, demonstrating significant economic benefits. Simultaneously, it recovers high-purity anhydrous calcium oxalate, which has a wide range of applications.

[0088] Although embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the above embodiments.

Claims

1. A recovery process for the preparation of high purity calcium oxalate from waste incineration fly ash, characterized in that, Includes the following steps: a. Perform dioxin detoxification treatment on fly ash from municipal solid waste incineration to ensure that the dioxin content in the treated fly ash is less than a predetermined threshold. b. Perform multi-stage countercurrent rinsing on the detoxified fly ash to dissolve calcium ions in the fly ash into the liquid phase; c. Perform heavy metal removal treatment on the liquid phase obtained in step b; The heavy metal removal treatment includes the addition of sulfides and iron salts, wherein the concentration of the sulfides is 20–100 mg / L and the concentration of the iron salts is 50–350 mg / L. d. Add oxalate to the liquid phase after heavy metal removal treatment to generate calcium oxalate precipitate, and then separate the solid and liquid phases to obtain crude calcium oxalate; e. Acid washing purification: Dilute crude calcium oxalate with deionized water at a ratio of 1:1, adjust the pH to 4-4.5 with 3-5 mol / L oxalic acid, dissolve non-calcium oxalate metal oxides, and separate solid and liquid. f. Alkali washing and neutralization: Dilute the acid-washed calcium oxalate with deionized water at a ratio of 1:1, adjust the pH to 7-7.5 with 3-5 mol / L sodium hydroxide, and obtain high-purity calcium oxalate by solid-liquid separation; g. Dry the high-purity calcium oxalate to obtain high-purity anhydrous calcium oxalate.

2. The process according to claim 1, characterized in that, In step a, the predetermined threshold is 50 ng-TEQ / kg.

3. The process of claim 1, wherein, In step b, the multi-stage countercurrent rinsing is a three-stage countercurrent rinsing, and the pH is not adjusted during the rinsing process.

4. The process of claim 1, wherein, The sulfide is sodium sulfide, and the iron salt is ferrous chloride heptahydrate or ferrous sulfate heptahydrate.

5. The process of claim 1, wherein, In step d, the oxalate is anhydrous sodium oxalate, and the concentration of residual calcium ions in the liquid phase at the time of addition is greater than 3000 mg / L.

6. The process of claim 1, wherein, In step g, the drying is high-temperature drying, and the resulting high-purity anhydrous calcium oxalate content is higher than 97%; if the purity does not meet the standard, repeat steps e to f.

7. The process of claim 1, wherein, The chlorine content of the fly ash after treatment is less than 1%.

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