Preparation method of fly ash solidified body suitable for high-temperature incineration environment

Abstract

The invention relates to the technical field of solid waste treatment and resource utilization, and discloses a preparation method of a fly ash solidified body suitable for a high-temperature incineration environment, and the raw materials of the solidified body comprise waste incineration fly ash, calcined hydrotalcite, inert refractory aggregate, metakaolin, alkali-activated liquid, deionized water, sodium carbonate and sodium hexametaphosphate. During preparation, the sodium carbonate and sodium hexametaphosphate aqueous solution is kneaded with the fly ash and the hydrotalcite, the hydrotalcite is reconstructed and adsorbs free chlorine in a weakly alkaline environment, and meanwhile, a polyphosphate protective layer is formed on the surface of a crystal; and then pumping a strong-alkalinity alkali excitation liquid for mixing, then adding metakaolin and inert refractory aggregate for mixing, and finally extruding, granulating and thermally curing. Component competitive reaction is avoided through a step-by-step mixing technology, crystal lattice locking of free chlorine and heavy metal at high temperature is achieved through intercalation adsorption of hydrotalcite and the packaging effect of a geopolymer three-dimensional network, and the high-temperature structural stability of a solidified body is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment and resource utilization technology, specifically a method for preparing fly ash solidified body suitable for high-temperature incineration environments. Background Technology

[0002] Municipal solid waste incineration generates large amounts of fly ash rich in soluble chlorides and heavy metals, which must be stabilized and solidified to prevent the migration of harmful substances with moisture and environmental pollution. Currently, conventional fly ash treatment technologies mostly employ silicate cement for physical encapsulation and chemical solidification. However, in high-temperature incineration applications such as cement kiln co-processing or high-temperature melting, the hydration products of conventional silicate cement are highly susceptible to dehydration, pyrolysis, and structural deterioration upon heating, leading to volume shrinkage and even cracking of the solidified matrix.

[0003] The destruction of the solidified structure not only causes the previously sealed heavy metals to be released again, but also leads to a large amount of free chloride ions accumulated inside the fly ash vaporizing and overflowing under high temperatures. The large amount of overflowing volatile chlorides will severely corrode high-temperature furnace equipment, affecting the continuous and stable operation of the incineration system, and is easily discharged with flue gas, causing secondary pollution. Existing conventional solidification technologies struggle to maintain the integrity of the matrix structure during high-temperature heating, and cannot simultaneously achieve efficient chloride ion locking and stable heavy metal preservation, failing to meet the practical engineering requirements for the safe disposal of fly ash in high-temperature environments.

[0004] Therefore, this invention proposes a method for preparing fly ash solidified body suitable for high-temperature incineration environments to overcome the shortcomings of existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing fly ash solidified bodies suitable for high-temperature incineration environments. This method solves the problem that conventional fly ash solidification methods are prone to structural degradation and pyrolysis under high-temperature incineration conditions, leading to the large-scale release of free chloride ions and heavy metals.

[0006] To address the above problems, the present invention provides the following technical solution:

[0007] A method for preparing solidified fly ash suitable for high-temperature incineration environments, wherein the raw materials for preparing the solidified fly ash include waste incineration fly ash, calcined hydrotalcite, inert refractory aggregate, metakaolin, alkali-activated liquid, deionized water, sodium carbonate, and sodium hexametaphosphate; the preparation method includes the following steps:

[0008] S1. Add deionized water, sodium carbonate and sodium hexametaphosphate into a biaxial high-strength kneader and stir until sodium carbonate and sodium hexametaphosphate are completely dissolved.

[0009] S2. Incineration fly ash and calcined hydrotalcite are fed into a dual-shaft high-strength kneader for pre-shearing and kneading to obtain plastic wet mud.

[0010] S3. Pump the alkali activation solution into the plastic wet mud at a uniform rate and continue shearing and mixing.

[0011] S4. The metakaolin and inert refractory aggregate are simultaneously and uniformly dropped into a twin-shaft high-strength kneader for continuous shearing and mixing to obtain a mixture.

[0012] S5. The mixture is fed into an extrusion granulator to be extruded into granules, and after constant temperature and humidity curing, fly ash solidification suitable for high-temperature incineration environment is obtained.

[0013] By employing the above technical solution, this invention relies on an extremely rigorous stepwise shearing and mixing process to control the reaction sequence, avoiding competitive reactions between components and constructing a solidified matrix capable of withstanding high temperatures. Specifically, in stages S1 and S2, sodium carbonate dissolves in deionized water, providing a weakly alkaline environment that promotes the absorption of moisture by calcined hydrotalcite to restore its layered bimetallic hydroxide structure. During this reconstruction, free chloride ions dissolved from waste incineration fly ash enter the hydrotalcite interlayer structure as compensating anions, resulting in the following intercalation reaction:

[0014] Mg 1-x Al x O 1+x / 2 +xCl - +(1+x / 2+m)H2O→Mg 1-x Al x (OH)2Cl x ·mH2O+xOH - ;

[0015] Where x represents the mole fraction of aluminum ions replacing magnesium ions, and its value ranges from 0.20 to 0.33; m represents the number of interlayer water molecules.

[0016] Along with the above reactions, sodium hexametaphosphate undergoes hydrolysis and dissociation. The resulting polyphosphate anions adhere to the surface of the reconstructed hydrotalcite crystals through electrostatic adsorption. Simultaneously, the polyphosphate anions coordinate with magnesium and aluminum ions on the surface, forming a dense polyphosphate interfacial protective layer. This protective layer can mitigate the leaching and erosion of the alumina framework in the layered structure of hydrotalcite by the subsequent strongly alkaline activation system. Subsequently, in stages S3 and S4, metakaolinite releases silicon-oxygen tetrahedral and aluminum-oxygen tetrahedral monomers under the depolymerization effect of the alkaline activation solution. These monomers further undergo condensation reactions to form an amorphous three-dimensional network polymer of aluminosilicates. This three-dimensional network structure not only binds and encapsulates the hydrotalcite crystals, fly ash particles, and inert refractory aggregates adsorbed with chloride ions, but also allows free heavy metal cations inside the fly ash to enter the three-dimensional framework of aluminosilicates through isomorphic substitution. Therefore, it effectively reduces the free chloride content of the fly ash system, prevents the formation of volatile chlorides at high temperatures, and maintains the structural integrity of the solidified body at high temperatures.

[0017] Preferably, the raw materials for preparation consist of the following components in parts by weight: waste incineration fly ash: 50-70 parts; calcined hydrotalcite: 3-8 parts; inert refractory aggregate: 120-220 parts; metakaolin: 25-45 parts; alkali-activated solution: 36-54 parts; deionized water: 15-25 parts; sodium carbonate: 1.0-2.0 parts; sodium hexametaphosphate: 0.3-0.8 parts.

[0018] By adopting the above technical solution, the raw and auxiliary materials achieved optimal chemical and physical synergy under specific mass ratios. This ensured both the sufficient adsorption and encapsulation of chloride ions by the hydrotalcite and the adequate encapsulation strength provided by the geopolymer matrix, achieving an optimal balance between curing cost and high-temperature stability.

[0019] Preferably, the alkaline activation solution is prepared from liquid sodium silicate and solid sodium hydroxide, and the total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution, calculated as oxides, is 1.4 to 1.8, and 36 to 54 parts of the alkaline activation solution contain 20 to 30 parts of deionized water.

[0020] By employing the above technical solution, the total molar ratio of silica to sodium oxide in the alkali-activated solution, calculated as oxides, is controlled within the range of 1.4 to 1.8. This ensures sufficient active silica-oxygen segments in the reaction system, thereby promoting cross-linking and polymerization of the active silica-oxygen segments with the alumina tetrahedra dissolved from metakaolin. Simultaneously, strictly limiting the water content ensures the solidified slurry possesses the flowability required for mixing and kneading operations, and avoids problems such as increased porosity and decreased mechanical strength in the final solidified body due to excessive water.

[0021] Preferably, the inert refractory aggregate is obtained by crushing waste high-alumina refractory bricks, the alumina mass fraction in the inert refractory aggregate is greater than or equal to 65wt%, the particle size distribution of the inert refractory aggregate is between 1.0mm and 3.0mm, and the fineness of the metakaolin is greater than or equal to 1000 mesh.

[0022] By adopting the above technical solution, since the alumina mass fraction in the aggregate is greater than or equal to 65 wt%, the aggregate itself does not undergo phase transformation shrinkage under high temperature conditions. Inert refractory aggregates with a particle size between 1.0 mm and 3.0 mm are introduced as rigid support nodes into the aluminosilicate matrix, interrupting the thermal stress transmission path within the matrix. This disperses the volume shrinkage stress of the polymer during the polymerization reaction and high-temperature heating stages, inhibiting the initiation of microcracks within the solidified body. Furthermore, metakaolin with a fineness greater than or equal to 1000 mesh possesses a large powder specific surface area, increasing the dissolution rate of metakaolin materials under strongly alkaline conditions.

[0023] Preferably, calcined hydrotalcite is prepared by the following steps:

[0024] The first step is to dissolve magnesium sulfate heptahydrate and aluminum sulfate octahydrate in deionized water to prepare a mixed salt solution. The molar ratio of magnesium ions to aluminum ions in the mixed salt solution is 3:1, and the total concentration of metal ions in the mixed salt solution is 1.0 to 1.5 mol / L.

[0025] The second step is to dissolve sodium hydroxide and sodium carbonate in deionized water to prepare an alkaline precipitant. The concentration of sodium hydroxide in the alkaline precipitant is 2.0 mol / L, and the concentration of sodium carbonate in the alkaline precipitant is 0.5 mol / L.

[0026] The third step involves simultaneously adding the mixed salt solution and alkaline precipitant dropwise into the reaction vessel under a constant temperature water bath of 60–65°C. The dropping rate is adjusted to maintain the pH value of the reaction system at 9.5–10.5, and the mixture is stirred and aged for 18–24 hours to obtain a suspension.

[0027] The fourth step is to filter the suspension and wash it with deionized water until the pH of the filtrate is 8.0-8.5, and then dry it for 24 hours to obtain the precursor powder.

[0028] The fifth step involves calcining the precursor powder at 500–550°C for 4–6 hours, followed by cooling to obtain calcined hydrotalcite.

[0029] By employing the above-mentioned technical solution and conducting a co-precipitation reaction using a mixed salt solution of a specific ratio under constant pH conditions, a magnesium-aluminum layered bimetallic hydroxide precursor with a fixed interlayer spacing can be synthesized. Subsequent intermediate-temperature calcination removes water molecules and carbonate anions from the precursor interlayers, causing the internal crystal structure of the precursor to collapse and transform into calcined hydrotalcite exhibiting amorphous solid solution characteristics. The constant control of crystallization temperature and precipitation pH during the preparation process increases the density of lattice defects generated in the material, thereby enhancing the rate of structural reconstruction reaction and the intercalation adsorption capacity for chloride ions in the calcined hydrotalcite.

[0030] Preferably, in step S1, the circulating cooling water jacket of the twin-shaft high-strength kneader is turned on, and the cooling water temperature of the circulating cooling water jacket is set to 15-20℃; in step S2, the kneading is continuously performed at a speed of 30-50 r / min for 15-20 min, and the core temperature of the plastic wet mud is controlled at 30-35℃ through the circulating cooling water jacket during the continuous kneading; in step S3, the pumped alkali activation liquid is pre-cooled to room temperature, and the material temperature inside the twin-shaft high-strength kneader is maintained at 30-35℃ for continuous shearing and mixing for 3-5 min; in step S4, metakaolin and inert refractory aggregate are simultaneously fed at a mass flow rate ratio of 25-45 to 120-220, and continuously sheared and mixed for 3-5 min.

[0031] By adopting the above technical solution, since both the structural reconstruction of hydrotalcite and the interfacial cross-linking reaction of polyphosphates are accompanied by heat release, the temperature of the plastic wet mud is controlled within the range of 30-35℃ using a circulating cooling water jacket. This not only maintains the thermodynamic conditions required for hydrotalcite crystal reconstruction but also prevents excessively high temperatures from causing deep hydrolysis of sodium hexametaphosphate and loss of its complexing and cross-linking ability. The set rotation speed and kneading time ensure that the solid particles achieve a uniform dispersion in the liquid phase, breaking down powder agglomeration and thus improving the contact adsorption efficiency of free chloride ions. An exothermic reaction occurs when the alkali-activated liquid is mixed with the plastic wet mud. Pre-cooling the alkali-activated liquid and controlling the material temperature can delay the initial setting time of the geopolymer gel, reserving sufficient time for subsequent mixing operations. By feeding metakaolin and inert refractory aggregate simultaneously at a specific flow rate ratio, the high-hardness inert refractory aggregate can act as a grinding medium in the biaxial meshing zone to perform in-situ shearing and grinding of the primary powder agglomerates generated by metakaolin after it comes into contact with alkaline solution. This accelerates the dissolution of amorphous silica and alumina on the surface of metakaolin and improves the uniformity of the geopolymer polymerization reaction.

[0032] Preferably, before step S2, the waste incineration fly ash is pretreated: untreated waste incineration fly ash with an initial free chloride ion mass fraction of 15wt% to 25wt% is mixed with deionized water at a liquid-to-solid ratio of 2 to 4 L / kg, mechanically stirred and washed for 30 to 60 minutes, and then solid-liquid separation is performed. The solid filter cake obtained from the solid-liquid separation is dried and crushed and passed through a 100-mesh sieve, and the residual chloride ion mass fraction of the pretreated waste incineration fly ash is controlled to be less than 2.0wt%.

[0033] By adopting the above technical solutions, the water washing and dechlorination pretreatment removes readily soluble chlorides in advance, reducing the chloride ion adsorption load in the subsequent hydrotalcite structure reconstruction stage. Controlling the residual chloride ion mass fraction to below 2.0 wt% ensures that the remaining free chloride ions are entirely within the theoretical intercalation adsorption capacity range of hydrotalcite, preventing excess free chloride ions from damaging the three-dimensional network structure of the geopolymer under the action of the alkaline activating solution or from vaporizing and overflowing at high temperatures. Controlling the particle size breaks up the hard agglomerates generated during the drying process, restoring the reactivity of the fly ash powder.

[0034] Preferably, before step S4, the inert refractory aggregate is pre-conditioned by soaking it completely in clean water for 24 hours, then draining or dehydrating it until there is no visible water reflection on the surface of the inert refractory aggregate, thus obtaining a saturated surface-dry aggregate with internal pores filled with water and no visible water reflection on the surface. The inert refractory aggregate that falls into the twin-shaft high-strength kneader in step S4 is a saturated surface-dry aggregate with internal pores filled with water and no visible water reflection on the surface.

[0035] By adopting the above technical solution, the state conditioning operation eliminates the disordered absorption of free water from the alkali-activating solution by the dry aggregate during the mixing process. The saturated surface-dry state of the aggregate, with its internal pores filled with water, prevents a localized decrease in the water-cement ratio of the geopolymer slurry during mixing, ensuring the fluidity and coating uniformity of the geopolymer slurry. Furthermore, the absence of visible water on the surface avoids the introduction of excess water into the aggregate, thus preventing dilution of the alkali-activating solution concentration at the interface. During the subsequent high-temperature curing stage, the water in the internal pores of the aggregate releases water through capillary action, providing an internal curing water source for the continuous hydration and three-dimensional network cross-linking of the surrounding geopolymer matrix, and reducing the self-shrinkage stress at the matrix interface.

[0036] Preferably, in step S5, the mixture is extruded by an extrusion granulator to prepare particles with an equivalent diameter of 15-30 mm; the constant temperature and humidity curing conditions are: the particles are tightly covered with an alkali-resistant film and continuously cured in a closed environment with a temperature of 58-62℃ and a relative humidity of 95% for 20-28 hours.

[0037] By employing the above technical solution, particles with an equivalent diameter of 15–30 mm are extruded, improving the stacking porosity of the particles in high-temperature processing equipment and ensuring the heat transfer efficiency of high-temperature flue gas. The particles are tightly covered with an alkali-resistant film and cured in a high-humidity, sealed environment at 58–62°C, suppressing excessively rapid evaporation of internal moisture and providing the thermal excitation energy required for the geopolymer condensation reaction. Constant temperature and humidity curing accelerates the three-dimensional cross-linking of the amorphous silicon-aluminum network, enabling the extruded particles to rapidly reach a stable mechanical property and chemical bonding state within 20–28 hours.

[0038] The present invention also provides a fly ash solidified body suitable for high-temperature incineration environment prepared by any of the above preparation methods.

[0039] By adopting the above technical solution, the fly ash solidified body inherits the unique microscopic interface protective layer and macroscopic refractory skeleton support, and has extremely excellent comprehensive performance in high temperature resistance to heavy metal release, resistance to free chlorine volatilization and thermal shock cracking, and has great prospects for industrial application.

[0040] This invention provides a method for preparing solidified fly ash suitable for high-temperature incineration environments. It has the following beneficial effects:

[0041] 1. This invention achieves effective locking of free chloride ions and heavy metals in fly ash under high-temperature conditions by introducing calcined hydrotalcite, sodium carbonate, and sodium hexametaphosphate. The weakly alkaline environment provided by sodium carbonate promotes the reconstruction and intercalation of calcined hydrotalcite to absorb free chloride ions. Subsequently, sodium hexametaphosphate cross-links on the surface of the hydrotalcite to form a polyphosphate protective layer, blocking the erosion of the hydrotalcite framework by the strong alkaline activating solution. Combined with the encapsulation effect of the amorphous aluminosilicate three-dimensional network generated by metakaolin polymerization on heavy metals, the solidified body can prevent the formation of volatile chlorides and the release of heavy metals in a high-temperature environment.

[0042] 2. The cured body of this invention possesses excellent high-temperature structural stability and crack resistance. By adding inert refractory aggregate with an alumina mass fraction greater than or equal to 65 wt% and a particle size distribution between 1.0 mm and 3.0 mm, and utilizing its characteristic of not undergoing phase transformation shrinkage under high-temperature conditions, rigid support nodes are constructed within the aluminosilicate matrix. These aggregate nodes interrupt the thermal stress transmission path within the matrix, disperse the volume shrinkage stress generated by the geopolymer gel during the polymerization reaction and high-temperature heating period, inhibit the initiation of internal microcracks, and maintain the overall mechanical structure of the cured body.

[0043] 3. This invention employs a stepwise shear mixing and aggregate condition adjustment process, ensuring the stability of the multi-component system reaction and the uniformity of the internal structure of the cured body. First, a weakly alkaline solution is used for hydrotalcite ion reconstruction and interface protection, followed by the introduction of a strongly alkaline activating liquid for geopolymer network construction. This sequential isolation of reactions avoids competitive reactions between components. The use of saturated surface-dry aggregates with internally filled pores and no visible water on the surface prevents the aggregates from absorbing free water from the slurry during mixing, maintaining a stable water-cement ratio. Furthermore, during the subsequent heat curing stage, the capillary release of water from the aggregates provides an internal curing water source, reducing the self-shrinkage stress at the interface of the cured matrix. Attached Figure Description

[0044] Figure 1 The following are the results of online rheological and thermodynamic monitoring of the mixing process of the present invention; wherein, (a) is a bar chart of the change of main shaft torque with mixing time, (b) is a bar chart of the change of pH value at the center of the material with mixing time, and (c) is a bar chart of the change of temperature at the center of the material with mixing time.

[0045] Figure 2 This is a comparison chart of the extrusion molding yield and physical defects of different samples of the present invention;

[0046] Figure 3 The results of the test on the synergistic sealing performance of chlorine and heavy metals under high temperature environment of the present invention are shown in the figure. Among them, (a) is a bar chart comparing the high temperature emission rates of free chlorine, lead and cadmium of different samples under the condition of calcination at 1000℃ for 2 hours, and (b) is a bar chart comparing the leaching concentration of lead element of different samples before and after calcination at 1000℃.

[0047] Figure 4 The figure shows the test results of residual mechanics and volumetric stress stability after high-temperature thermal shock according to the present invention; wherein, (a) is the bearing capacity retention rate curve of different samples after three 800℃ thermal shock cycles, and (b) is the comparison curve of sample mass loss rate after thermal shock cycles. Detailed Implementation

[0048] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0050] The fly ash from waste incineration is collected from the bag filter dust collector of municipal solid waste incineration power plant. Its initial free chloride ion mass fraction is 15% to 25%, and its main chemical components include silicon dioxide, aluminum oxide and free calcium oxide. The particle size distribution is between 0.1 μm and 50 μm.

[0051] Metakaolin (CAS No.: 92704-41-1) is obtained by calcining raw kaolin ore at 700℃ to 800℃ to remove hydroxyl groups. Its fineness is greater than or equal to 1000 mesh, and its main internal structure is an amorphous aluminosilicate network.

[0052] Inert refractory aggregate is obtained by mechanical crushing and screening of waste high-alumina refractory bricks. Its alumina mass fraction is greater than or equal to 65%, and the particle size distribution is between 1.0 mm and 3.0 mm.

[0053] Liquid sodium silicate (CAS No.: 1344-09-8) is in the form of an aqueous solution, with a molar ratio of silicon dioxide to sodium oxide of 1.4 to 1.8 and a solid content of 35% to 50%.

[0054] Sodium hexametaphosphate (CAS No.: 10124-56-8) has the molecular formula (NaPO3)6. It is a cyclic inorganic polymer composed of six phosphate groups, with a degree of polymerization of 6. It is a white crystalline powder with a purity greater than 99.0%.

[0055] Preparation Example 1: This preparation example provides a method for preparing calcined hydrotalcite, including the following steps:

[0056] (1) Dissolve magnesium sulfate heptahydrate and aluminum sulfate octahydrate in deionized water to prepare a mixed salt solution, wherein the molar ratio of magnesium ions to aluminum ions is 3:1 and the total concentration of metal ions is 1.25 mol / L;

[0057] (2) Dissolve sodium hydroxide and sodium carbonate in deionized water to prepare an alkaline precipitant, wherein the concentration of sodium hydroxide is 2.0 mol / L and the concentration of sodium carbonate is 0.5 mol / L;

[0058] (3) Turn on the reactor equipped with a stirring device and a constant temperature water bath, set the temperature of the constant temperature water bath to 62°C, and add the mixed salt solution and alkaline precipitant to the reactor simultaneously by a micro-tip pump. Adjust the dropping rate to keep the pH value of the reaction system at 10.0. After the dropping is completed, continue to stir and age at 62°C for 21 hours to obtain a suspension.

[0059] (4) Filter the suspension and wash the precipitate with deionized water until the pH of the filtrate is 8.0 to 8.5. Then place the precipitate in a 90°C drying oven and dry for 24 hours to obtain the precursor powder.

[0060] (5) The precursor powder was placed in a muffle furnace and calcined at a constant temperature of 525°C for 5 hours. After naturally cooling to room temperature, calcined hydrotalcite was obtained.

[0061] Preparation Example 2: This preparation example provides a method for preparing calcined hydrotalcite, including the following steps:

[0062] (1) Dissolve magnesium sulfate heptahydrate and aluminum sulfate octahydrate in deionized water to prepare a mixed salt solution, wherein the molar ratio of magnesium ions to aluminum ions is 3:1 and the total concentration of metal ions is 1.0 mol / L;

[0063] (2) Dissolve sodium hydroxide and sodium carbonate in deionized water to prepare an alkaline precipitant, wherein the concentration of sodium hydroxide is 2.0 mol / L and the concentration of sodium carbonate is 0.5 mol / L;

[0064] (3) Turn on the reactor equipped with a stirring device and a constant temperature water bath, set the temperature of the constant temperature water bath to 60°C, and add the mixed salt solution and alkaline precipitant to the reactor simultaneously by a micro-tip pump. Adjust the dropping rate to keep the pH value of the reaction system at 9.5. After the dropping is completed, continue to stir and age at 60°C for 18 hours to obtain a suspension.

[0065] (4) Filter the suspension and wash the precipitate with deionized water until the pH of the filtrate is 8.0 to 8.5. Then place the precipitate in a forced-air drying oven at 80°C and dry for 24 hours to obtain the precursor powder.

[0066] (5) The precursor powder was placed in a muffle furnace and calcined at a constant temperature of 500°C for 4 hours. After naturally cooling to room temperature, calcined hydrotalcite was obtained.

[0067] Preparation Example 3: This preparation example provides a method for preparing calcined hydrotalcite, including the following steps:

[0068] (1) Dissolve magnesium sulfate heptahydrate and aluminum sulfate octahydrate in deionized water to prepare a mixed salt solution, wherein the molar ratio of magnesium ions to aluminum ions is 3:1 and the total concentration of metal ions is 1.5 mol / L;

[0069] (2) Dissolve sodium hydroxide and sodium carbonate in deionized water to prepare an alkaline precipitant, wherein the concentration of sodium hydroxide is 2.0 mol / L and the concentration of sodium carbonate is 0.5 mol / L;

[0070] (3) Turn on the reactor equipped with a stirring device and a constant temperature water bath, set the temperature of the constant temperature water bath to 65°C, and add the mixed salt solution and alkaline precipitant to the reactor simultaneously by a micro-tip pump. Adjust the dropping rate to keep the pH value of the reaction system at 10.5. After the dropping is completed, continue to stir and age at 65°C for 24 hours to obtain a suspension.

[0071] (4) Filter the suspension and wash the precipitate with deionized water until the pH of the filtrate is 8.0 to 8.5. Then place the precipitate in a 100°C drying oven and dry for 24 hours to obtain the precursor powder.

[0072] (5) The precursor powder was placed in a muffle furnace and calcined at a constant temperature of 550°C for 6 hours. After naturally cooling to room temperature, calcined hydrotalcite was obtained.

[0073] Example 1: This example provides a method for preparing a fly ash solidified body suitable for high-temperature incineration environments, including the following steps:

[0074] (1) Take a sufficient amount of waste incineration fly ash and mix it with deionized water at a liquid-solid ratio of 3L / kg. Mechanically stir and wash at room temperature for 45 minutes, and then separate the solid and liquid through a plate and frame filter press. Dry the resulting solid filter cake at 105℃ and mechanically crush it to pass through a 100-mesh sieve. Accurately weigh 60 kg of the treated fly ash for later use. After this step, the soluble chloride ion removal rate of the fly ash is greater than 90%, and the residual chloride ion mass fraction is less than 2.0%.

[0075] (2) Soak 150 kg of inert refractory aggregate with a particle size distribution between 1.0 mm and 3.0 mm in clean water for 24 hours. After taking it out, place it on a ventilation net to drain naturally until there is no visible water reflection on the surface of the aggregate, and obtain saturated surface-dry aggregate. Seal it for later use.

[0076] (3) Turn on the twin-shaft high-power kneader with a circulating cooling water jacket and set the jacket cooling water temperature to 18°C; add 19 kg of deionized water, 1.5 kg of sodium carbonate and 0.5 kg of sodium hexametaphosphate to the kneader and turn on the stirring for 3 minutes until the solute is completely dissolved.

[0077] (4) Add 60 kg of fly ash weighed in step (1) and 5 kg of calcined hydrotalcite obtained in Preparation Example 1 to the above kneader and continue to shear and knead at a speed of 40 rpm for 18 minutes. During this period, the temperature of the material center is controlled at 32°C by the cooling jacket to obtain plastic wet mud.

[0078] (5) Keep the kneader speed at 40 rpm and the material temperature at 32°C. Pump 45 kg of pre-prepared and cooled to room temperature alkaline activation solution into the plastic wet mud through a high-pressure spray nozzle. The alkaline activation solution is prepared by liquid sodium silicate and solid sodium hydroxide. The total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution (i.e., the modulus of the alkaline activation solution) is 1.6. The 45 kg alkaline activation solution contains 25 kg of water. The uniform pumping process lasts for 4 minutes, during which synchronous shearing and kneading are maintained.

[0079] (6) Start the joint control system and simultaneously turn on the twin-screw loss-in-weight feeder at the bottom of the powder silo and the belt weighing feeder at the bottom of the wet hopper. Put 35 kg of metakaolin and 150 kg of saturated surface-dry aggregate prepared in step (2) into the twin-shaft meshing zone of the twin-shaft high-strength kneader at a mass flow rate ratio of 35:150 within 4 minutes. During this period, the kneader is continuously sheared and mixed at a speed of 40 rpm to obtain the mixture.

[0080] (7) The mixture obtained in step (6) is fed into a roller extrusion granulator and extruded into granules with an equivalent diameter of 20 mm. After being spread out and tightly covered with an alkali-resistant film, it is pushed into a constant temperature and humidity curing chamber and continuously heat-cured for 24 hours in a closed environment with a temperature of 60°C and a relative humidity of 95%, thus obtaining fly ash solidified body.

[0081] Example 2: This example provides a method for preparing a fly ash solidified body suitable for high-temperature incineration environments, including the following steps:

[0082] (1) Take a sufficient amount of waste incineration fly ash and mix it with deionized water at a liquid-solid ratio of 2L / kg. Mechanically stir and wash at room temperature for 30 minutes, and then separate the solid and liquid through a plate and frame filter press. Dry the resulting solid filter cake at 105℃ and mechanically crush it to pass through a 100-mesh sieve. Accurately weigh 50 kg of the treated fly ash for later use. After this step, the soluble chloride ion removal rate of the fly ash is greater than 90%, and the residual chloride ion mass fraction is less than 2.0%.

[0083] (2) 120 kg of inert refractory aggregate with a particle size distribution between 1.0 mm and 3.0 mm was completely soaked in clean water for 24 hours. After being taken out, it was placed in a centrifugal dewatering machine to remove surface moisture until there was no visible water reflection, and saturated surface-dry aggregate was obtained and sealed for later use.

[0084] (3) Turn on the twin-shaft high-power kneader with a circulating cooling water jacket and set the jacket cooling water temperature to 15°C; add 15 kg of deionized water, 1.0 kg of sodium carbonate and 0.3 kg of sodium hexametaphosphate to the kneader and turn on the stirring for 3 minutes until the solute is completely dissolved.

[0085] (4) Add 50 kg of fly ash weighed in step (1) and 3 kg of calcined hydrotalcite obtained in preparation example 2 to the above kneader, and continue to shear and knead at a speed of 30 rpm for 15 minutes. During this period, the temperature of the material center is controlled at 30°C by the cooling jacket to obtain plastic wet mud.

[0086] (5) Keep the kneader speed at 30 rpm and the material temperature at 30°C. Pump 36 kg of pre-prepared and cooled to room temperature alkaline activation solution into the plastic wet mud through a high-pressure nozzle. The alkaline activation solution is prepared by liquid sodium silicate and solid sodium hydroxide. The total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution (i.e., the modulus of the alkaline activation solution) is 1.4. The 36 kg alkaline activation solution contains 20 kg of water. The pumping process continues for 3 minutes, during which synchronous shearing and kneading are maintained.

[0087] (6) Start the joint control system and simultaneously turn on the twin-screw loss-in-weight feeder at the bottom of the powder silo and the belt weighing feeder at the bottom of the wet hopper. Put 25 kg of metakaolin and 120 kg of saturated surface-dry aggregate prepared in step (2) into the twin-shaft meshing zone of the twin-shaft high-strength kneader at a mass flow rate ratio of 25:120 within 3 minutes. During this period, the kneader is continuously sheared and mixed at a speed of 30 rpm to obtain the mixture.

[0088] (7) The mixture obtained in step (6) is fed into a roller extrusion granulator and extruded into granules with an equivalent diameter of 15 mm. After being spread out and tightly covered with an alkali-resistant film, it is pushed into a constant temperature and humidity curing chamber and continuously heat-cured for 20 hours in a closed environment with a temperature of 58°C and a relative humidity of 95% to obtain fly ash solidified body.

[0089] Example 3: This example provides a method for preparing a fly ash solidified body suitable for high-temperature incineration environments, including the following steps:

[0090] (1) Take a sufficient amount of waste incineration fly ash and mix it with deionized water at a liquid-solid ratio of 4L / kg. Mechanically stir and wash at room temperature for 60 minutes, and then perform solid-liquid separation through a plate and frame filter press. Dry the resulting solid filter cake at 105℃ and mechanically crush it to pass through a 100-mesh sieve. Accurately weigh 70 kg of the treated fly ash for later use. After this step, the soluble chloride ion removal rate of the fly ash is greater than 90%, and the residual chloride ion mass fraction is less than 2.0%.

[0091] (2) Soak 220 kg of inert refractory aggregate with a particle size distribution between 1.0 mm and 3.0 mm in clean water for 24 hours. After taking it out, place it on a ventilation net to drain naturally until there is no visible water reflection on the surface of the aggregate, and obtain saturated surface-dry aggregate. Seal it for later use.

[0092] (3) Turn on the twin-shaft high-power kneader with a circulating cooling water jacket and set the jacket cooling water temperature to 20°C; add 25 kg of deionized water, 2.0 kg of sodium carbonate and 0.8 kg of sodium hexametaphosphate to the kneader and turn on the stirring for 3 minutes until the solute is completely dissolved.

[0093] (4) Add 70 kg of fly ash weighed in step (1) and 8 kg of calcined hydrotalcite obtained in preparation example 3 to the above kneader and continue to shear and knead at a speed of 50 rpm for 20 minutes. During this period, the temperature of the material center is controlled at 35°C by the cooling jacket to obtain plastic wet mud.

[0094] (5) Keep the kneader speed at 50 rpm and the material temperature at 35°C. Pump 54 kg of pre-prepared and cooled to room temperature alkaline activation solution into the plastic wet mud through a high-pressure nozzle. The alkaline activation solution is prepared by liquid sodium silicate and solid sodium hydroxide. The total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution (i.e., the modulus of the alkaline activation solution) is 1.8. The 54 kg alkaline activation solution contains 30 kg of water. The pumping process continues for 5 minutes, during which synchronous shearing and kneading are maintained.

[0095] (6) Start the joint control system and simultaneously turn on the twin-screw loss-in-weight feeder at the bottom of the powder silo and the belt weighing feeder at the bottom of the wet hopper. Put 45 kg of metakaolin and 220 kg of saturated surface-dry aggregate prepared in step (2) into the twin-shaft meshing zone of the twin-shaft high-strength kneader at a mass flow rate ratio of 45 to 220 within 5 minutes. During this period, the kneader is continuously sheared and mixed at a speed of 50 rpm to obtain the mixture.

[0096] (7) The mixture obtained in step (6) is fed into a double roller extrusion granulator and extruded into granules with an equivalent diameter of 30 mm. After being spread out and tightly covered with an alkali-resistant film, it is pushed into a constant temperature and humidity curing chamber and continuously heat-cured for 28 hours in a closed environment with a temperature of 62°C and a relative humidity of 95% to obtain fly ash solidified body.

[0097] Example 4: This example provides a method for preparing a fly ash solidified body suitable for high-temperature incineration environments, including the following steps:

[0098] (1) Take a sufficient amount of waste incineration fly ash and mix it with deionized water at a liquid-solid ratio of 3L / kg. Mechanically stir and wash at room temperature for 40 minutes, and then perform solid-liquid separation through a plate and frame filter press. Dry the resulting solid filter cake at 105℃ and mechanically crush it to pass through a 100-mesh sieve. Accurately weigh 65 kg of the treated fly ash for later use. After this step, the soluble chloride ion removal rate of the fly ash is greater than 90%, and the residual chloride ion mass fraction is less than 2.0%.

[0099] (2) Soak 180 kg of inert refractory aggregate with a particle size distribution between 1.0 mm and 3.0 mm in clean water for 24 hours. After taking it out, place it on a ventilation net to drain naturally until there is no visible water reflection on the surface of the aggregate, and obtain saturated surface-dry aggregate. Seal it for later use.

[0100] (3) Turn on the twin-shaft high-power kneader with a circulating cooling water jacket and set the jacket cooling water temperature to 18°C; add 22 kg of deionized water, 1.2 kg of sodium carbonate and 0.6 kg of sodium hexametaphosphate to the kneader and turn on the stirring for 3 minutes until the solute is completely dissolved.

[0101] (4) Add 65 kg of fly ash weighed in step (1) and 6 kg of calcined hydrotalcite obtained in Preparation Example 1 to the above kneader and continue to shear and knead at a speed of 45 rpm for 16 minutes. During this period, the temperature of the material center is controlled at 33°C by the cooling jacket to obtain plastic wet mud.

[0102] (5) Keep the kneader speed at 45 rpm and the material temperature at 33°C. Pump 50 kg of pre-prepared and cooled to room temperature alkaline activation solution into the plastic wet mud through a high-pressure nozzle. The alkaline activation solution is prepared by liquid sodium silicate and solid sodium hydroxide. The total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution (i.e., the modulus of the alkaline activation solution) is 1.5. The 50 kg alkaline activation solution contains 28 kg of water. The uniform pumping process lasts for 4 minutes, during which synchronous shearing and kneading are maintained.

[0103] (6) Start the joint control system and simultaneously turn on the twin-screw loss-in-weight feeder at the bottom of the powder silo and the belt weighing feeder at the bottom of the wet hopper. Put 30 kg of metakaolin and 180 kg of saturated surface-dry aggregate prepared in step (2) into the twin-shaft meshing zone of the twin-shaft high-strength kneader at a mass flow rate ratio of 30 to 180 within 4 minutes. During this period, the kneader is continuously sheared and mixed at a speed of 45 rpm to obtain the mixture.

[0104] (7) The mixture obtained in step (6) is fed into a roller extrusion granulator and extruded into granules with an equivalent diameter of 25 mm. After being spread out and tightly covered with an alkali-resistant film, it is pushed into a constant temperature and humidity curing chamber and continuously heat-cured for 24 hours in a closed environment with a temperature of 60°C and a relative humidity of 95% to obtain fly ash solidified body.

[0105] Comparative Example 1:

[0106] Compared with Example 1, the difference is that sodium hexametaphosphate was not added in step (3), but the rest are the same.

[0107] Comparative Example 2:

[0108] Compared with Example 1, the difference lies in the change of the material addition sequence and mixing method. Specifically, the step-by-step feeding process of steps (3) to (6) is omitted. 19 kg of deionized water, 1.5 kg of sodium carbonate, 0.5 kg of sodium hexametaphosphate, 60 kg of fly ash, 5 kg of calcined hydrotalcite, 45 kg of alkali-activated liquid, 35 kg of metakaolin, and 150 kg of saturated surface-dry aggregate are mixed in a kneader in one go and continuously sheared and kneaded for 29 minutes. The subsequent granulation and curing operations are the same as in step (7).

[0109] Comparative Example 3:

[0110] Compared with Example 1, the difference is that in step (2), the inert refractory aggregate was not soaked in water and allowed to drain naturally, while in step (6), the inert refractory aggregate that was completely dried at room temperature was used directly. All other aspects are the same.

[0111] Comparative Example 4:

[0112] Compared with Example 1, the difference lies in the change of the synchronous feeding method in step (6). Specifically, 35 kg of metakaolin is first dropped into a twin-shaft high-strength kneader and sheared and mixed for 2 minutes, and then 150 kg of saturated surface-dry aggregate is added and sheared and mixed for another 2 minutes. The rest are the same.

[0113] Comparative Example 5:

[0114] Compared with Example 1, the difference is that during the operation of steps (3) to (5), the circulating cooling water jacket of the twin-shaft high-strength kneader is always closed and the temperature of the material center is not controlled. All other aspects are the same.

[0115] Comparative Example 6:

[0116] Compared with Example 1, the difference is that in step (7), the mixture was not fed into a roller extrusion granulator to prepare particles with an equivalent diameter of 20 mm. Instead, the mixture obtained in step (6) was directly poured into a cubic steel mold with an internal size of 150 mm × 150 mm × 150 mm. After the surface was tightly covered with an alkali-resistant film, it was pushed into a constant temperature and humidity curing room for 24 hours for heat curing. After demolding, fly ash solidified body was obtained. The rest were the same.

[0117] Test Example 1:

[0118] Test objective: To characterize the effects of different feeding sequences and temperature control conditions on the rheological dynamics and thermodynamic state of the mixture, and to verify the feasibility of the cold pre-kneading and aggregate carrier composite feeding mechanism.

[0119] The experimental steps are as follows:

[0120] Example 1, Comparative Example 2, Comparative Example 4 and Comparative Example 5 were selected as test objects. The mixing equipment was cleaned and the cooling jacket was confirmed to be in the corresponding set state.

[0121] A dynamic torque sensor is connected in series with the main shaft drive motor of the dual-shaft high-strength kneader to collect the output torque of the main shaft throughout the process, reflecting the rheological and viscous state of the material.

[0122] An online pH composite electrode equipped with an automatic cleaning device and a PT100 thermocouple are fixedly installed in the lower part of the kneader cavity. The probe insertion depth is adjusted so that its working end is in the center of the material in the meshing zone.

[0123] Start the equipment and run it according to the feeding sequence of each group. Control the online monitoring system to continuously record the data from the initial water addition to the final discharge at a sampling frequency of 1Hz, and export the torque, pH value and temperature parameters at different mixing stages.

[0124] The experimental results are shown in Table 1:

[0125] Table 1: Online monitoring data of the reaction system at different mixing time points

[0126]

[0127] in conclusion:

[0128] Based on the data in Table 1 and referring to the appendix Figure 1 The rheological state and thermodynamic evolution path of the mixed system are related to the timing of component introduction and temperature control methods. When monitoring torque changes, it was observed that in Example 1, after the simultaneous addition of metakaolin and aggregate at the 26th minute, the torque rose to 312.5 N·m and remained within a stable range. In Comparative Example 4, after the powder was added alone, the metakaolin surface, stimulated by a strong alkali, underwent a coagulation reaction, resulting in a resistance peak exceeding 800 N·m in torque. This could cause motor overload and prevent hydration of the powder. In Example 1, the simultaneous falling of saturated surface-dry aggregate and powder utilized the physical grinding action of the coarse aggregate in the biaxial region to break up the initial agglomerates, maintaining a rheologically stable state.

[0129] From the perspective of the chemical and thermodynamic environment of the system, Example 1 established a pH range maintained between 9.8 and 9.9 for the first 20 minutes. This weakly alkaline liquid environment met the conditions for the reconstruction of the hydrotalcite structure and the adsorption of free chlorine, and also provided time for sodium hexametaphosphate to self-assemble on the crystal surface to form a polyphosphate protective layer. Comparative Example 2 used a one-time mixing method, and the initial pH of the system reached 13.52. The strong alkaline environment bypassed the protective layer construction stage and destroyed the aluminum-oxygen octahedral framework of the hydrotalcite. Comparative Example 5, lacking external cooling, experienced heat accumulation. When the high-modulus alkaline solution entered the reaction zone, the exothermic effect and mechanical shear heat combined to raise its temperature to 71.5°C at 26 minutes. This temperature rise triggered the evaporation of free water and the early condensation of the geopolymer slurry, which was reflected in the data as the torque climbing to 442.1 N·m in the later stage. Example 1 controlled the temperature of the entire process below 32.5°C through a cold water jacket, slowing down the mass transfer kinetics rate and ensuring the slurry plasticity required for the subsequent extrusion granulation process.

[0130] Test Example 2:

[0131] Test objective: To evaluate the effect of aggregate moisture content on the rheological properties of high solids content geopolymer slurry and the final particle forming quality, and to verify the engineering feasibility of saturated surface-dry aggregate in the extrusion granulation process.

[0132] The experimental steps are as follows:

[0133] The total material discharged from the double-roller extrusion granulator in step (7) of Examples 1, 2, 3, 4 and Comparative Example 3 was selected as the test object. The discharged material was collected and weighed and recorded.

[0134] The collected material is sieved using a standard sieve with a set aperture to remove unformed fragments, powders, and large, agglomerated pieces. Regular particles that meet the equivalent diameter requirements are retained and weighed. The percentage of qualified particles out of the total feed is calculated to obtain the primary pelletizing pass rate.

[0135] Five hundred samples were randomly selected from the qualified particles in each group using the quartering method as the observation objects. The surface macromorphology and cross-sectional structure of the samples were observed by visual inspection and vernier calipers.

[0136] The number of particles with surface cracks exceeding 2 mm in length, localized powdering and peeling, or internal dry powder cores not encapsulated by the geopolymer gel was counted. This number was divided by the total sample size to calculate the physical defect rate.

[0137] The experimental results are shown in Table 2:

[0138] Table 2: Statistics on extrusion molding yield and physical defects of different samples

[0139]

[0140] in conclusion:

[0141] Based on the data in Table 2 and referring to the appendix Figure 2 The first-pass pelletizing success rate of Examples 1 to 4 ranged from 95.8% to 98.1%, with a corresponding physical defect rate of less than 2.3%. High-solids-content geopolymer slurries are highly sensitive to free water content; fluctuations in moisture content can alter the apparent viscosity of the material. The refractory aggregates used in the examples underwent pretreatment, resulting in internal pores filled with water and no visible water on the surface. When the aggregates were mixed with metakaolin and a strong alkali activating solution, they did not release water externally, nor did they absorb free water from the geopolymer gel. The internal moisture balance of the material maintained the plastic flowability of the mixture within the extruder, ensuring the integrity of the extruded particle structure.

[0142] The pelletizing pass rate of Comparative Example 3 decreased to 37.6%, and the screened particles exhibited numerous appearance defects. When dry aggregate was directly mixed into the system, the internal pores of the aggregate absorbed surrounding liquid moisture. This moisture migration reduced the water content of the slurry surrounding the aggregate, increasing the internal friction angle of the material. Due to the decreased material flowability, the operating resistance of the equipment increased when pushing through the die holes of the double-roll extruder. After the demolding stress was released, the extruded particles developed dry cracks extending into the interior. Furthermore, in some areas, insufficient moisture to support the hydration reaction prevented the formation of a continuous geopolymer gel network, exposing powder spalling and loosely structured dry powder cores upon dissection. Macroscopically, up to 62.4% of the unformed discharge in Comparative Example 3 was mostly loose powder or brittle fragments with no strength, and it caused significant equipment jamming and current overload during double-roll extrusion. The comparative experimental data reflect the influence of the aggregate's moisture content on the rheology and green body physical integrity of the extrusion pelletizing system.

[0143] Test Example 3:

[0144] Test objective: To determine the emission rate and leaching concentration of free chlorine and heavy metals under high temperature conditions of 1000℃, and to verify the influence of the sodium hexametaphosphate interface layer and specific feeding sequence on the lattice locking stability of the multiphase system.

[0145] The experimental steps are as follows:

[0146] The cured particles from Examples 1 to 4, Comparative Example 1 and Comparative Example 2, which had been cured for 28 days, were selected as test objects. They were crushed using a planetary ball mill and passed through a 150-mesh standard sieve to collect the sample powder.

[0147] Each powder sample, weighing 20.03 to 20.08 grams, was placed in a platinum crucible, which was then transferred to the heating zone of a horizontal tube resistance furnace. Under a continuous atmosphere of high-purity nitrogen, the furnace temperature was raised to 1000°C at a rate of 10°C per minute, and then calcined at the specified temperature for 2 hours.

[0148] During heating and constant-temperature roasting, the exhaust port of the tubular furnace is led out through a polytetrafluoroethylene pipeline and connected in series to a gas washing bottle group containing a mixed absorbent of 5% dilute nitric acid and hydrogen peroxide to continuously collect the tail gas volatilized from the reaction.

[0149] After calcination and cooling, the total chloride ion concentration in the tail gas absorption liquid was quantitatively analyzed using ion chromatography, while the lead and cadmium concentrations in the absorption liquid were determined using inductively coupled plasma mass spectrometry. The absolute mass of each element was converted to the initial element content in the original powder to obtain the corresponding high-temperature efflux rate.

[0150] According to the HJ / T 300 standard "Leaching Toxicity of Solid Waste - Acetic Acid Buffer Solution Method", the untreated powder and the residue after calcination at 1000℃ and natural cooling were extracted by tumbling and shaking. After filtration through a 0.45-micron filter membrane, the leaching concentrations of lead and cadmium were determined.

[0151] The experimental results are shown in Table 3:

[0152] Table 3: High-temperature efflux rate and leaching toxicity data of different samples

[0153]

[0154] in conclusion:

[0155] Based on the data in Table 3 and referring to the appendix Figure 3The high-temperature chloride ion evaporation rates in Examples 1 to 4 ranged from 1.15% to 1.42%, corresponding to low levels of lead and cadmium volatilization at 1000°C and leaching concentrations in the acidic buffer solution. Preliminary mechanism investigations revealed a strong correlation between the stability of the multiphase system at high temperatures and the chemical evolution pathway of the micro-interface at room temperature. Specifically, in the early cold kneading stage of the mixing process, the weakly alkaline liquid film constructed by free water and sodium carbonate facilitates the reconstructing of free chloride in fly ash into the layered bimetallic hydroxide lattice. Sodium hexametaphosphate molecules added to the system undergo chemical adsorption on the surface of the layered double hydroxide crystal, cross-linking and polymerizing to form a polyphosphate interface layer. When a high-modulus strong alkaline solution is pumped into the reaction system, the outer phosphate layer preferentially dissolves with the alkaline solution. The time difference in reaction kinetics delays the erosion of the aluminum-oxygen octahedral framework inside the layered double hydroxide by the high pH environment, allowing heavy metals and chloride ions to remain within the inorganic lattice under high-temperature heating conditions. The lead leaching concentrations before and after roasting (≤0.071 mg / L) in the examples were both lower than the lead leaching concentration limit (0.25 mg / L) specified in GB 18598 Hazardous Waste Landfill Pollution Control Standard, meeting environmental compliance requirements and high-temperature resistance indicators.

[0156] The test results of Comparative Example 1 without sodium hexametaphosphate showed that the high-temperature chloride ion escape rate increased to 45.62%, and the lead leaching concentration in the calcined residue was 4.82 mg / L. The lack of an external phosphate buffer layer caused the reconstructed hydrotalcite structure to undergo localized dissolution upon contact with a strong alkaline activating solution. Previously captured chloride ions and heavy metals were released again into the capillary pore liquid of the geopolymer gel. Elements in a free or weakly bound state vaporized as metal chlorides during tube furnace calcination and were discharged with the exhaust gas. Heavy metals remaining inside the matrix dissolved during oscillating leaching due to the lack of lattice binding.

[0157] In Comparative Example 2, which employed a one-step mixing process of powder and alkali solution, the initial pH of the reaction system exceeded 13. This high alkalinity environment hindered the crystal remodeling process of free chlorine adsorbed by hydrotalcite, preventing sodium hexametaphosphate from completing its self-assembly coating on the solid surface. The chloride ion volatilization rate in Comparative Example 2 reached 68.35%, and the lead leaching concentration increased to 8.35 mg / L after calcination. These comparative data, from the opposite perspective, verify the irreplaceable role of the pre-cold kneading step in constructing the chemical protective layer and maintaining the high-temperature lattice stability of the overall solidification system.

[0158] Test Example 4:

[0159] Test objective: To evaluate the structural integrity and compressive strength retention of the cured body under high-temperature thermal shock cycling conditions, and to verify the physical buffering effect of granulation size characteristics and cold-state pre-temperature control steps on the multiphase system's resistance to thermal expansion and phase transformation shrinkage stress.

[0160] The experimental steps are as follows:

[0161] The granulated particles of Examples 1 to 4, which were cured for 28 days, the granulated particles of Comparative Example 5, which were produced without opening the cooling jacket for temperature control, and Comparative Example 6, which used the same formula but was cast into a 150 mm cubic module, were selected as test objects.

[0162] The initial bearing capacity of each specimen was determined using a computer-controlled electronic universal testing machine at room temperature of 25°C. For the particle samples of Examples 1 to 4 and Comparative Example 5, 30 particles were randomly selected to test their single-particle crushing force and the average value was taken. For the large-size modular sample of Comparative Example 6, its cubic compressive strength was determined according to relevant specifications, and the mechanical reference data were recorded.

[0163] After weighing the initial mass of each group of samples, the samples were placed in a muffle furnace preheated to 800°C and kept at a constant temperature in air for 2 hours.

[0164] After the holding time is reached, the sample is removed from the furnace and placed directly in a room temperature environment to cool naturally to 25°C. The above heating and cooling process is repeated to complete a total of 3 thermal shock cycles.

[0165] After the thermal shock cycle, the residual material after natural cooling was sieved using a standard test sieve with a 2 mm aperture. The bottom layer debris and powder generated by thermal stress cracking and spalling were collected and accurately weighed. Simultaneously, the residual particles retained by the sieve, maintaining the integrity of the main structure, were also weighed. The physical spalling mass loss rate of the sample was calculated by dividing the mass of the spalling debris under the sieve by the total mass of the dry material over and under the sieve, thus eliminating the intrinsic weight loss interference caused by the chemical dehydration of the gel structure.

[0166] The macroscopic crack morphology of the main body surface of each sample is directly observed, and the mechanical data of the residual particles in the main body are measured according to the method in the second step to calculate the final load-bearing capacity retention rate.

[0167] The experimental results are shown in Table 4:

[0168] Table 4: Mechanical and physical spalling characteristics of different samples after three thermal shock cycles at 800℃

[0169]

[0170] in conclusion:

[0171] Based on the data in Table 4 and referring to the appendix Figure 4Different cured bodies with different molding sizes and temperature control histories exhibit varying resistance to structural degradation after undergoing 800°C thermal shock cycling. At 800°C, the hydrated aluminosilicate gel within the geopolymer undergoes free water evaporation and dehydration condensation of structural hydroxyl groups, leading to a gradual transformation of the matrix into a ceramic-like phase accompanied by significant volume shrinkage. The granulated particles from Examples 1 to 4 showed a physical spalling mass loss rate of only 1.8% to 3.2% after three thermal shock cycles, with a load-bearing capacity retention rate exceeding 80.8%. Only non-penetrating microcracks were observed on the particle surface. The granulation size of ten to thirty millimeters selected in these examples geometrically shortened the escape path of internal water vapor, reducing the vapor pressure accumulated within the pores. The rigid network constructed by the aggregate carrier within the matrix provided support, dispersing the shrinkage stress generated during the phase transformation process and the external thermal shock stress at the microscopic interface, mitigating macroscopic physical spalling caused by stress concentration.

[0172] Comparative Example 5, which lacked pre-cooling temperature control, exhibited structural deterioration during thermal shock cycling. Test records showed that its load-bearing capacity retention decreased to 16.5%, its physical spalling mass loss rate reached 24.6%, and a large amount of pulverized residue appeared after sieving. The lack of temperature control during the mixing stage led to early non-uniform condensation of the slurry, resulting in an internal gel network with initial micro-defects. Under alternating temperature shocks between 800℃ and room temperature, the initial micro-cracks propagated and interconnected, causing macroscopic spalling of the outer layer of the particles and a decrease in overall strength.

[0173] The test results of Comparative Example 6, with varying molding dimensions, reflect the failure mechanism of the macroscopic thermal stress gradient. The 150 mm cubic module collapsed and fractured during the first heating process, with its structural load-bearing capacity dropping to zero. When large-volume materials are rapidly heated, a significant temperature gradient arises between the surface and the core region, with the exterior expanding while the interior remains in a relatively cool, contracting state. Water vapor generated during internal chemical dehydration cannot escape in time due to the long mass transfer path, forming high pressure in the core region. The superposition of thermal stress and internal vapor pressure exceeds the tensile limit of the geopolymer matrix during its phase transition. The test data demonstrate that the miniaturized molding size, combined with the internal aggregate network, is the physical basis for maintaining the mechanical stability of the multiphase system under conditions of severe temperature fluctuations.

[0174] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing solidified fly ash suitable for high-temperature incineration environments, characterized in that, The raw materials for preparing the solidified fly ash include waste incineration fly ash, calcined hydrotalcite, inert refractory aggregate, metakaolin, alkali-activated liquid, deionized water, sodium carbonate, and sodium hexametaphosphate; the preparation method includes the following steps: S1. Add deionized water, sodium carbonate and sodium hexametaphosphate to a biaxial high-strength kneader and stir until the sodium carbonate and sodium hexametaphosphate are completely dissolved. S2. In the dual-shaft high-strength kneader, incineration fly ash and calcined hydrotalcite are added to the pre-shearing kneading process to obtain plastic wet mud. S3. Pump the alkali activation solution into the plastic wet mud at a uniform rate and continue shearing and mixing. S4. The metakaolin and inert refractory aggregate are simultaneously and uniformly fed into the twin-shaft high-strength kneader for continuous shearing and mixing to obtain a mixture. S5. The mixture is fed into an extrusion granulator to be extruded into granules, and after constant temperature and humidity curing, the fly ash solidified body suitable for high-temperature incineration environment is obtained.

2. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, The raw materials for preparation consist of the following components in parts by weight: Waste incineration fly ash: 50-70 parts; Calcined hydrotalcite: 3-8 parts; Inert refractory aggregate: 120-220 parts; Metakaolin: 25-45 parts; Alkali activation solution: 36-54 parts; Deionized water: 15-25 parts; Sodium carbonate: 1.0–2.0 parts; Sodium hexametaphosphate: 0.3–0.8 parts.

3. The method for preparing fly ash solidified body suitable for high-temperature incineration environments according to claim 2, characterized in that, The alkaline activation solution is prepared from liquid sodium silicate and solid sodium hydroxide. The total molar ratio of silicon dioxide to sodium oxide in the alkaline activation solution, calculated as oxides, is 1.4 to 1.8, and 36 to 54 parts of the alkaline activation solution contain 20 to 30 parts of deionized water.

4. The method for preparing fly ash solidified body suitable for high-temperature incineration environments according to claim 1, characterized in that, The inert refractory aggregate is obtained by crushing waste high-alumina refractory bricks. The alumina mass fraction in the inert refractory aggregate is greater than or equal to 65 wt%, and the particle size distribution of the inert refractory aggregate is between 1.0 mm and 3.0 mm. The fineness of the metakaolin is greater than or equal to 1000 mesh.

5. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, The calcined hydrotalcite is prepared by the following steps: The first step is to dissolve magnesium sulfate heptahydrate and aluminum sulfate octahydrate in deionized water to prepare a mixed salt solution. The molar ratio of magnesium ions to aluminum ions in the mixed salt solution is 3:1, and the total concentration of metal ions in the mixed salt solution is 1.0 to 1.5 mol / L. The second step involves dissolving sodium hydroxide and sodium carbonate in deionized water to prepare an alkaline precipitant, wherein the concentration of sodium hydroxide in the alkaline precipitant is 2.0 mol / L and the concentration of sodium carbonate in the alkaline precipitant is 0.5 mol / L. The third step involves simultaneously adding the mixed salt solution and the alkaline precipitant dropwise into the reaction vessel under a constant temperature water bath of 60–65°C. The dropping rate is adjusted to maintain the pH value of the reaction system at 9.5–10.5, and the mixture is continuously stirred and aged for 18–24 hours to obtain a suspension. The fourth step is to filter the suspension and wash it with deionized water until the pH of the filtrate is 8.0-8.5, and then dry it for 24 hours to obtain the precursor powder. The fifth step involves calcining the precursor powder at 500–550°C for 4–6 hours, followed by cooling to obtain the calcined hydrotalcite.

6. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, In step S1, the circulating cooling water jacket of the biaxial high-strength kneader is turned on, and the cooling water temperature of the circulating cooling water jacket is set to 15-20℃. In step S2, the kneading is continuously performed at a speed of 30-50 r / min for 15-20 min, and the core temperature of the plastic wet mud is controlled at 30-35℃ through the circulating cooling water jacket during the continuous kneading. In step S3, the alkali-activated liquid pumped in is pre-cooled to room temperature, and the material temperature inside the biaxial high-strength kneader is maintained at 30-35℃ for continuous shearing and mixing for 3-5 min. In step S4, the metakaolin and the inert refractory aggregate are simultaneously fed at a mass flow rate ratio of 25-45 to 120-220, and continuously sheared and mixed for 3-5 min.

7. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, Before step S2, the waste incineration fly ash is pretreated: untreated waste incineration fly ash with an initial free chloride ion mass fraction of 15wt% to 25wt% is mixed with deionized water at a liquid-to-solid ratio of 2 to 4 L / kg, mechanically stirred and washed for 30 to 60 minutes, and then solid-liquid separation is performed. The solid filter cake obtained from the solid-liquid separation is dried and crushed and passed through a 100-mesh sieve to control the residual chloride ion mass fraction of the pretreated waste incineration fly ash to be less than 2.0wt%.

8. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, Before step S4, the inert refractory aggregate is pre-conditioned by immersing it completely in clean water for 24 hours, then draining or dehydrating it until there is no visible water reflection on the surface of the inert refractory aggregate, thus obtaining a saturated surface-dry aggregate with internal pores filled with water and no visible water reflection on the surface. The inert refractory aggregate that falls into the biaxial high-strength kneader in step S4 is the saturated surface-dry aggregate with internal pores filled with water and no visible water reflection on the surface.

9. The method for preparing fly ash solidified body suitable for high-temperature incineration environment according to claim 1, characterized in that, In step S5, the mixture is extruded by the extrusion granulator to prepare particles with an equivalent diameter of 15-30 mm; the constant temperature and humidity curing conditions are as follows: the particles are tightly covered with an alkali-resistant film and continuously cured in a closed environment with a temperature of 58-62℃ and a relative humidity of 95% for 20-28 hours.

10. A fly ash solidified body suitable for high-temperature incineration environment, prepared by the preparation method according to any one of claims 1-9.

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