Process for separating and recovering calcium hydroxide from waste incineration fly ash

Abstract

The present application relates to waste incineration technical field, specifically to a kind of calcium hydroxide separation and recovery process in waste incineration fly ash, comprising the following steps: (1) mechanical activation pretreatment;(2) homogenization reaction;(3) low-temperature drying;(4) thermochemical conversion;(5) selective solvent extraction;(6) solvent recovery;(7) digestion reaction;(8) product preparation.This process realizes the efficient separation of calcium hydroxide and calcium chloride by the double separation mechanism of thermochemical conversion and selective solvent extraction, and the recovery rate of calcium hydroxide is more than 85%;Selective extraction is carried out using ethanol-water mixed solvent, and the solvent can be recycled by distillation, and the amount of wastewater generated is reduced by more than 80%;Through the double separation mechanism of thermochemical conversion and selective solvent extraction, chlorine ions and part of heavy metals are removed, and the purity of the product is more than 90%;The recovered calcium hydroxide can be used as flue gas desulfurizer, wastewater treatment agent, etc., to realize the resource utilization of hazardous waste.

Description

Technical Field

[0001] This invention relates to the field of waste incineration technology, specifically to a process for separating and recovering calcium hydroxide from fly ash in waste incineration. Background Technology

[0002] Fly ash from waste incineration is fine particulate matter collected by the flue gas purification system during the incineration of municipal solid waste, and is classified as hazardous waste (HW18). According to the test report, the calcium content in the fly ash is as high as 25.8%. 40.1% (28.8% as CaO) 38.0%, mainly from the slaked lime injected during flue gas desulfurization. The fly ash has a high chlorine content (12.79%). (21.07%), mainly existing in the form of soluble chloride salts, including NaCl, KCl, CaCl2, etc.

[0003] Calcium compounds in fly ash exist in complex forms, mainly including: ① Calcium hydroxide Ca(OH)2: derived from slaked lime injected during flue gas desulfurization, it is one of the main forms of calcium in fly ash; ② Calcium chloride CaCl2: generated by the reaction of slaked lime with HCl in flue gas, it is easily soluble in water; ③ Calcium hydroxide CaClOH: generated by the reaction of Ca(OH)2 and CaCl2 at room temperature, it is an important intermediate product in fly ash; ④ Calcium sulfate CaSO4: generated by the reaction of slaked lime with SO2 in flue gas; ⑤ Calcium carbonate CaCO3: generated by the reaction of calcium hydroxide with CO2.

[0004] Currently, domestic and international technologies for the separation and recovery of calcium compounds from fly ash mainly fall into the following categories: 1. Water washing method: Water washing is the most common method for fly ash pretreatment. However, this method suffers from calcium hydroxide dissolution and loss, with a recovery rate of only 30%. 50%. 2. Acid leaching: Acid leaching can dissolve most of the metallic compounds in fly ash, including calcium compounds. However, acid leaching is non-selective, and all calcium compounds will dissolve, making it impossible to directly recover calcium hydroxide. Furthermore, it consumes a large amount of acid, is costly, and generates acidic wastewater that requires treatment. 3. Carbonation method: This method utilizes the reaction of CO2 with calcium hydroxide in fly ash to produce calcium carbonate. The product is calcium carbonate, not calcium hydroxide, and calcium hydroxide cannot be directly recovered. 4. Physical separation methods: These include particle size classification, gravity separation, magnetic separation, and flotation. Physical separation can be used as a pretreatment method, but it has low separation efficiency and poor product purity.

[0005] 5. Thermal Treatment Method: Current research on the thermal treatment of fly ash both domestically and internationally (such as ultrafast carbothermal processes and melting processes) focuses on "destructive removal"—that is, completely volatilizing or destroying heavy metals and dioxins through extremely high temperatures (above 1000℃). Under this technological guidance, the research approach of those skilled in the art is that "the higher the temperature, the better the removal effect; the shorter the time, the lower the energy consumption." This pursuit of high-temperature destruction completely ignores the directional transformation logic of calcium compounds in fly ash. At high temperatures, the target product Ca(OH)2 will inevitably be sintered or decomposed; therefore, existing thermal treatment technologies cannot achieve the selective recovery of calcium hydroxide.

[0006] Currently, the aforementioned existing technologies still have some shortcomings, mainly in the following aspects: (1) Main disadvantages of water washing method: ① Loss of calcium hydroxide due to dissolution: Ca(OH)2 has a certain solubility in water (0.16 g / 100g water at 20℃). During the water washing process, some calcium hydroxide will dissolve into the aqueous phase, resulting in a decrease in recovery rate. According to literature reports, the recovery rate of calcium hydroxide by water washing method is only 30%-50%. ② Large amount of wastewater generated: Water washing requires a large amount of water for washing, and the liquid-solid ratio is usually 3-10:1, generating a large amount of saline wastewater that needs to be treated, increasing treatment costs and environmental risks. ③ Low product purity: The calcium hydroxide product recovered by water washing often contains impurities such as chloride ions and heavy metals, resulting in low product purity and limited application.

[0007] (2) The main disadvantages of acid leaching: ① Lack of selectivity: Acid leaching dissolves all calcium compounds in fly ash, making it impossible to selectively recover calcium hydroxide, and the product needs further purification. ② High acid consumption and cost: Acid leaching requires a large amount of acid, resulting in high operating costs. ③ Generation of acidic wastewater: The acidic wastewater generated by acid leaching needs to be neutralized, increasing treatment costs.

[0008] (3) The main disadvantages of the carbonation method: ① The product is calcium carbonate instead of calcium hydroxide: The product of the carbonation method is calcium carbonate, which cannot be directly recovered as calcium hydroxide. Further calcination and digestion are required to obtain calcium hydroxide. ② Incomplete reaction: The calcium carbonate produced during the carbonation process will coat the fly ash particles, hindering the reaction. The actual conversion rate is lower than the theoretical value.

[0009] (4) The main disadvantages of physical sorting: ① Low separation efficiency: Fly ash particles are small and the physical properties of different components are small, resulting in low physical sorting efficiency. ② Poor product purity: Physical sorting cannot achieve complete separation of calcium compounds from other components, resulting in poor product purity. Summary of the Invention

[0010] In view of the above-mentioned shortcomings and deficiencies of the existing technology, the present invention provides a process for separating and recovering calcium hydroxide from waste incineration fly ash.

[0011] Specifically, the technical solution of the present invention is as follows: A process for separating and recovering calcium hydroxide from fly ash from waste incineration includes the following steps: (1) Mechanical activation pretreatment The fly ash was ball-milled for 30-60 minutes at a speed of 200-400 rpm. Mechanical activation increases the specific surface area of ​​the fly ash particles, promotes the reaction of Ca(OH)₂ and CaCl₂, and increases the formation rate of CaClOH. Simultaneously, mechanical activation homogenizes the fly ash particles, which is beneficial for subsequent heat treatment and solvent extraction.

[0012] (2) Homogenization reaction After ball milling, the fly ash is left at room temperature for 24-48 hours to allow Ca(OH)₂ and CaCl₂ to react fully to form CaClOH; the reaction equation is: Ca(OH)₂ + CaCl₂ → 2CaClOH. This homogenization reaction makes the calcium compounds in the fly ash more uniform in form, creating conditions for subsequent thermochemical transformation.

[0013] (3) Low temperature drying The homogenized fly ash is dried at 100-150℃ for 2-4 hours to remove free water and some of the water of crystallization. Low-temperature drying can avoid premature decomposition of CaClOH and reduce the energy consumption of subsequent heat treatment.

[0014] (4) Thermochemical transformation The dried fly ash is heat-treated at 550-580℃ for 30-60 minutes to decompose CaClOH into CaO and CaCl2; the reaction equation is: 2CaClOH → CaO + CaCl2 + H2O. Controlling the heat treatment temperature within the range of 550-580℃ ensures complete decomposition of CaClOH while preventing the decomposition of Ca(OH)2. The HCl gas generated during the heat treatment process can be collected and treated through a flue gas purification system.

[0015] (5) Selective solvent extraction Heat-treated fly ash is mixed with an ethanol-water mixture (ethanol concentration of 60%-80%) at a liquid-to-solid ratio of 5-10:1 and stirred at room temperature for 30-60 minutes to allow CaCl2 to selectively dissolve into the liquid phase. Then, solid-liquid separation is performed using a filter press to obtain an extract containing CaCl2 and a solid residue containing CaO.

[0016] (6) Solvent recovery The extract is distilled to recover the ethanol solvent for recycling; the distillation residue is a CaCl2 solution, which can be further evaporated and crystallized to obtain calcium chloride product.

[0017] (7) Digestion reaction The solid residue is mixed with water at a water-cement ratio of 3-4:1 and stirred at 60-80℃ for 1-2 hours to digest CaO and generate Ca(OH)2. The reaction equation is: CaO + H2O → Ca(OH)2. After the digestion reaction, solid-liquid separation is performed by a filter press to obtain calcium hydroxide slurry.

[0018] (8) Product preparation Calcium hydroxide slurry is spray-dried or oven-dried to obtain calcium hydroxide product with a purity of over 90%, which can be used as a flue gas desulfurization agent, wastewater treatment agent, etc.

[0019] The specific technical principles involved in the calcium hydroxide separation and recovery process from waste incineration fly ash of this invention include: (1) Thermochemical conversion principle: Ca(OH)2 and CaCl2 in fly ash react to form CaClOH at room temperature. CaClOH decomposes into CaO and CaCl2 at 550-580℃, while Ca(OH)2 decomposes into CaO above 580℃. By controlling the heat treatment temperature within the range of 550-580℃, the complete decomposition of CaClOH can be achieved, while the decomposition of Ca(OH)2 is avoided, thereby converting the calcium compounds in fly ash into two forms: CaO and CaCl2.

[0020] (2) Selective solvent extraction principle: CaCl2 is readily soluble in ethanol-water mixed solvent, while CaO, Ca(OH)2, CaCO3, and CaSO4 have extremely low solubility in ethanol-water mixed solvent. By adjusting the ethanol concentration (60%-80%), selective dissolution of CaCl2 can be achieved, while insoluble substances such as CaO remain in the solid phase.

[0021] (3) Digestion reaction principle: CaO in the solid phase reacts with water to produce Ca(OH)2. The reaction equation is: CaO + H2O → Ca(OH)2. By controlling the digestion conditions, high-purity calcium hydroxide products can be obtained.

[0022] Compared with existing technologies, the calcium hydroxide separation and recovery process in fly ash from waste incineration of this invention solves at least the following technical problems: (1) Addressing the challenge of separating calcium hydroxide and calcium chloride: Waste incineration fly ash contains a large amount of slaked lime (Ca(OH)2) and calcium chloride (CaCl2). Although the solubility of the two in water differs significantly, traditional water washing processes result in the partial dissolution and loss of calcium hydroxide, with a recovery rate of only 30%. 50%. The process of this invention converts Ca(OH)₂ and CaCl₂ into CaO and CaCl₂ through thermochemical conversion, utilizing both in ethanol. Selective separation can be achieved by utilizing differences in solubility in water-based mixed solvents, which can increase the recovery rate of calcium hydroxide to over 85%.

[0023] (2) Regarding the problem of large wastewater generation: Traditional water washing processes require a large amount of water for washing, and the liquid-to-solid ratio is usually 3. A ratio of 10:1 results in a large amount of saline wastewater that requires treatment. The process of this invention uses ethanol. Selective extraction is performed using a water-mixed solvent, and the solvent can be recovered and recycled through distillation, reducing wastewater generation by more than 80%. (3) Regarding the problem of low product purity: Calcium hydroxide products recovered by traditional methods often contain impurities such as chloride ions and heavy metals, resulting in low product purity and limited applications. The process of this invention can effectively remove chloride ions and some heavy metals through a dual separation mechanism of thermochemical conversion and selective solvent extraction, achieving a product purity of over 90%. (4) Addressing the challenge of controlling the transformation of calcium compounds: Calcium compounds in fly ash exhibit complex transformation relationships. Ca(OH)2 and CaCl2 react at room temperature to form CaClOH (calcium oxychloride), which makes separation difficult. The process of this invention utilizes the thermal decomposition characteristics of CaClOH to achieve the directional transformation of calcium compounds by controlling the heat treatment temperature, thus creating conditions for selective separation.

[0024] The calcium hydroxide separation and recovery process in fly ash from waste incineration of the present invention has at least the following beneficial effects: (1) High selectivity: A dual separation mechanism of thermochemical conversion and selective solvent extraction achieves efficient separation of calcium hydroxide and calcium chloride. Compared with the traditional water washing method, the recovery rate of calcium hydroxide increases from 30%-50% to over 85%.

[0025] (2) Low wastewater generation: Selective extraction is performed using an ethanol-water mixed solvent, and the solvent can be recovered and recycled through distillation. Compared with the traditional water washing method, the amount of wastewater generated is reduced by more than 80%.

[0026] (3) High product purity: Through a dual separation mechanism of thermochemical conversion and selective solvent extraction, chloride ions and some heavy metals are effectively removed. Compared with traditional methods, the product purity is increased from 60%-70% to over 90%.

[0027] (4) Resource utilization: The recovered calcium hydroxide can be used as a flue gas desulfurization agent, wastewater treatment agent, etc., and the recovered calcium chloride can be used as an industrial raw material, realizing the resource utilization of hazardous waste.

[0028] (5) Controllable operating costs: Although this process requires heat treatment and solvent recovery equipment, the solvent can be recycled, and the main consumption is energy cost. Compared with the traditional acid leaching method, it does not require a large amount of acid and the operating cost is lower.

[0029] (6) High product value: The calcium hydroxide product recovered by this process has high purity and can be used as an industrial raw material. The product value is higher than that of the low-purity product recovered by traditional methods.

[0030] (7) Cost savings in disposal: After treatment, fly ash is reduced in volume and weight, thus reducing the cost of hazardous waste disposal. Attached Figure Description

[0031] The accompanying drawings in this application are intended to supplement the textual description in the specification with graphics, and to further explain the technical solution of this application. They do not constitute an undue limitation on this application.

[0032] Figure 1 This is a flow chart of the process for separating and recovering calcium hydroxide from fly ash from waste incineration according to the present invention.

[0033] Figure 2 This is a technical principle block diagram of the key steps in the process of separating and recovering calcium hydroxide from fly ash of waste incineration according to the present invention.

[0034] Figure 3 The temperature-time sequence diagram is shown. The entire process consists of five stages: (1) mechanical activation stage (0-30 min, room temperature); (2) drying stage (30-90 min, heating to 100-150℃); (3) heating stage (90-150 min, heating to 550℃); (4) thermal decomposition stage (150-210 min, maintaining 550-580℃); (5) cooling stage (210-300 min, cooling to room temperature). Among them, the thermal decomposition stage is the key step, which requires precise temperature control within the range of 550-580℃ to achieve complete decomposition of CaClOH while avoiding the decomposition of Ca(OH)2.

[0035] Figure 4 This is a solubility comparison graph. Different calcium compounds exhibit different solubilities in pure water and a 70% ethanol-water mixture. The graph shows that CaCl2 has high solubility in both solvents (74.5 g / 100 g in pure water, 45.2 g / 100 g in 70% ethanol-water), while Ca(OH)2, CaCO3, CaSO4, and CaO have very low solubility in both solvents (<0.2 g / 100 g). This solubility difference provides a theoretical basis for selective solvent extraction. By using an ethanol-water mixture, CaCl2 can be selectively dissolved, while insoluble substances such as CaO remain in the solid phase. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0037] like Figure 1-4As shown, a process for separating and recovering calcium hydroxide from waste incineration fly ash includes the following steps: (1) Mechanical activation pretreatment The fly ash was ball-milled for 30-60 minutes at a speed of 200-400 rpm. Mechanical activation increases the specific surface area of ​​the fly ash particles, promotes the reaction of Ca(OH)₂ and CaCl₂, and increases the formation rate of CaClOH. Simultaneously, mechanical activation homogenizes the fly ash particles, which is beneficial for subsequent heat treatment and solvent extraction.

[0038] (2) Homogenization reaction After ball milling, the fly ash is left at room temperature for 24-48 hours to allow Ca(OH)₂ and CaCl₂ to react fully to form CaClOH; the reaction equation is: Ca(OH)₂ + CaCl₂ → 2CaClOH. This homogenization reaction makes the calcium compounds in the fly ash more uniform in form, creating conditions for subsequent thermochemical transformation.

[0039] (3) Low temperature drying The homogenized fly ash is dried at 100-150℃ for 2-4 hours to remove free water and some of the water of crystallization. Low-temperature drying can avoid premature decomposition of CaClOH and reduce the energy consumption of subsequent heat treatment.

[0040] (4) Thermochemical transformation The dried fly ash is heat-treated at 550-580℃ for 30-60 minutes to decompose CaClOH into CaO and CaCl2; the reaction equation is: 2CaClOH → CaO + CaCl2 + H2O. Controlling the heat treatment temperature within the range of 550-580℃ ensures complete decomposition of CaClOH while preventing the decomposition of Ca(OH)2. The HCl gas generated during the heat treatment process can be collected and treated through a flue gas purification system.

[0041] The temperature range of 550-580℃ and the processing time of 30-60 minutes are narrow process windows discovered through extensive and meticulous experimental research to achieve the specific goal of "selective separation of calcium hydroxide." ① In order to recover calcium hydroxide, the present invention must strictly control the temperature to prevent its decomposition, which requires the reaction system to be carried out in a temperature range much lower than that of traditional heat treatment.

[0042] ②) 550-580℃ is an extremely narrow equilibrium window: This invention requires the complete decomposition of CaClOH while Ca(OH)2 must not decompose. According to thermodynamic properties, the decomposition temperature of CaClOH is approximately 550℃. If the temperature is below this, CaClOH decomposes incompletely, and during subsequent solvent extraction, chlorine remains encapsulated in the form of CaClOH, making effective elution impossible (product purity is only about 70%). Furthermore, Ca(OH)2 begins to significantly decompose into CaO above 580℃. This narrow window of only 30℃ (550-580℃) represents a critical equilibrium achieved between the mutually restrictive conditions of "complete dechlorination" and "retention of calcium hydroxide."

[0043] ③ 30-60 minutes is a specific kinetic requirement for solid-state lattice reconstruction: Existing ultrafast carbothermal processes utilize flash evaporation in 0.03 seconds, the logic of which is to allow the material to instantly vaporize and leave the system. The reaction of this invention (2CaClOH → CaO + CaCl2 + H2O) is a solid-state decomposition and lattice reconstruction reaction. Fly ash is a complex multiphase mixture with low thermal conductivity. If the reaction time is too short (<30 minutes), thermal equilibrium cannot be reached inside the particles, and the internal CaClOH will not decompose, leading to incomplete reaction. If the reaction time is too long (>60 minutes), it will cause severe sintering of the fly ash particles, and may even promote the solid-state reaction of unreacted SiO2, Al2O3, and CaO to form inert silicates / aluminates, resulting in a sharp decrease in the activity of subsequent digestion reactions. 30-60 minutes is the optimal kinetic time to ensure consistent heat transfer inside and outside the solid-state system, complete reaction, and avoid over-sintering.

[0044] Table 1. Effect of heat treatment temperature on the decomposition rate of CaClOH and the recovery rate of calcium hydroxide (treatment time: 45 min) Table 2. Effect of heat treatment time on fly ash lattice reconstruction and product activity (treatment temperature: 560℃) (5) Selective solvent extraction Heat-treated fly ash is mixed with an ethanol-water mixture (ethanol concentration of 60%-80%) at a liquid-to-solid ratio of 5-10:1 and stirred at room temperature for 30-60 minutes to allow CaCl2 to selectively dissolve into the liquid phase. Then, solid-liquid separation is performed using a filter press to obtain an extract containing CaCl2 and a solid residue containing CaO.

[0045] The experimental verification and mechanism explanation of the key solvent extraction parameters are as follows: This step uses a 60%-80% ethanol-water mixed solvent with a liquid-to-solid ratio of 5-10:1 to extract the heat-treated fly ash. While this may seem similar to the existing technology (CN120247075A) which uses anhydrous ethanol to extract a "pure mixed salt," there is a fundamental difference. The parameters of this invention are specifically designed for the "complex multiphase fly ash solid phase containing unreacted CaO / Ca(OH)2," and were discovered through meticulous research, revealing a "selective dissolution window." Those skilled in the art, limited by the bias of existing "anhydrous extraction" techniques, would find it difficult to conceive of the parameter combination of this invention.

[0046] ① The fundamental difference between the object being processed and the purpose of extraction can easily lead to technical bias. The existing technology (anhydrous extraction) suffers from a technical bias: CN120247075A treats a pure mixed salt (CaCl2, NaCl, KCl) obtained after evaporating fly ash leachate, which does not contain CaO and Ca(OH)2. Its core logic is to use anhydrous ethanol to dissolve CaCl2 (and its water of crystallization) from NaCl / KCl. Under the guidance of this technology, those skilled in the art would instinctively believe that water is an impurity, and that extremely high-purity anhydrous ethanol must be used to prevent the introduction of moisture that could lead to the dissolution of NaCl / KCl or the caking of CaCl2.

[0047] The specific processing target of this invention is the solid phase of fly ash after heat treatment. This solid phase contains not only the target extract, CaCl2, but also a large amount of unreacted CaO, Ca(OH)2, and inert substances such as CaCO3 and CaSO4. If existing technology uses anhydrous ethanol, its extremely low polarity prevents it from wetting the porous fly ash particles, resulting in the inability of CaCl2 trapped within the fly ash pores to dissolve, leading to a very low extraction rate. Furthermore, if too much water is added, Ca(OH)2 and CaO will dissolve into the liquid phase, causing subsequent loss of calcium hydroxide product and a decrease in purity. Therefore, it is essential to find an extremely precise solvent polarity window.

[0048] ② Experimental verification of "precise polarity control" of ethanol concentration (60%-80%) To demonstrate the irreplaceability of 60%-80% concentration, this invention conducted extraction experiments with different ethanol concentrations on the same batch of heat-treated fly ash (containing approximately 25% CaCl2 and approximately 50% CaO / Ca(OH)2) under a liquid-to-solid ratio of 8:1: Table 3 The 60%-80% concentration specified in this invention is key to overcoming the "anhydrous" bias of existing technologies. An appropriate amount of water (20%-40%) acts as a "wetting agent" and "polarity modifier," breaking down the surface tension of fly ash particles and allowing the solvent to penetrate deep into the porous framework to thoroughly wash out CaCl2. Simultaneously, a sufficient proportion of ethanol lowers the dielectric constant of the system to an extremely low level, inhibiting the dissociation and dissolution of Ca(OH)2. This "precise polarity window" is the result of specialized research on complex fly ash solid phases; existing technologies for treating pure salts do not require consideration of this issue.

[0049] ③ Experimental verification of solid-liquid mass transfer at liquid-to-solid ratios (5-10:1) Existing technologies target pure mixed salts that have been ground into powder, where dissolution and mass transfer are extremely easy. However, this invention deals with fly ash particles that may slightly agglomerate after heat treatment at 550-580℃, resulting in high mass transfer resistance due to their internal pores. A liquid-to-solid ratio that is too low cannot displace the internal CaCl2, while a ratio that is too high increases the energy consumption for solvent recovery.

[0050] Table 4 The technical logic of CN120247075A is "water removal and purification" (purifying calcium chloride from pure salt using anhydrous ethanol). Those skilled in the art, limited by this approach, are prone to fall into the misconception that "water must be strictly prevented from entering" when encountering fly ash dechlorination problems, resulting in the inability of CaCl2 to dissolve from the fly ash interior (as shown in the table above, the anhydrous ethanol extraction rate is only 45.2%). This invention takes the opposite approach, creatively introducing 20%-40% water to form a mixed solvent, and using a mass transfer liquid-solid ratio of 5-10:1 to achieve precise control of "wetting and breaking down cell walls with water, and inhibiting dissolution with ethanol." This is a creative achievement that can only be obtained through in-depth research on the solid phase of specific fly ash.

[0051] (6) Solvent recovery The extract is distilled to recover the ethanol solvent for recycling; the distillation residue is a CaCl2 solution, which can be further evaporated and crystallized to obtain calcium chloride product.

[0052] (7) Digestion reaction The solid residue is mixed with water at a water-cement ratio of 3-4:1 and stirred at 60-80℃ for 1-2 hours to digest CaO into Ca(OH)2; the reaction equation is: CaO + H2O → Ca(OH)2. After the digestion reaction, solid-liquid separation is performed using a filter press to obtain calcium hydroxide slurry.

[0053] (8) Product preparation Calcium hydroxide slurry is spray-dried or oven-dried to obtain calcium hydroxide product with a purity of over 90%, which can be used as a flue gas desulfurization agent, wastewater treatment agent, etc.

[0054] Application Examples I. Project Overview Object to be processed: Fly ash from a municipal solid waste incineration plant Processing capacity: 1 ton II. Process Steps and Technical Parameters (1) Mechanical activation pretreatment Raw material: 1000 kg fly ash Equipment: Ball mill; Parameters: Rotation speed 300 rpm, Time 45 min. Function: Increases specific surface area and promotes the transition from Ca(OH)2 + CaCl2 to CaClOH.

[0055] (2) Homogenization reaction Conditions: Room temperature, sealed, and left to stand. Time: 36 hours Objective: To unify the calcium form in fly ash to CaClOH.

[0056] (3) Low temperature drying Temperature: 120℃; Time: 3h.

[0057] Objective: To remove free water and prevent premature decomposition of CaClOH.

[0058] (4) Thermochemical transformation (core) Temperature: 560℃ (controlled at 550–580℃); Time: 45 min.

[0059] Reaction: CaClOH → CaO + CaCl2 Flue gas: HCl collection and treatment (5) Ethanol Water selective extraction Solvent: 70% ethanol + 30% water; Liquid-to-solid ratio: 8:1 Temperature: room temperature; stirring: 45 min; separation: pressure filtration. Results: CaCl2 / NaCl / KCl all entered the liquid phase; CaO / inert substances remained in the solid phase. (6) Solvent distillation recovery Distillation temperature: 80℃ Ethanol recovery rate: 95% Residual liquid: Mixed salt solution (CaCl2 + NaCl + KCl) (7) CaO digestion reaction Water-cement ratio: 3.5:1; temperature: 70℃; time: 1.5 h.

[0060] Reaction: CaO + H₂O → Ca(OH)₂ (8) Solid-liquid separation + drying Filtration: Filter press (to remove SiO2, CaSO4, CaCO3, and heavy metal residues) Drying: Spray drying Product: High-purity calcium hydroxide.

[0061] Comparative experiment 1. Experimental Objective It has been demonstrated that, under the same batch of fly ash and the same solvent system, if the pre-processing steps of "mechanical activation + homogenization + 550℃ pyrolysis reconstruction" of this invention are not performed, and the chlorine present in the form of CaClOH and the chlorine encapsulated therein cannot be effectively removed, resulting in the purity of the final calcium hydroxide product remaining at around 70%. However, after thermochemical reconstruction, the product purity can jump to over 90%.

[0062] 2. Experimental equipment and materials Raw material: 1000 kg of fly ash from the same batch at a municipal solid waste incineration plant.

[0063] Equipment: planetary ball mill, tubular rotary kiln (with tail gas absorption), vacuum filter press, constant temperature water bath, reaction vessel with stirring, spray drying tower, X-ray diffractometer (XRD), scanning electron microscope (SEM-EDS), chloride ion selective electrode, chemical titration apparatus.

[0064] 3. Experimental Procedure (1) Direct ethanol washing method Direct extraction: Take 100 kg of raw fly ash, without any ball milling or heat treatment. Directly add 700 kg of 70% ethanol-water mixed solvent (liquid-solid ratio 7:1).

[0065] Separation: After stirring at room temperature for 45 minutes, the mixture is sent to a filter press for solid-liquid separation.

[0066] Digestion and drying: The solid residue obtained by pressure filtration was added to water (water-cement ratio 3.5:1) and digested at 70°C for 1.5 hours. The digested slurry was filtered again, and the filter cake was sent to a spray drying tower to dry, obtaining the final calcium hydroxide product (denoted as sample A).

[0067] (2) Method of the present invention Mechanical activation and homogenization: Take 100 kg of the same batch of original fly ash, put it into a ball mill (300 rpm, 45 min), and then let it stand at room temperature for 36 h in a sealed container for homogenization.

[0068] Low-temperature drying: Place in a 120℃ oven and dry for 3 hours.

[0069] Thermochemical conversion: Place in a tube furnace and heat-treat at 560°C for 45 minutes (the tail gas is used to absorb HCl with alkaline solution), then cool to room temperature.

[0070] Extraction: Add 700 kg of 70% ethanol-water mixed solvent, stir at room temperature for 45 minutes, and then filter by pressure.

[0071] Digestion and drying: The subsequent digestion, filtration and drying steps were exactly the same as in Experiment 1, and the final calcium hydroxide product was obtained (denoted as Sample B).

[0072] 4. Experimental Data Table 5 Comparison of core technical indicators between Sample A and Sample B Table 6 Comparison of XRD semi-quantitative phase analysis and microstructure The experimental results above demonstrate that the process of this invention achieves the following key technologies: (1) Precise control of heat treatment temperature The heat treatment temperature is the most critical parameter in this technology. The decomposition temperature of CaClOH is 550℃, and that of Ca(OH)2 is 580℃. The heat treatment temperature needs to be precisely controlled within the range of 550-580℃ to achieve complete decomposition of CaClOH while avoiding the decomposition of Ca(OH)2. If the temperature is below 550℃, CaClOH will not decompose completely, affecting the subsequent solvent extraction effect; if the temperature is above 580℃, Ca(OH)2 will decompose significantly, and fly ash is prone to sintering, leading to a sharp decrease in the digestibility of the generated CaO. This not only increases energy consumption but also seriously affects the recovery rate and quality of the final product.

[0073] (2) Optimization of ethanol concentration Ethanol concentration is a key parameter affecting selective extraction efficiency. If the ethanol concentration is too low (<60%), the solubility of CaCl2 is high, but the solubility of Ca(OH)2 also increases, leading to decreased selectivity. If the ethanol concentration is too high (>80%), the solubility of CaCl2 decreases, resulting in lower extraction efficiency. An ethanol concentration within the range of 60%-80% can achieve efficient CaCl2 extraction while maintaining high selectivity.

[0074] (3) The effect of mechanical activation Mechanical activation is a crucial pretreatment step in this process. Ball milling increases the specific surface area of ​​fly ash particles, promoting the reaction between Ca(OH)₂ and CaCl₂ and increasing the formation rate of CaClOH. Simultaneously, mechanical activation homogenizes the fly ash particles, which is beneficial for subsequent heat treatment and solvent extraction. The effectiveness of mechanical activation is influenced by parameters such as ball milling time, rotation speed, and ball-to-particle ratio, and needs to be optimized based on the characteristics of the fly ash.

[0075] (4) Solvent recovery and recycling Solvent recovery and recycling are key to the economic viability of this technology. Recovering ethanol solvent through distillation can significantly reduce operating costs and wastewater generation. Temperature and vacuum levels must be controlled during distillation to ensure complete ethanol recovery and effective separation of CaCl2.

[0076] (5) Behavioral control of heavy metals Fly ash contains a certain amount of heavy metals (Pb, Zn, Cu, Cd, Cr, etc.), and the behavior of these heavy metals needs to be monitored during the separation and recovery process. Most heavy metals will accumulate in the solid phase during heat treatment, while a small amount may volatilize into the gas phase. During solvent extraction, heavy metals are mainly retained in the solid phase, with a small amount potentially dissolving into the liquid phase. By controlling process parameters, it can be ensured that the heavy metal content in the recovered product meets the standards.

[0077] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A process for separating and recovering calcium hydroxide from waste incineration fly ash, characterized by, Includes the following steps: (1) Mechanical activation pretreatment (2) Homogenization reaction After ball milling, the fly ash is left at room temperature for 24-48 hours to allow Ca(OH)2 and CaCl2 to react fully to form CaClOH; the reaction equation is: Ca(OH)2 + CaCl2 → 2CaClOH; (3) Low temperature drying (4) Thermochemical transformation The dried fly ash is heat-treated at 550-580℃ to decompose CaClOH into CaO and CaCl2; the reaction equation is: 2CaClOH → CaO + CaCl2 + H2O. (5) Selective solvent extraction The heat-treated fly ash was mixed with an ethanol-water mixture at a liquid-solid ratio of 5-10:1 and stirred at room temperature to allow CaCl2 to selectively dissolve into the liquid phase. Then, solid-liquid separation was performed using a filter press to obtain an extract containing CaCl2 and a solid residue containing CaO. (6) Solvent recovery (7) Digestion reaction (8) Product preparation.

2. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: Specifically, step (1) involves ball milling the fly ash for 30-60 minutes at a speed of 200-400 rpm.

3. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: Specifically, step (3) involves drying the homogenized fly ash at 100-150℃ for 2-4 hours to remove free water and some water of crystallization.

4. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: In step (4), the heat treatment time is 30-60 minutes.

5. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: In step (5), the ethanol concentration in the ethanol-water mixed solvent is 60%-80%.

6. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: In step (5), stir for 30-60 minutes at room temperature.

7. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: Specifically, step (6) involves distilling the extract to recover the ethanol solvent for recycling; the distillation residue is a CaCl2 solution, which is further evaporated and crystallized to obtain calcium chloride product.

8. The process for separating and recovering calcium hydroxide from waste incineration fly ash according to claim 1, characterized in that: The specific step (7) is to mix the solid residue with water at a water-cement ratio of 3-4:1 and stir at 60-80℃ for 1-2 hours to digest CaO to generate Ca(OH)2. The reaction equation is: CaO + H2O → Ca(OH)2. After the digestion reaction, solid-liquid separation is carried out by a filter press to obtain calcium hydroxide slurry.

9. The process for separating and recovering calcium hydroxide from fly ash from waste incineration according to claim 1, characterized in that: Specifically, step (8) involves spray drying or baking the calcium hydroxide slurry to obtain a calcium hydroxide product with a purity of over 90%.

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