Method for preparing high-flame-retardance superfine calcium-aluminum hydrotalcite by extracting aluminum element from fly ash

Abstract

The invention provides a method for preparing high-flame-retardance superfine calcium-aluminum hydrotalcite by extracting an aluminum element from fly ash, and belongs to the technical field of comprehensive utilization of byproducts, the method comprises the following steps: uniformly mixing fly ash, sodium carbonate secondary salty mud and a low-temperature active agent, and carrying out ultralow-temperature calcination, ultrasonic-assisted alkaline leaching and coprecipitation to obtain the high-flame-retardance superfine calcium-aluminum hydrotalcite. The superfine calcium-aluminum hydrotalcite with the high flame retardance is prepared. According to the method, the thermoelectric by-product fly ash and the sodium carbonate secondary salty mud are used as raw materials to prepare the high-flame-retardance superfine calcium-aluminum hydrotalcite, the method is simple, the effect is good, the production cost is low, and high-added-value utilization of the fly ash and the secondary salty mud is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of resource utilization, and in particular relates to a method for extracting aluminum elements from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite. Background Art

[0002] Calcium aluminum hydrotalcite is a material with good flame retardant properties. Calcium aluminum hydrotalcite is a synthetic hydrocalumite that uses calcium to replace magnesium. It has good flame retardancy. When heated, the hydroxyl groups and interlayer ions of its structural hydrated layers will be released in the form of water and carbon dioxide, which plays a role in reducing the concentration of combustion gases and blocking oxygen, and can release flame retardants in a lower temperature range; it also has the characteristics of large heat absorption, which is conducive to reducing the high temperature generated during combustion. It can be used as a halogen-free high smoke suppression flame retardant and is widely used in plastics, rubber, coatings and other fields. In practical applications, the flame retardant properties of calcium aluminum hydrotalcite will be affected by many factors such as chemical composition, crystal structure, particle size and particle size distribution and dispersibility. This leads to the fact that its preparation method and raw materials will significantly affect the flame retardant properties of calcium aluminum hydrotalcite. Calcium aluminum hydrotalcite has better long-term thermal stability and lower preparation cost than magnesium aluminum hydrotalcite. Calcium aluminum hydrotalcite is superior to magnesium aluminum hydrotalcite in thermal stability, adsorption performance, early strength agent application, wide application prospects, and biological and environmental compatibility.

[0003] Meanwhile, the preparation of calcium aluminum hydrotalcite in the prior art is mainly carried out by hydrothermal method, microwave synthesis method and other methods. However, in the process of preparing calcium aluminum hydrotalcite by these methods, soluble salts such as calcium nitrate and aluminum nitrate are usually used as the main raw materials, and the mass fraction of available calcium and aluminum elements in such raw materials does not exceed 20%; and in the preparation process, on the one hand, the synthesis of calcium aluminum hydrotalcite by the traditional method does not follow the "fast nucleation mechanism", the radius of calcium ions is quite different from that of magnesium ions, and it is difficult to replace the magnesium element in the hydrotalcite, and the formation rate of CaCO3 is much higher than that of calcium aluminum hydrotalcite. On the other hand, the thermal stability of CaCO3 is much higher than that of calcium aluminum hydrotalcite, making it easier for carbonate or hydroxide to form stable CaCO3 impurities with calcium and aluminum ions. Therefore, the atomic utilization rate of calcium aluminum hydrotalcite prepared by the prior art is low, and many by-products are generated. It is difficult to separate the by-products from the main product calcium aluminum hydrotalcite, which leads to poor flame retardancy of the obtained calcium aluminum hydrotalcite product.

[0004] In summary, it is necessary to develop a preparation method that can improve the flame retardant properties of calcium aluminum hydrotalcite. Summary of the invention

[0005] In view of the above problems, the present invention provides a method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite, characterized in that the method comprises mixing fly ash, soda ash secondary salt mud (containing more than 97% calcium carbonate) and a low-temperature activator (i.e., ammonium sulfate) uniformly, calcining at ultra-low temperature, and then subjecting the obtained cooked material powder to ultrasonic-assisted alkali leaching and co-precipitation to obtain the highly flame-retardant ultrafine calcium aluminum hydrotalcite.

[0008] Furthermore, the method comprises the following specific steps:

[0009] S1. Ultra-low temperature calcination

[0010] The secondary salt mud produced as a by-product of the soda ash produced by the ammonia-soda process is dried and ground to obtain the soda ash secondary salt mud;

[0011] After fly ash, soda ash secondary salt mud and low-temperature activator are mixed evenly, the obtained mixed powder is subjected to ultra-low temperature calcination to obtain cooked material;

[0012] Among them, fly ash mainly contains mullite (i.e. 3Al2O3·2SiO2) and aluminum oxide, and the chemical reaction formulas involved in the ultra-low temperature calcination process include:

[0013] CaCO3=CaO+CO2↑;

[0014] 4(NH4)2SO4+Al2O3=2NH4Al(SO4)2+3H2O↑+6NH3↑;

[0015] 7(3Al2O3·2SiO2)+64CaO=3(12CaO·7Al2O3)+14(2CaO·SiO2);

[0016] S2, ultrasonic assisted alkali leaching

[0017] The cooked material is fully ground into powder, and the obtained cooked material powder is placed in a sodium bicarbonate solution for ultrasonic-assisted alkali leaching. After the ultrasonic-assisted alkali leaching is completed, the filtrate is filtered and collected, and the pH value of the obtained filtrate is adjusted to obtain solution A;

[0018] Among them, in the process of ultrasonic-assisted alkaline leaching and subsequent pH adjustment, the chemical reaction formula involved is:

[0019] CaO+H2O=Ca(OH)2;

[0020] Al2O3+2HCO3 - =2AlO2 - +H2O+2CO2↑;

[0021] Al 3++4OH - =AlO2 - +2H2O;

[0022] S3. Coprecipitation

[0023] Take calcium chloride and dissolve it in water to make solution B;

[0024] Solution A and solution B are sprayed into the spray tower in opposite directions, and hot CO2 flue gas is introduced into the deionized water at the bottom of the spray tower for carbonization;

[0025] After carbonization is completed, it is aged and filtered, and the filtrate and filter cake are collected;

[0026] The chemical reactions involved in the spray tower include:

[0027] CO2+H2O=H2CO3;

[0028] 6Ca 2+ +2AlO2 - +8OH - +2HCO3 - +8H2O=Ca6Al2(OH) 16 (HCO3)2·4H2O;

[0029] The filter cake is dried to obtain the highly flame-retardant ultrafine calcium aluminum hydrotalcite;

[0030] The filtrate is returned to step S2 and used as a solvent for dissolving sodium bicarbonate to prepare a sodium bicarbonate solution.

[0031] Furthermore, in step S3, the flow rate of the introduced CO2 hot flue gas is 0.08-0.12 m / s.

[0032] Furthermore, during the ultra-low temperature calcination process, the weight ratio of fly ash, soda ash secondary salt mud and low temperature activator is 1:0.2-0.3:0.6-0.7;

[0033] The ultra-low temperature calcination temperature is 300-400°C and the time is more than 2 hours.

[0034] Furthermore, during the ultrasonic-assisted alkali leaching process, the ultrasonic power of the ultrasonic-assisted alkali leaching is above 400 W and the time is 30 to 40 minutes.

[0035] Furthermore, in step S2, the concentration of the sodium bicarbonate solution is 9.6-10 wt %; and the pH value of solution A is 12-13.

[0036] Furthermore, in step S3, the concentration of calcium chloride in solution B is 1.75-2.75 wt %.

[0037] The weight ratio of the cooked material powder used to prepare solution A to the calcium chloride in solution B is 1:0.08-0.15.

[0038] Furthermore, in step S3, the injection flow rates of solution A and solution B are both 1.8-2 m / h.

[0039] Furthermore, in step S3, the temperature of the bottom of the spray tower is 70-90°C;

[0040] The aging temperature is 70-90°C and the time is 2-3h;

[0041] The drying temperature is 60-80°C.

[0042] Furthermore, in step S3, the spray tower is a conventional spray tower, including a tower kettle, a discharge port and a first pipe for CO2 hot flue gas are provided at the bottom of the tower kettle, the outlet of the first pipe in the tower kettle is close to a position near the bottom of the tower kettle, and nozzles are oppositely arranged at the top of the tower kettle. The nozzles have a spray plane, and water outlet holes are evenly distributed on the spray plane. The aperture of the water outlet holes is 2 mm, and the angle between the two spray planes is 75°.

[0043] The beneficial effects of the method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite of the present invention are:

[0044] The invention uses fly ash, a byproduct of thermoelectricity, and soda ash secondary salt mud as raw materials to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite, which has a simple method, good effect, and low production cost, and realizes high added value utilization of fly ash and secondary salt mud;

[0045] The highly flame-retardant ultrafine calcium-aluminum hydrotalcite prepared by the present invention contains a considerable amount of structural water in its structure, and has bicarbonate between layers. When heated, water and carbon dioxide are released to achieve the purpose of flame retardancy. After the water and carbon dioxide are released, the nano-scale hydrotalcite is transformed into a porous solid base with a relatively high specific surface area, which has a very strong adsorption effect on smoke and toxic acidic gases generated by combustion.

[0046] The invention introduces CO2 hot flue gas to significantly increase the bicarbonate content in the prepared highly flame-retardant ultrafine calcium aluminum hydrotalcite, and the increase in bicarbonate can further improve the flame retardant properties of the highly flame-retardant ultrafine calcium aluminum hydrotalcite.

[0047] The highly flame-retardant ultrafine calcium aluminum hydrotalcite prepared by the present invention has a particle size of 0.5 to 1 μm after testing, which is much lower than the particle size (6 to 7 μm) of calcium aluminum hydrotalcite prepared by the traditional co-precipitation method, and a lower particle size is conducive to its uniform dispersion in PVC.

[0048] The oxygen index of the PVC material sample made of ordinary calcium aluminum hydrotalcite is about 30%, and the smoke density level is 70. However, the oxygen index of the PVC material sample made of the same amount of highly flame retardant ultrafine calcium aluminum hydrotalcite prepared by the present invention can reach about 38-40%, and the smoke density level can be reduced to 62-65.

[0049] Instruction Manual

[0050] Figure 1 It is a specific structural diagram of the spray tower used in the present invention;

[0051] Figure 2 This is an electron microscope image of the highly flame-retardant ultrafine calcium aluminum hydrotalcite prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0052] The technical solutions in the embodiments of the present invention are described clearly and completely below. In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0053] The present invention discloses a method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash, wherein the highly flame-retardant ultrafine calcium aluminum hydrotalcite is prepared by calcination, alkali leaching and coprecipitation, and specifically comprises the following steps:

[0054] S1. Ultra-low temperature calcination

[0055] The secondary salt mud produced as a by-product of the soda ash produced by the ammonia-soda process is dried and ground to obtain the soda ash secondary salt mud (containing more than 90% calcium carbonate);

[0056] After the fly ash, a byproduct of thermoelectricity, the secondary salt mud of soda ash and the low-temperature activator (i.e. ammonium sulfate) are mixed evenly in a weight ratio of 1:0.2-0.3:0.6-0.7, the obtained mixed powder is put into a hollow paddle dryer and ultra-low temperature calcined at 300-400°C for more than 2 hours to obtain cooked material;

[0057] The ultra-low temperature calcination temperature of the present invention is much lower than the calcination temperature (1000° C.) of the traditional limestone method, which is beneficial to reduce energy consumption and production costs;

[0058] Among them, fly ash mainly contains mullite (i.e. 3Al2O3·2SiO2) and aluminum oxide, and the chemical reaction formulas involved in the ultra-low temperature calcination process include:

[0059] CaCO3=CaO+CO2↑;

[0060] 4(NH4)2SO4+Al2O3=2NH4Al(SO4)2+3H2O↑+6NH3↑;

[0061] 7(3Al2O3·2SiO2)+64CaO=3(12CaO·7Al2O3)+14(2CaO·SiO2);

[0062] S2, ultrasonic assisted alkali leaching

[0063] Taking sodium bicarbonate to prepare a sodium bicarbonate solution with a concentration of 9.6-10wt%;

[0064] The cooked material is fully ground into powder, and the obtained cooked material powder is placed in a sodium bicarbonate solution for ultrasonic-assisted alkali leaching, wherein the weight ratio of the cooked material powder to the sodium bicarbonate in the sodium bicarbonate solution is 1:0.48-0.50, and the ultrasonic power of the ultrasonic-assisted alkali leaching is above 400 W and the time is 30-40 min. After the ultrasonic-assisted alkali leaching is completed, the filtrate is filtered and collected, and a small amount of NaOH is added to the obtained filtrate to adjust the pH value to 12-13, so as to obtain solution A.

[0065] In step S2, the use of ultrasound-assisted alkali leaching can effectively shorten the alkali leaching time, and the alkali leaching time is shortened from 3 to 4 hours of ordinary alkali leaching to 30 to 40 minutes, which is beneficial to improving production efficiency.

[0066] Among them, in the process of ultrasonic-assisted alkaline leaching and subsequent pH adjustment, the chemical reaction formula involved is:

[0067] CaO+H2O=Ca(OH)2;

[0068] Al2O3+2HCO3 - =2AlO2 - +H2O+2CO2↑;

[0069] Al 3+ +4OH - =AlO2 - +2H2O.

[0070] S3. Coprecipitation

[0071] Calcium chloride is dissolved in water to prepare solution B with a calcium chloride concentration of 1.75 to 2.75 wt%.

[0072] Solution A and solution B are sprayed into a spray tower from nozzles in opposite directions, and CO2 hot flue gas is introduced into the deionized water at the bottom of the spray tower for carbonization. The introduction of CO2 hot flue gas is beneficial to the reduction of the particle size of the hydrotalcite powder. The volume ratio of solution A to deionized water is 2:1. The injection flow rates of solution A and solution B are both 1.8-2 m / h, the flow rate of the introduced CO2 hot flue gas is 0.08-0.12 m / s, and the temperature of the spray tower bottom is 70-90°C.

[0073] After carbonization is completed, the mixture is aged at 70-90°C for 2-3 hours, filtered, and the filtrate and filter cake are collected; the aging time is shortened to 2-3 hours compared to the traditional aging time of 8-10 hours, which is beneficial to improving production efficiency;

[0074] The chemical reactions involved in the spray tower include:

[0075] CO2+H2O=H2CO3;

[0076] 6Ca 2+ +2AlO2 - +8OH - +2HCO3 - +8H2O=Ca6Al2(OH) 16 (HCO3)2·4H2O;

[0077] The filter cake is dried at 60-80℃ to obtain a product containing a large amount of HCO3 2- Highly flame retardant ultrafine calcium aluminum hydrotalcite.

[0078] The filtrate is returned to step S2 and used as a solvent for dissolving sodium bicarbonate to prepare a sodium bicarbonate solution.

[0079] Among them, the specific structure of the spray tower used in the present invention is as follows Figure 1 As shown, the spray tower is a conventional spray tower, including a tower kettle, a discharge port and a first pipe for CO2 hot flue gas are arranged at the bottom of the tower kettle, the outlet of the first pipe in the tower kettle is close to the bottom of the tower kettle, a pH meter is arranged at the lower part of the tower kettle, and oppositely arranged nozzles are arranged at the top of the tower kettle. The nozzles have a spray plane, and water outlet holes are evenly distributed on the spray plane. The diameter of the water outlet holes is 2 mm, and the angle between the two spray planes is 75°.

[0080] S4. Application

[0081] Highly flame-retardant ultrafine calcium aluminum hydrotalcite was used as a flame retardant, and a PVC material sample was prepared in a conventional manner, wherein the addition amount of the highly flame-retardant ultrafine calcium aluminum hydrotalcite in the PVC material sample was 4%.

[0082] The highly flame-retardant ultrafine calcium-aluminum hydrotalcite prepared by the present invention contains a considerable amount of structural water in its structure, has bicarbonate between layers, and releases water and carbon dioxide when heated, thereby achieving the purpose of flame retardancy; after the water and carbon dioxide are released, the nano-scale hydrotalcite is transformed into a porous solid base with a relatively high specific surface area, which has a very strong adsorption effect on smoke and toxic acidic gases generated by combustion.

[0083] The invention introduces CO2 hot flue gas to significantly increase the bicarbonate content in the prepared highly flame-retardant ultrafine calcium aluminum hydrotalcite. The increase in bicarbonate can further improve the flame retardant properties of the highly flame-retardant ultrafine calcium aluminum hydrotalcite.

[0084] The particle size of the highly flame-retardant ultrafine calcium aluminum hydrotalcite prepared by the present invention is tested to be 0.5-1 μm, which is much lower than the particle size (6-7 μm) of calcium aluminum hydrotalcite prepared by the traditional co-precipitation method, and the lower particle size is conducive to its uniform dispersion in PVC.

[0085] The oxygen index of the PVC material sample made of ordinary calcium aluminum hydrotalcite is about 30%, and the smoke density level is 70. However, the oxygen index of the PVC material sample made of the same amount of highly flame retardant ultrafine calcium aluminum hydrotalcite prepared by the present invention can reach about 38-40%, and the smoke density level can be reduced to 62-65.

[0086] Example 1 A method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite

[0087] This embodiment is a method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite, which specifically includes the following steps:

[0088] S1. Ultra-low temperature calcination

[0089] The secondary salt mud produced as a by-product of the soda ash produced by the ammonia-soda process is dried and ground to obtain the soda ash secondary salt mud;

[0090] 400.00 g of fly ash, 80.00 g of soda ash secondary salt mud and 240.0 g of ammonium sulfate were mixed evenly, and the obtained mixed powder was put into a hollow paddle dryer and calcined at 300° C. for 2 h to obtain cooked material.

[0091] S2, ultrasonic assisted alkali leaching

[0092] Take sodium bicarbonate and add water to prepare a sodium bicarbonate solution with a concentration of 9.6wt%;

[0093] The cooked material was fully ground into powder, 500 g of the cooked material powder was placed in a beaker, 2500 mL of a 9.6 wt% sodium bicarbonate solution was added, and ultrasonic-assisted alkali leaching was performed for 30 min at an ultrasonic power of 400 W. After the ultrasonic-assisted alkali leaching was completed, the filtrate was filtered and collected, and 8 g of NaOH was added to the obtained filtrate to adjust the pH value to 12 to obtain solution A.

[0094] S3. Coprecipitation

[0095] Take 67.6 g of calcium chloride with a purity of 74% and dissolve it in 2500 mL of water to prepare solution B with a calcium chloride concentration of 1.95 wt%.

[0096] Solution A and solution B are sprayed into the spray tower from nozzles in opposite directions, and CO2 hot flue gas is introduced into the deionized water at the bottom of the spray tower for carbonization; wherein the volume ratio of solution A to deionized water is 2:1; the injection flow rates of solution A and solution B are both 2m / h, the flow rate of the introduced CO2 hot flue gas is 0.1m / s, and the bottom temperature of the spray tower is 70°C.

[0097] After carbonization, the mixture was aged at 70°C for 2 h, filtered, and the filtrate and filter cake were collected;

[0098] The filter cake is dried at 70℃ to obtain a product containing a large amount of HCO3 2- The high flame retardant ultrafine calcium aluminum hydrotalcite has an average particle size of 0.65μm. Figure 2 As shown, it can be seen that the highly flame-retardant ultrafine calcium aluminum hydrotalcite prepared in this embodiment has a lamellar structure, small thin sheets of hydrotalcite are gathered together, and the lamellar structures are connected to each other.

[0099] The filtrate is returned to step S2 and used as a solvent for dissolving sodium bicarbonate to prepare a sodium bicarbonate solution.

[0100] S4. Application

[0101] Highly flame-retardant ultrafine calcium aluminum hydrotalcite was used as a flame retardant, and a PVC material sample was prepared in a conventional manner, wherein the addition amount of the highly flame-retardant ultrafine calcium aluminum hydrotalcite in the PVC material sample was 4%.

[0102] The oxygen index of the PVC material sample prepared in this embodiment is 38.1%, and the smoke density level is reduced to 64.8.

[0103] Embodiments 2 to 5 A method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite

[0104] Examples 2 to 5 are respectively a method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite. The steps are basically the same as those in Example 1, and the only difference is the amount of raw materials and process parameters. See Table 1 for details:

[0105] Table 1 List of process parameters in Examples 2 to 5

[0106]

[0107]

[0108] The other parts of Examples 2 to 5 are the same as those of Example 1 and will not be described again.

[0109] Comparative Examples 1-2 A method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite

[0110] Comparative Examples 1 and 2 are respectively a method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite. The steps are basically the same as those in Example 1, and the raw materials used are also the same batch as those in Example 1. The only difference is that:

[0111] In step S3 of comparative example 1, the flow rate of the CO2 hot flue gas is 0.01 m / s, and the other steps and parameters are exactly the same as those of example 1, and a highly flame-retardant ultrafine calcium aluminum hydrotalcite is prepared, and the average particle size is 3.86 μm. The prepared highly flame-retardant ultrafine calcium aluminum hydrotalcite is then used to prepare a PVC material sample according to the method in step S4 of example 1. The oxygen index of the PVC material sample prepared in this comparative example is 36.5%, and the smoke density level is 68.2.

[0112] In step S3 of comparative example 2, the flow rate of the CO2 hot flue gas is 0.2 m / s, and the other steps and parameters are exactly the same as those of example 1, and a highly flame-retardant ultrafine calcium aluminum hydrotalcite is prepared, and the average particle size is 1.98 μm. The prepared highly flame-retardant ultrafine calcium aluminum hydrotalcite is then used to prepare a PVC material sample according to the method in step S4 of example 1. The oxygen index of the PVC material sample prepared in this comparative example is 34.2%, and the smoke density level is 69.5.

[0113] 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 ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Claims

1. A method for extracting aluminum from fly ash to prepare highly flame-retardant ultrafine calcium aluminum hydrotalcite, characterized in that: The method comprises the following steps: uniformly mixing fly ash, soda ash secondary salt mud and a low-temperature activator, calcining at ultra-low temperature, and then subjecting the obtained cooked material powder to ultrasonic-assisted alkali leaching and co-precipitation to obtain the highly flame-retardant ultrafine calcium aluminum hydrotalcite.

2. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 1, characterized in that: The method comprises the following specific steps: S1. Ultra-low temperature calcination After fly ash, soda ash secondary salt mud and low-temperature activator are mixed evenly, the obtained mixed powder is subjected to ultra-low temperature calcination to obtain cooked material; S2, ultrasonic assisted alkali leaching Grind the cooked material into powder, put the cooked material powder into sodium bicarbonate solution for ultrasonic-assisted alkali leaching, filter and collect the filtrate after the ultrasonic-assisted alkali leaching, adjust the pH value of the filtrate to obtain solution A; S3. Coprecipitation Take calcium chloride and dissolve it in water to make solution B; Solution A and solution B are sprayed into the spray tower in opposite directions, and hot CO2 flue gas is introduced into the deionized water at the bottom of the spray tower for carbonization; After carbonization is completed, it is aged and filtered, and the filtrate and filter cake are collected; The filter cake is dried to obtain the highly flame-retardant ultrafine calcium aluminum hydrotalcite; The filtrate is returned to step S2 and used as a solvent for dissolving sodium bicarbonate to prepare a sodium bicarbonate solution.

3. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2, characterized in that: In step S3, the flow rate of the CO2 hot flue gas is 0.08-0.12 m / s.

4. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to any one of claims 1 to 3, characterized in that: During the ultra-low temperature calcination process, the weight ratio of fly ash, soda ash secondary salt mud and low temperature activator is 1:0.2-0.3:0.6-0.7; The ultra-low temperature calcination temperature is 300-400°C and the time is more than 2 hours.

5. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to any one of claims 1 to 3, characterized in that: During the ultrasonic-assisted alkali leaching process, the ultrasonic power of the ultrasonic-assisted alkali leaching is above 400 W and the time is 30 to 40 minutes.

6. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2 or 3, characterized in that: In step S2, the concentration of the sodium bicarbonate solution is 9.6-10 wt %; The pH value of solution A is 12-13.

7. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2 or 3, characterized in that: In step S3, the concentration of calcium chloride in solution B is 1.75-2.75 wt %; The weight ratio of the cooked material powder used to prepare solution A to the calcium chloride in solution B is 1:0.08-0.

15.

8. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2 or 3, characterized in that: In step S3, the injection flow rates of solution A and solution B are both 1.8-2 m / h.

9. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2 or 3, characterized in that: In step S3, the temperature of the bottom of the spray tower is 70-90°C; The aging temperature is 70-90°C and the time is 2-3h; The drying temperature is 60-80°C.

10. The method for preparing highly flame-retardant ultrafine calcium aluminum hydrotalcite by extracting aluminum from fly ash according to claim 2 or 3, characterized in that: In step S3, the spray tower is a conventional spray tower, including a tower kettle, a discharge port and a first pipe for CO2 hot flue gas are provided at the bottom of the tower kettle, the outlet of the first pipe in the tower kettle is close to the bottom of the tower kettle, and nozzles arranged opposite to each other are provided at the top of the tower kettle. The nozzles have a spray plane, and water outlet holes are evenly distributed on the spray plane. The diameter of the water outlet holes is 2 mm, and the angle between the two spray planes is 75°.

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