Method for preparing nano calcium carbonate from fly ash washing liquid based on membrane aeration bio-membrane reactor

Abstract

The invention discloses a method for preparing nano calcium carbonate from fly ash washing liquid based on a membrane aeration bio-membrane reactor. The membrane aeration bio-membrane reactor comprises a reaction container and a membrane assembly arranged in the reaction container. And introducing a fly ash washing liquid into the reaction container, introducing a gas containing carbon dioxide into the membrane assembly, and carrying out carbonation reaction on the carbon dioxide penetrating through the membrane assembly and the fly ash washing liquid in the reaction container to obtain a suspension containing nano calcium carbonate. According to the method, calcium pollutants in the fly ash are directly converted into a nano calcium carbonate product with a high additional value, so that harmlessness and recycling of the fly ash washing liquid are realized; the method is simple in process and mild in reaction condition, the purity of the recycled nano calcium carbonate is up to 96% or above, the average particle size is 80-100 nm, and a brand new technical approach is provided for green and high-valued treatment of the fly ash.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a method for preparing nano-calcium carbonate from fly ash washing liquid based on a membrane aerated biofilm reactor. Background Technology

[0002] Municipal solid waste incineration fly ash is classified as hazardous waste due to its high content of persistent organic pollutants such as soluble chlorides, heavy metals, and dioxins. Currently, fly ash disposal primarily involves stabilization / solidification followed by safe landfilling. However, landfill disposal not only consumes significant land resources but also carries the risk of long-term pollutant leaching, essentially failing to achieve the harmlessness and resource recovery of waste. With the advancement of "zero-waste city" construction and increasingly stringent environmental protection requirements, developing treatment technologies that can achieve fly ash reduction, harmlessness, and resource recovery has become an urgent need for the industry.

[0003] Water washing pretreatment is a crucial initial step in the resource utilization of fly ash, effectively removing high levels of soluble chloride salts (such as NaCl and KCl), thus clearing the way for subsequent resource utilization (e.g., as a building material raw material). Currently, domestic and international research has systematically optimized key parameters of the water washing process, such as the liquid-to-solid ratio, number of washes, and stirring intensity, confirming that multi-stage countercurrent washing can efficiently dechlorinate and reduce water consumption. However, while removing chloride salts, the water washing process also leads to a significant loss of calcium from the fly ash. Fly ash washing liquid typically contains a high concentration of calcium ions; direct disposal not only wastes valuable calcium resources but also increases the load and cost of subsequent wastewater treatment systems due to its high hardness.

[0004] In existing technologies, for wastewater rich in calcium ions, the common method is to add chemical agents such as sodium carbonate to precipitate and recover the calcium carbonate. However, this method requires additional chemical agents, introduces new impurity ions, and the resulting calcium carbonate product has low purity, and its crystal form and particle size are difficult to control. It is typically only suitable as a low-value building material, resulting in poor economic viability. Therefore, seeking a novel recovery technology that can directly convert calcium ions in fly ash washing liquid into high-value-added products is crucial for improving the economic and environmental benefits of the entire fly ash resource utilization process. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing nano-calcium carbonate from fly ash washing liquid based on the shortcomings of the prior art, in order to address the deficiencies of the prior art.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0007] A method for preparing nano-calcium carbonate from fly ash washing liquid based on a membrane aerated biofilm reactor, wherein the membrane aerated biofilm reactor includes a reaction vessel and a membrane module disposed in the reaction vessel; fly ash washing liquid is introduced into the reaction vessel, and carbon dioxide-containing gas is introduced into the membrane module; the carbon dioxide permeates through the membrane module and reacts with the fly ash washing liquid in the reaction vessel to undergo a carbonation reaction, thereby obtaining a suspension containing nano-calcium carbonate.

[0008] The hydraulic retention time of the fly ash washing solution in the MABR is not limited. Those skilled in the art can comprehensively determine the appropriate hydraulic retention time based on the treatment purpose of the fly ash washing solution, system operating costs, and on-site operable engineering parameters to achieve the optimal balance between treatment effect and operating economy. For example, the hydraulic retention time can be set to 15 to 90 minutes.

[0009] Preferably, the membrane aerated biofilm reactor is externally connected to a storage tank; the outlet of the storage tank is connected to the inlet of the membrane aerated biofilm reactor, and the inlet of the storage tank is connected to the outlet of the membrane aerated biofilm reactor, so that a circulating flow path is formed between the storage tank and the reaction vessel.

[0010] Preferably, the fly ash washing liquid enters the reaction vessel through the outlet of the storage tank; after the carbon dioxide and the fly ash washing liquid undergo a carbonation reaction in the reaction vessel, the resulting reaction liquid flows into the storage tank from the outlet of the membrane aerated biofilm reactor, mixes with the liquid in the storage tank, and is then reintroduced into the reaction vessel to carry out a cyclic carbonation reaction, so as to maximize the reuse of calcium ions in the fly ash washing liquid.

[0011] The total time for the cyclic carbonation reaction is not limited. Those skilled in the art can comprehensively determine the appropriate total time for the cyclic carbonation reaction based on the purpose of fly ash washing liquid treatment, system operating costs, and on-site operable engineering parameters to achieve the optimal balance between treatment effect and operational economy. For example, the total time for the cyclic carbonation reaction can be set to 15 to 90 minutes.

[0012] Preferably, the outlet of the storage tank is connected to the inlet of the membrane aerated biofilm reactor via a pipeline, and a peristaltic pump is installed on the pipeline to control the speed of the peristaltic pump so that the flow rate of the liquid in the reaction vessel is 200 mL / min.

[0013] The membrane module is a bundled membrane module composed of hollow fiber membranes; the hollow fiber membrane is made of polyetheretherketone; and the pore size of the hollow fiber membrane is less than 0.4 nm.

[0014] Preferably, the hollow fiber membrane has a porosity greater than 46%; the inner diameter of the hollow fiber membrane is 0.4 mm and the outer diameter is 1.0 mm.

[0015] Hollow fiber membranes made of polyetheretherketone (PEEK) selectively allow carbon dioxide to pass through. When carbon dioxide gas is introduced into the membrane module, it permeates through the hollow fiber membrane into the fly ash washing solution and reacts with calcium ions to form calcium carbonate.

[0016] The fly ash washing solution is obtained by solid-liquid separation after washing fly ash with water; the fly ash is fly ash obtained from the incineration of municipal solid waste; the calcium ion concentration in the fly ash washing solution is 2000 ~ 10000 mg / L, preferably 4000 ~ 6000 mg / L.

[0017] Preferably, the method for washing the fly ash is a three-stage countercurrent washing: fly ash is mixed with secondary washing liquid in a certain proportion for primary washing; the mixture after primary washing is subjected to solid-liquid separation to obtain primary washing liquid and primary fly ash washing residue; the primary fly ash washing residue is mixed with tertiary washing liquid for secondary washing; the mixture after secondary washing is subjected to solid-liquid separation to obtain secondary washing liquid and secondary fly ash washing residue; the secondary washing liquid is reused in primary washing; the secondary fly ash washing residue is mixed with water for tertiary washing; the mixture after tertiary washing is subjected to solid-liquid separation to obtain tertiary washing liquid and tertiary fly ash washing residue; the tertiary washing liquid is reused in secondary washing.

[0018] Preferably, during the three-stage countercurrent water washing, the liquid-to-solid ratio of the first-stage water washing is 4-16 mL: 1 g, the volume of water used in the second and third-stage water washing is equal to the volume of water used in the first-stage water washing, and each stage of water washing is carried out for 10-60 min under the conditions of stirring at a speed of 450-1100 rpm and a temperature of 30-55℃.

[0019] Preferably, the fly ash washing solution is a first-grade washing solution.

[0020] Wherein, the carbon dioxide-containing gas is pure carbon dioxide or a mixture of carbon dioxide and other non-reactive gases; preferably, the mixture of carbon dioxide and other non-reactive gases is flue gas, biogas, or a mixture of carbon dioxide and air; when the carbon dioxide-containing gas is introduced into the membrane module, the aeration pressure is 5 to 35 kPa.

[0021] Preferably, the carbon dioxide-containing gas is pure carbon dioxide; when the carbon dioxide-containing gas is introduced into the membrane module, the aeration pressure is 15-20 kPa.

[0022] The carbonation reaction is carried out at a temperature of 40 to 50°C, preferably 45°C.

[0023] Preferably, the liquid temperature in the storage tank is maintained at 40-50°C; the storage tank is equipped with a stirring device with a stirring speed of 300-600 rpm.

[0024] Preferably, one end of the membrane module is provided with an air inlet for introducing carbon dioxide-containing gas into the hollow fiber membrane cavity of the membrane module; the other end of the membrane module is provided with an air outlet for discharging gas and condensate and impurities inside the hollow fiber membrane.

[0025] The suspension containing nano-calcium carbonate is subjected to a separation and purification process to obtain nano-calcium carbonate; preferably, the separation and purification process includes any one or a combination of several processes selected from solid-liquid separation, washing, and drying.

[0026] Preferably, the drying process involves vacuum drying at 80-105°C for 1-3 hours. The washing process involves washing with pure water.

[0027] The purity of the nano-calcium carbonate is above 96%, and the average particle size is 80 ~ 100 nm.

[0028] This invention also claims protection for the application of the aforementioned nano-calcium carbonate as a flue gas desulfurizing agent.

[0029] The key parameters of the hollow fiber membrane in a MABR membrane module, such as pore size, porosity, and dimensions, play a decisive role in the quality of the final calcium carbonate product by regulating the mass transfer behavior of CO2. Selecting a PEEK membrane with a specific pore size (less than 0.4 nm) and porosity (>46%) ensures that CO2 diffuses into the reaction solution in a controllable and uniform manner, thereby precisely controlling the nucleation and growth process of calcium carbonate at the microscale. This unique mass transfer mechanism effectively avoids local supersaturation and inhibits disordered crystal aggregation, which is the fundamental reason for obtaining high-purity, uniformly sized, and regularly morphologically regular nano-calcium carbonate products.

[0030] This invention provides a method for calcium recovery and nano-calcium carbonate preparation from fly ash washing liquid based on a MABR (Metal-Oxide-Blood Blender). This method utilizes a hollow fiber membrane with selective carbon dioxide permeability as a controllable carbonation reaction interface, allowing carbon dioxide gas to be controllably transferred into the fly ash washing liquid to react with calcium ions. This not only directly converts calcium pollutants into high-purity, nano-sized calcium carbonate products, achieving high-value recovery of calcium resources, but also avoids the addition of chemical reagents, simplifying the process. The nano-calcium carbonate prepared by this method has high purity and uniform particle size, making it suitable as a high-quality chemical filler or functional material. The resulting wastewater is also easier to treat due to the efficient removal of calcium ions. This technological approach provides a novel solution for the harmless treatment and full-component resource utilization of municipal solid waste incineration fly ash.

[0031] Beneficial effects:

[0032] (1) This invention is the first to apply MABR to the treatment of fly ash washing liquid. By utilizing its selective permeability to CO2, a controllable gas-liquid reaction interface is constructed, realizing the efficient and mild conversion of calcium ions into nano-calcium carbonate. The reaction conditions are easy to control, and the process flow is simple. (2) This invention directly converts calcium pollutants in fly ash into high-value-added nano-calcium carbonate products with a purity of over 98%, uniform particle size, and an average particle size of 80-100 nm. This realizes the high-value resource utilization of hazardous waste and significantly reduces the scaling risk and treatment load of subsequent water treatment systems. (3) This method does not require the addition of additional precipitants (such as sodium carbonate), avoiding the introduction of new impurity ions. The process is green and environmentally friendly. It synergistically treats industrial waste gas containing CO2, achieving the dual environmental benefits of "treating waste with waste". Attached Figure Description

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention as described above or otherwise will become clearer.

[0034] Figure 1 This is a schematic diagram of the apparatus used to prepare calcium carbonate using fly ash washing liquid in Examples 1 to 5 and Example 7.

[0035] Figure 2 This is a process flow diagram of the method for recovering calcium from fly ash washing liquid and preparing nano-calcium carbonate based on MABR in Example 1.

[0036] Figure 3 The graphs show the calcium ion removal rate and calcium carbonate mass of fly ash washing liquid with different calcium ion concentrations after MABR treatment in Example 2; where graph a shows the calcium ion concentration and pH value in the effluent, and graph b shows the decalcification efficiency and calcium carbonate mass.

[0037] Figure 4 The graphs show the calcium ion removal rate and calcium carbonate mass of fly ash washing liquid after MABR treatment under different aeration pressures in Example 3; where graph a shows the calcium ion concentration and pH value in the effluent, and graph b shows the decalcification efficiency and calcium carbonate mass.

[0038] Figure 5 The graphs show the calcium ion removal rate and calcium carbonate mass of the fly ash washing liquid after MABR treatment for different times in Example 4; where graph a shows the calcium ion concentration and pH value in the effluent, and graph b shows the decalcification efficiency and calcium carbonate mass.

[0039] Figure 6The graphs show the calcium ion removal rate and calcium carbonate mass of fly ash washing liquid after MABR treatment at different reaction temperatures in Example 5; where graph a shows the calcium ion concentration and pH value in the tail water, and graph b shows the decalcification efficiency and calcium carbonate mass.

[0040] Figure 7 This is a scanning electron microscope image of the nano-calcium carbonate prepared under optimal conditions in Example 7.

[0041] Figure 8 The X-ray diffraction pattern of nano-calcium carbonate prepared under optimal conditions in Example 7 is shown.

[0042] Figure 9 The particle size distribution diagrams are for the nano-calcium carbonate prepared in Examples 1 and 7. Detailed Implementation

[0043] The present invention will be further described below with reference to the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0044] The MABR used in the following examples was purchased from Jiangsu Julan Nanotechnology Co., Ltd. Its cylindrical reaction vessel houses the membrane module. The cylindrical reaction vessel is 8.5 cm long, has a base diameter of 2.8 cm, and an effective volume of 30 mL. The membrane module consists of a bundle of 60 hollow fiber membrane filaments made of PEEK. The average pore size of a single filament is less than 0.4 nm, the porosity is greater than 46%, the inner diameter of the filament is 0.4 mm, and the outer diameter is 1.0 mm. The effective length of a single filament is 8.5 cm. The MABR membrane module uses a flow-channel (open) aeration method. Gas is input from one end of the membrane module, flows through the inner cavity of the hollow fiber membrane, and then exits directly from the other end. Carbon dioxide diffuses through the membrane wall into the fly ash washing liquid in the reaction vessel during the gas flow through the filaments. This MABR has an acid and alkali resistance range of pH 2 to 14, making it suitable for the carbonation reaction system of this invention.

[0045] The following examples illustrate the method for preparing nano-calcium carbonate based on a MABR and a beaker connected to the MABR. A schematic diagram of the apparatus is shown below. Figure 1 As shown, the inlet of the MABR is connected to a sealed beaker via a hose. A peristaltic pump is installed on the hose, and the flow rate of fly ash washing liquid in the MABR is controlled to be 200 mL / min by adjusting the speed of the peristaltic pump. The outlet of the MABR is connected to the beaker via a hose, so that a circulation path is formed between the beaker and the MABR.

[0046] Example 1

[0047] A method for preparing nano-calcium carbonate from fly ash washing liquid based on MABR includes the following steps:

[0048] Step 1: Three-stage countercurrent water washing of municipal solid waste incineration fly ash. The specific steps are as follows: The first batch of fly ash is mixed with deionized water at a liquid-to-solid ratio of 8 mL: 1 g and stirred at 45℃ and 750 rpm for 1 h for primary water washing. The mixture after primary water washing is subjected to solid-liquid separation to obtain primary water washing liquid and primary fly ash water washing residue. The primary fly ash water washing residue is mixed with deionized water for secondary water washing, with the same amount of deionized water used and water washing conditions as primary water washing. The mixture after secondary water washing is subjected to solid-liquid separation to obtain secondary water washing liquid and secondary fly ash water washing residue. The secondary water washing liquid is reused for the primary water washing of the next batch of fly ash. The secondary fly ash water washing residue is mixed with deionized water for tertiary water washing, with the same amount of deionized water used and water washing conditions as primary water washing. The mixture after tertiary water washing is subjected to solid-liquid separation to obtain tertiary water washing liquid and tertiary fly ash water washing residue. The tertiary water washing liquid is reused for the secondary water washing of the next batch of fly ash. The second batch of fly ash was mixed with the secondary washing liquid obtained from the washing of the previous batch of fly ash at a liquid-to-solid ratio of 8 mL: 1 g. The mixture was stirred at 45℃ and 750 rpm for 1 h for primary washing. The mixture after primary washing was subjected to solid-liquid separation to obtain primary washing liquid and primary fly ash washing residue. The primary fly ash washing residue was mixed with the tertiary washing liquid obtained from the washing of the previous batch of fly ash for secondary washing. The amount of tertiary washing liquid used and the washing conditions were the same as those for primary washing. The mixture after secondary washing was subjected to solid-liquid separation to obtain secondary washing liquid and secondary fly ash washing residue. The secondary washing liquid was reused for the primary washing of the next batch of fly ash. The secondary fly ash washing residue was mixed with deionized water for tertiary washing. The amount of deionized water used and the washing conditions were the same as those for primary washing. The mixture after tertiary washing was subjected to solid-liquid separation to obtain tertiary washing liquid and tertiary fly ash washing residue. The tertiary washing liquid was reused for the secondary washing of the next batch of fly ash. This process is repeated for each batch of fly ash, involving a three-stage countercurrent water wash. Starting with the second batch of fly ash, the primary wash solution obtained is used as the fly ash washing solution to prepare nano-calcium carbonate. The calcium ion concentration in the primary wash solution was measured to be approximately 6000 mg / L.

[0049] Step 2: Take 1 L of the fly ash washing solution obtained in Step 1 and inject it into a beaker connected to the MABR. Incubate in a 45°C water bath with stirring at 600 rpm, then pump it into the MABR reaction vessel, controlling the flow rate of the washing solution at 200 mL / min. Simultaneously, introduce pure carbon dioxide gas into the MABR membrane module, controlling the aeration pressure at 15 kPa. The fly ash washing solution reacts with carbon dioxide in the MABR. The reaction solution overflows from the MABR outlet into the beaker, mixes evenly with the remaining fly ash washing solution in the beaker, and is then reintroduced into the reaction vessel. This allows the fly ash washing solution and carbon dioxide to repeatedly undergo a carbonation reaction in the membrane aerated biofilm reactor, fully utilizing the calcium ions in the fly ash washing solution. The 1 L fly ash washing solution runs for a total of 45 minutes in the device consisting of the MABR and the beaker. During this process, CO2 permeates through the membrane wall and reacts with the calcium ions in the fly ash washing solution to form a white calcium carbonate precipitate.

[0050] Step 3: After the operation is completed, collect the suspension in the reactor from the outlet of the MABR, filter it, wash it with deionized water, and then vacuum dry it at 105°C for 2 hours to obtain a white powdery nano-calcium carbonate product.

[0051] The process flow diagram of the MABR-based method for calcium recovery from fly ash washing liquid and preparation of nano-calcium carbonate is shown in the figure below. Figure 2 As shown.

[0052] Example 2

[0053] The difference between this embodiment and Embodiment 1 lies only in the three-stage countercurrent water washing conditions in step 1, to investigate the effect of calcium ion concentration in the fly ash washing solution on the calcium ion reuse and calcium carbonate quality. The only difference between the three-stage countercurrent water washing conditions in this embodiment and Embodiment 1 is that, in the three-stage countercurrent water washing process of the fly ash in this embodiment, the liquid-to-solid ratios for the first-stage water washing are set to 16 mL: 1 g, 12 mL: 1 g, 6 mL: 1 g, and 4 mL: 1 g, respectively, and the washing conditions are: stirring at 40°C and 750 rpm for 45 minutes. The final calcium ion concentrations in the obtained fly ash washing solutions are 2000, 4000, 8000, and 10000 mg / L, respectively.

[0054] After the operation was completed, the resulting calcium carbonate suspension was filtered, and the filtrate was collected to determine the calcium ion concentration and pH value. The decalcification efficiency was calculated according to the following formula. The filter residue was collected, washed with deionized water, and then vacuum dried at 105°C for 2 hours to obtain a white powdery nano-calcium carbonate product, which was then weighed.

[0055]

[0056] In the formula, represents the concentration of residual calcium ions in the filtrate, in mg / L; represents the concentration of calcium ions in the initial fly ash washing solution, in mg / L.

[0057] After MABR treatment, the calcium ion concentration and pH value of the final filtrate from fly ash washing solutions with different calcium ion concentrations are as follows: Figure 3 As shown in Figure a, the mass and decalcification efficiency of the obtained nano-calcium carbonate are as follows: Figure 3 As shown in b. The results indicate that a moderate calcium ion concentration is beneficial for CO2 absorption and carbonation reactions, thereby consuming more H2. + This significantly increased the pH value of the system.

[0058] Example 3

[0059] The only difference between this embodiment and Example 1 is the CO2 gas aeration pressure in step 2, in order to investigate the effect of carbonation driving force on the reuse of calcium ions and the quality of calcium carbonate in fly ash washing liquid. The aeration pressure of the introduced CO2 gas was controlled at 5, 10, 20, 25 and 30 kPa, respectively, and other reaction conditions were the same as in Example 1.

[0060] After the operation was completed, the resulting calcium carbonate suspension was filtered, and the filtrate was collected to determine the calcium ion concentration and pH value, and the decalcification efficiency was calculated. The filter residue was collected, washed with deionized water, and then vacuum dried at 105°C for 2 hours to obtain a white powdery nano-calcium carbonate product, which was then weighed.

[0061] Under different aeration pressures, the calcium ion concentration and pH value in the final filtrate of fly ash washing liquid after MABR treatment are as follows: Figure 4 As shown in Figure a, the mass and decalcification efficiency of the obtained nano-calcium carbonate are as follows: Figure 4 As shown in b, the results indicate that as the aeration pressure of CO2 gas increased from 5 kPa to 30 kPa, the mass of calcium carbonate, the decalcification efficiency, and the pH value of the filtrate all showed a trend of first increasing and then decreasing, while the calcium ion concentration in the filtrate showed a trend of first decreasing and then increasing. This suggests that a suitable aeration pressure is more conducive to the recovery of calcium ions in the washing liquid.

[0062] Example 4

[0063] The only difference between this embodiment and Embodiment 1 is that the total running time of the fly ash washing solution in the device consisting of the MABR and the beaker in step 2 is 15, 30, 60, 75, and 90 minutes, respectively, while other reaction conditions remain unchanged, in order to examine the effect of the reaction process on the recovery of calcium ions in the fly ash washing solution and the quality of calcium carbonate.

[0064] After the operation was completed, the resulting calcium carbonate suspension was filtered, and the filtrate was collected to determine the calcium ion concentration and pH value, and the decalcification efficiency was calculated. The filter residue was collected, washed with deionized water, and then vacuum dried at 105°C for 2 hours to obtain a white powdery nano-calcium carbonate product, which was then weighed.

[0065] After fly ash washing liquid was treated with MABR for different times, the calcium ion concentration and pH value in the final filtrate were as follows: Figure 5 As shown in Figure a, the mass and decalcification efficiency of the obtained nano-calcium carbonate are as follows: Figure 5 As shown in b, as the reaction time increased to 45–60 minutes, the residual calcium concentration in the filtrate decreased significantly to a lower level of 1200–1400 mg / L, while the pH value rose to 7.8–8.0 and tended to stabilize. This marks the completion of the main stage of the carbonation reaction, where calcium ions in the liquid phase react with dissolved CO2 (in the form of carbonate ions) to form calcium carbonate precipitate.

[0066] Example 5

[0067] The only difference between this embodiment and Example 1 is that the water bath temperature of the beaker in step 2 is controlled at 35, 40, 50, and 55°C, while other reaction conditions remain unchanged, in order to investigate the effect of different reaction temperatures on the reuse of calcium ions in fly ash washing solution and the quality of calcium carbonate.

[0068] After the operation was completed, the resulting calcium carbonate suspension was filtered, and the filtrate was collected to determine the calcium ion concentration and pH value, and the decalcification efficiency was calculated. The filter residue was collected, washed with deionized water, and then vacuum dried at 105°C for 2 hours to obtain a white powdery nano-calcium carbonate product, which was then weighed.

[0069] At different temperatures, the calcium ion concentration and pH value in the final filtrate of fly ash washing liquid after MABR treatment are as follows: Figure 6 As shown in Figure a, the mass and decalcification efficiency of the obtained nano-calcium carbonate are as follows: Figure 6 As shown in b. The results indicate that appropriate heating is beneficial for accelerating the carbonation reaction kinetics and improving decalcification efficiency; however, when the temperature is too high, the significant decrease in CO2 solubility may become a limiting factor for calcium ion recovery, and may also affect the stability of calcium carbonate precipitate, thus leading to a slight decrease in decalcification efficiency.

[0070] Example 6

[0071] Based on the experimental results of Examples 1 to 5 above, the CO2 gas aeration pressure and the Ca content in the fly ash washing liquid were taken. 2+ Concentration and reaction temperature are the variable factors, and decalcification efficiency is the evaluation index. The L9(3) method is used. 4 The experiment was designed using an orthogonal array, and the experimental design and results are shown in Table 1.

[0072] Table 1 Orthogonal experimental design and results

[0073]

[0074] The analysis of variance is shown in Table 2. It can be seen that the CO2 gas aeration pressure and the Ca content in the fly ash washing liquid are related. 2+ The concentration had a significant effect on the decalcification efficiency. p <0.05).

[0075] Table 2 Analysis of Variance

[0076]

[0077] In the table, "*" indicates a significant difference ( p <0.05)

[0078] The range analysis is shown in Table 3. Table 3 shows that the order of importance of the three factors affecting decalcification efficiency is: Ca in fly ash washing solution... 2+ The optimal conditions for achieving the highest decalcification efficiency are: Ca concentration > CO2 gas aeration pressure > reaction temperature. 2+ The concentration was 4000 mg / L, the CO2 gas aeration pressure was 20 kPa, and the reaction temperature was 45℃.

[0079] Table 3 Range Analysis

[0080]

[0081] Example 7

[0082] Under the optimal reaction conditions obtained in Example 6 (Ca in fly ash washing solution) 2+ Nano-sized calcium carbonate was prepared using MABR (with a concentration of 4000 mg / L, a CO2 gas aeration pressure of 20 kPa, and a reaction temperature of 45℃). Other reaction conditions were the same as in Example 1.

[0083] The final prepared nano-calcium carbonate product was observed by SEM and detected by XRD. The results are as follows: Figure 7 and Figure 8 As shown. From Figure 7 It can be seen that the carbonation product exhibits a typical hierarchical structure of "nano-primary particles – micron-scale secondary aggregates": at low magnification, the samples are mostly irregular blocky / cauliflower-shaped aggregates with rough surfaces and numerous pores / gaps, indicating that a large number of nucleations occurred during carbonation, accompanied by obvious particle adhesion and secondary aggregation; at higher magnification, the aggregates are composed of a large number of fine particles, with the primary particle size ranging from submicron to nanometer, showing the characteristics of highly supersaturated nucleation under rapid precipitation conditions. Meanwhile, regular near-cubic crystals can be observed locally, indicating the presence of highly crystallized CaCO3 crystals in the product. Figure 8The strongest diffraction peak appears at 2θ≈29.4°, with a series of characteristic peaks at 39°, 43°, 47°, 48°, and 57°. The overall diffraction peaks match well with those shown on the standard card for CaCO3, indicating that the main crystalline phase of the product is crystalline calcium carbonate. The spectrum has a stable baseline and no obvious impurity peaks, indicating high sample purity. Furthermore, while the diffraction peaks are relatively sharp, they still exhibit some broadening, reflecting that the grain size is at the nanoscale and possesses a certain degree of crystallinity, consistent with the structural characteristics of nano-CaCO3.

[0084] The particle size distribution of the nano-calcium carbonate prepared in Examples 1 and 7 was measured, and the results are as follows: Figure 9 The average particle size is less than 100 nm.

[0085] The purity of the calcium carbonate prepared in Examples 1 and 7 was determined by acid-base titration. The purity of the calcium carbonate prepared in Example 1 was 96.1%, and the purity of the calcium carbonate prepared in Example 7 was 98.4%.

[0086] Comparative Example 1

[0087] The traditional chemical precipitation method for preparing calcium carbonate involves taking the same fly ash washing liquid as in Example 1, adding sodium carbonate solid at a molar ratio of 1:1 to calcium ions, and reacting at room temperature and a stirring rate of 500 rpm for 45 minutes. After the reaction, the resulting suspension is subjected to solid-liquid separation, washing, and drying in the same manner as in Example 1 to obtain the calcium carbonate product.

[0088] Testing revealed that the calcium carbonate product obtained by this method had a purity of only 92.1%, a wide particle size distribution (1 ~ 10 μm), and an irregular morphology. Furthermore, this method introduces sodium ions, increasing the complexity of wastewater treatment.

[0089] This invention provides a method for preparing nano-calcium carbonate from fly ash washing liquid using a membrane-aerated biofilm reactor. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method for preparing nano-calcium carbonate from fly ash washing liquid based on a membrane aerated biofilm reactor, wherein the membrane aerated biofilm reactor comprises a reaction vessel and a membrane module disposed in the reaction vessel, characterized in that, The fly ash washing liquid is introduced into the reaction vessel, and a carbon dioxide-containing gas is introduced into the membrane module. The carbon dioxide permeates through the membrane module and reacts with the fly ash washing liquid in the reaction vessel to undergo a carbonation reaction, resulting in a suspension containing nano-calcium carbonate.

2. The method according to claim 1, characterized in that, The membrane aerated biofilm reactor is externally connected to a storage tank; the outlet of the storage tank is connected to the inlet of the membrane aerated biofilm reactor, and the inlet of the storage tank is connected to the outlet of the membrane aerated biofilm reactor, so that a circulation path is formed between the storage tank and the reaction vessel.

3. The method according to claim 2, characterized in that, The fly ash washing liquid enters the reaction vessel through the outlet of the storage tank; after the carbon dioxide and the fly ash washing liquid undergo a carbonation reaction in the reaction vessel, the resulting reaction liquid flows into the storage tank from the outlet of the membrane aerated biofilm reactor, mixes with the liquid in the storage tank, and is then introduced back into the reaction vessel to carry out a cyclic carbonation reaction.

4. The method according to claim 1, characterized in that, The membrane module is a bundled membrane module composed of hollow fiber membranes; the hollow fiber membrane is made of polyetheretherketone; and the pore size of the hollow fiber membrane is less than 0.4 nm.

5. The method according to any one of claims 1 to 3, characterized in that, The fly ash washing solution is obtained by solid-liquid separation after washing fly ash with water; the fly ash is fly ash obtained from the incineration of municipal solid waste; the calcium ion concentration in the fly ash washing solution is 2000 ~ 10000 mg / L.

6. The method according to any one of claims 1 to 3, characterized in that, The carbon dioxide-containing gas is pure carbon dioxide or a mixture of carbon dioxide and other non-reactive gases; preferably, the mixture of carbon dioxide and other non-reactive gases is flue gas, biogas, or a mixture of carbon dioxide and air; when the carbon dioxide-containing gas is introduced into the membrane module, the aeration pressure is 5 to 35 kPa.

7. The method according to any one of claims 1 to 3, characterized in that, The carbonation reaction is carried out at a temperature of 40 to 50°C.

8. The method according to claim 2 or 3, characterized in that, The liquid temperature in the storage tank is maintained at 40~50℃; the storage tank is equipped with a stirring device with a stirring speed of 300~600 rpm.

9. The method according to claim 1, characterized in that, The suspension containing nano-calcium carbonate is subjected to a separation and purification process to obtain nano-calcium carbonate; preferably, the separation and purification process includes any one or a combination of several processes selected from solid-liquid separation, washing, and drying.

10. The method according to claim 9, characterized in that, Nano-sized calcium carbonate was obtained through separation and purification processes, with an average particle size of 80-100 nm.

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