Method for synthesizing sodalite by taking kaolin as raw material in specific atmosphere through step-by-step heat treatment in specific atmosphere

Abstract

The invention discloses a method for synthesizing sodalite by taking kaolin as a raw material in a specific atmosphere through step-by-step heat treatment and application of the sodalite. The method comprises the following steps: mixing kaolin with an alkali activator, a structure-directing agent (such as chloride or sulfate) and a water-containing medium to form slurry; then in the atmosphere containing inert gas, water vapor, oxygen and optional reducing gas, pretreatment is conducted for a certain time at the low temperature (such as 80-250 DEG C), then the temperature is increased to 700-750 DEG C at the temperature increasing rate of 10-25 DEG C / min, and a reaction is conducted to generate sodalite crystals. The prepared sodalite is high in purity (greater than 85%), can be used as a stable curing carrier, effectively contains radionuclides such as Cs < + > and Sr < 2 + > through ion exchange and lattice curing, and has good curing performance.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic material preparation technology, and in particular to a method for synthesizing sodalite by stepwise heat treatment of kaolinite under a specific atmosphere. Sodalite can be used as a nuclide containment matrix, etc. Background Technology

[0002] Sodalite is an aluminosilicate mineral with a cage-like β-cage structure, typically represented by the ideal chemical formula Na8(AlSiO4)6Cl2. Its unique structure endows sodalite with excellent ion exchange and adsorption properties, making it suitable for applications in catalysis, gas separation, and the adsorption of specific ions (such as the radioactive nuclide Cs). + 、Sr 2+ and anion Cl - It has shown application potential in the inclusion and solidification of nuclear waste, and has attracted attention, especially in the field of safe disposal of nuclear waste.

[0003] Currently, the main methods for synthesizing sodalite include hydrothermal synthesis, high-temperature solid-state sintering, and sol-gel methods.

[0004] Hydrothermal synthesis is a common method for preparing zeolite materials, but it typically suffers from long reaction times, high energy consumption, high requirements for equipment pressure resistance, and the potential generation of alkaline wastewater, making it unsuitable for large-scale production. High-temperature solid-state sintering, while relatively simple, often requires extremely high reaction temperatures (e.g., above 800℃), resulting in huge energy consumption. Furthermore, the product is prone to melting or forming non-target crystalline phases (such as nepheline), making it difficult to control product uniformity and purity. The sol-gel method can prepare high-purity sodalite under relatively mild conditions, but its raw material costs (such as organosilicon sources and aluminum alkoxides) are usually high, and the process steps are relatively cumbersome, limiting its application in low-cost, large-scale production.

[0005] Kaolin, an abundant and inexpensive natural aluminosilicate mineral, is an ideal source of silicon and aluminum for synthesizing aluminosilicate functional materials such as sodalite. Existing research on the synthesis of sodalite using kaolin (or its heat-treated product metakaolin) typically involves the activation treatment of kaolin and its reaction with alkalis and salts under hydrothermal or high-temperature conditions. However, these kaolin-based synthetic methods still face the following challenges:

[0006] First, some methods still require high reaction temperatures or long reaction times to obtain high-purity sodalite, affecting economic efficiency. Second, the purity of the sodalite in the product is sometimes difficult to guarantee, often accompanied by the formation of unreacted raw materials, intermediate phases, or other zeolite phases (such as zeolite A, nepheline, etc.), affecting the performance of the final product. Research on the precise control of the reaction atmosphere and its influence on kaolin conversion and sodalite crystal phase formation, as well as on improving the reaction process and product quality through optimizing the heating program (such as stepwise heating and controlling the heating rate), is still insufficient and lacks depth and systematicity. In particular, under atmospheric or near-atmospheric pressure conditions, how to effectively coordinate the effects of the atmosphere and heat treatment program to promote the reconstruction of the kaolin framework and the directional formation of high-purity sodalite remains a technical challenge. Summary of the Invention

[0007] To address the problems of high energy consumption, high cost, harsh reaction conditions, and difficulty in controlling product purity in existing sodalite synthesis methods, especially when using inexpensive kaolin as a raw material to synthesize sodalite, which still faces technical challenges such as high reaction temperature, long reaction time, low product purity, and insufficient research on the synergistic effect of atmosphere and heat treatment procedures, this invention aims to provide a method for synthesizing sodalite using kaolin as a raw material through stepwise heat treatment under a specific atmosphere and its application. This method can efficiently synthesize high-purity sodalite under relatively mild conditions.

[0008] To achieve the above objectives, the present invention provides a method for synthesizing sodalite by stepwise heat treatment of kaolin as raw material under a specific atmosphere, comprising the following steps:

[0009] (1) Kaolin, alkali activator and structure directing agent are mixed to obtain a solid mixture;

[0010] (2) Add an aqueous medium to the solid mixture obtained in step (1) to form a slurry;

[0011] (3) The slurry obtained in step (2) is placed in a heating device and heat-treated under a specific atmosphere, wherein the specific atmosphere includes inert gas, water vapor and oxygen;

[0012] The heat treatment includes: first heating to the pretreatment temperature (80-250℃) at a heating rate of 10-25℃ / min and holding the temperature for a period of time; then continuing to heat to the final reaction temperature (700-750℃) at a heating rate of 10-25℃ / min and holding the temperature at this reaction temperature for 0.5-3 hours, during which the kaolinite undergoes aluminosilicate framework structure reconstruction to generate sodalite crystals.

[0013] As a preferred embodiment of the present invention, the structure guiding agent in step (1) is selected from at least one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate.

[0014] In a preferred embodiment of the present invention, the alkali activator is selected from one or more strong alkalis such as sodium hydroxide, potassium hydroxide, and calcium hydroxide.

[0015] As a preferred embodiment of the present invention, in step (1), kaolin, alkali activator and structure directing agent are mixed in a mass ratio of 1:(0.3-0.5):(0.1-0.2) to obtain a solid mixture; preferably, ammonium chloride is added at the same time or after mixing kaolin, sodium hydroxide and structure directing agent, wherein the mass ratio of ammonium chloride to kaolin is (0.05-0.15):1.

[0016] As a preferred embodiment of the present invention, the aqueous medium in step (2) is deionized water or ammonia solution, and the mass ratio of the aqueous medium to the solid mixture is (1.5-3.0):1.

[0017] As a preferred embodiment of the present invention, the specific atmosphere in step (3) optionally further comprises one or more reducing gases selected from CO and NH3; when the reducing gas is NH3, its volume fraction is 1-5.5%; the volume fraction of the water vapor is 40-60%; the volume fraction of the oxygen is 3-10%, and the remainder is nitrogen or an inert gas. Alternatively, when the reducing gas is CO, its volume fraction is 0.5-5%; the volume fraction of the water vapor is 40-60%; the volume fraction of the oxygen is 3-10%, and the remainder is mainly nitrogen or an inert gas.

[0018] As a preferred embodiment of the present invention, the pretreatment temperature in step (3) is preferably 180°C, the heating rate before reaching the pretreatment temperature is 10-25°C / min, and the constant temperature time after reaching the pretreatment temperature is 0.5-3.0 hours.

[0019] As a preferred embodiment of the present invention, the heat treatment in step (3) involves loading the slurry into a container and the reaction pressure being atmospheric pressure to 1 MPa.

[0020] The present invention also provides a sodalite synthesized by any of the foregoing methods, wherein X-ray diffraction analysis shows that the mass fraction of the sodalite phase is greater than 85%.

[0021] This invention further provides the application of sodalite as a stable solidification carrier in the encapsulation of radionuclides through ion exchange and lattice solidification, wherein the radionuclides include Cs. + 、Sr 2+ Co 2+ At least one of a cation or a chlorine-containing radioactive anion.

[0022] Furthermore, when the sodalite is used as the containing matrix for the nuclide, the molar percentage of radioactive cations replacing sodium ions in the sodalite crystal is 5-35%, and the lattice distortion rate is less than 2%; or, when the nuclide contains cesium ions, the 28-day standard leaching rate of cesium ions by the sodalite is less than 5 × 10⁻⁶. -4 g / (m 2 ·d).

[0023] The method of this invention uses inexpensive and readily available kaolin as the main raw material. Combined with a specific atmosphere and an optimized stepwise heat treatment procedure, the reaction can be carried out under relatively mild conditions (such as a final reaction temperature of 700-750℃, at atmospheric or near atmospheric pressure), which effectively reduces production energy consumption and equipment requirements, and is conducive to achieving low-cost synthesis of sodalite.

[0024] By precisely controlling the raw material ratio, reaction atmosphere (including inert gas, water vapor, oxygen, and optional reducing gases such as CO or NH3), and stepwise heat treatment regime (including the temperature and time of the pretreatment stage and the final reaction stage), this invention can significantly improve the purity of sodalite, making the mass fraction of sodalite phase greater than 85%, while effectively suppressing the formation of by-products.

[0025] The sodalite prepared in this invention is effective against specific radionuclides (such as Cs). + 、Sr 2+ (e.g., etc.) exhibits good containment performance and chemical stability, and can efficiently replace nuclides into its cage structure through ion exchange and stabilize and solidify them through the crystal lattice. Therefore, it has a low leaching rate and a considerable molar percentage of nuclide solidification, making it a promising candidate for stable solidification carriers in fields such as high-level radioactive waste liquid solidification and geological disposal of nuclear waste.

[0026] This invention provides a new technical approach for the low-temperature and efficient conversion of kaolin into high-purity sodalite through the synergistic effect of introducing a specific atmosphere and stepwise heat treatment, and offers a useful reference for the conversion and utilization of similar aluminosilicate minerals. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the experimental apparatus used for synthesizing sodalite in an embodiment of the present invention;

[0028] The attached figures are labeled as follows: 1-Air cylinder; 2-Air rotor flow meter; 3-Nitrogen cylinder; 4-Nitrogen rotor flow meter; 5-Water storage tank; 6-Metering pump; 7-Evaporator; 8-Homemade steam generator; 9-Heating equipment; 10-Sample boat; 11-Gas washing bottle. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining this invention and are not intended to limit the scope of protection of this invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0030] The core of this invention lies in achieving efficient and low-cost synthesis of high-purity sodalite from kaolin under mild conditions of atmospheric or near-atmospheric pressure by precisely and synergistically controlling the reaction atmosphere and stepwise heat treatment process. (Refer to...) Figure 1 This document illustrates a schematic diagram of an apparatus used in this invention to synthesize sodalite through stepwise thermal treatment of kaolinite under a specific atmosphere. The apparatus mainly includes a gas supply system (air cylinder 1 and nitrogen cylinder 3 in this embodiment), a gas flow control system (air rotor flowmeter 2 and nitrogen rotor flowmeter 4 in this embodiment), a liquid supply and evaporation system, i.e., a self-made steam generator (in this embodiment, a self-made steam generator 8 composed of a water storage tank 5, a metering pump 6, and an evaporator 7), a main reaction device (heating device 9, such as a tubular furnace in this embodiment), a sample carrier device (sample boat 10 in this embodiment), and a tail gas treatment system (gas washing bottle 11 in this embodiment). During the reaction, the gas from the cylinders mixes and vaporizes with the liquid (water or ammonia) precisely delivered by the metering pump in the evaporator, forming a reaction atmosphere with specific components, which is then introduced into the tubular furnace to react with the sample.

[0031] The specific steps and principles of the method of this invention are described below:

[0032] (1) Raw material mixing and slurry preparation: Kaolin as the silicon-aluminum source, sodium hydroxide (or other strong alkali) as the alkali activator, and salts (such as sodium chloride, sodium sulfate, etc.) as structure directing agents are mixed to obtain a solid mixture. The role of the alkali activator is to destroy the stable layered silicon-oxygen tetrahedron and aluminum-oxygen octahedron structure of kaolin during heat treatment, forming an active intermediate. The anions (such as Cl-, SO42-) in the structure directing agent will enter the β-cage structure center of sodalite, playing a role in stabilizing the crystal lattice and template. Subsequently, an aqueous medium (such as deionized water or ammonia water) is added to form a slurry, which can ensure uniform mixing of materials and provide the necessary hydrothermal microenvironment in subsequent heat treatment, promoting ion migration and phase transformation.

[0033] Preferably, in the above steps, kaolin, alkali activator, and structure directing agent are mixed at a mass ratio of 1:(0.3-0.5):(0.1-0.2) to obtain a solid mixture. Preferably, ammonium chloride is added simultaneously or after mixing kaolin, sodium hydroxide, and the structure directing agent, with the mass ratio of ammonium chloride to kaolin being (0.05-0.15):1. During heat treatment, ammonium chloride decomposes upon heating, generating ammonia and hydrogen chloride in situ. The generated ammonia creates a local atmosphere favorable for the formation of sodalite crystals in the material gaps, while the generated hydrogen chloride reacts with the alkali activator to supplement the formation of structure directing agent chloride, thereby synergistically promoting the synthesis of high-purity sodalite.

[0034] (2) Stepwise heat treatment under a specific atmosphere: This is the key step of the present invention.

[0035] Specific Atmosphere: Water vapor (preferably 40-60% by volume) is crucial for achieving the low-temperature reaction. It creates a "hydrothermal" environment at atmospheric pressure, significantly reducing the temperature required for the structural reconstruction of kaolin to form sodalite. The presence of oxygen (preferably 3-10%) may help regulate the redox potential of the system and inhibit the formation of certain byproducts. Optionally, the introduction of reducing gases, such as NH3 or CO, can further optimize the reaction environment; as shown in the examples, the introduction of NH3 significantly improved the purity of sodalite. The remainder is nitrogen or other inert gases, serving as balancing and carrier gases.

[0036] Stepwise heat treatment: This invention employs a two-step heating mode of "pretreatment + reaction". First, the material is heated to a lower pretreatment temperature (80-250℃, preferably 180℃) at a certain heating rate (10-25℃ / min) and held at this temperature for a period of time (0.5-3.0 hours). The purpose of this stage is to allow the kaolinite to orderly remove structural water and transform into metakaolinite, and to undergo a preliminary pre-mineralization reaction with alkali and water vapor to form an ordered reaction precursor. This step avoids material inhomogeneity and disordered reconstruction of amorphous phases caused by rapid heating, laying the foundation for the subsequent directional generation of high-purity sodalite. Subsequently, the temperature is increased to the final reaction temperature (700-750℃) at a heating rate of 10-25℃ / min and held at this temperature for 0.5-3 hours. At this time, the active precursor formed by the pretreatment can be rapidly and efficiently reconstructed into the target sodalite crystals.

[0037] Through the synergistic effect of the above technical solutions, this invention overcomes the shortcomings of traditional methods that rely on high temperature and high pressure and have low product purity, and provides an effective technical path for the large-scale and low-cost preparation of sodalite.

[0038] The kaolin used in this embodiment is #1 chiral powder kaolin produced by Yangshan Kaolin Company. Its main chemical components were determined by X-ray fluorescence spectroscopy (XRF) analysis, and the results are shown in Table 1.

[0039] Table 1 shows the main component content of the kaolin used.

[0040]

[0041] Example 1

[0042] This embodiment employs the basic technical solution of the present invention. This embodiment uses a step-by-step heat treatment process while maintaining consistent reaction atmosphere and other conditions, aiming to demonstrate the crucial role of the step-by-step heat treatment process in the present invention.

[0043] (1) Raw material preparation and mixing: Weigh 1.0g of #1 chiral powdered kaolin, weigh 0.5g of powdered sodium hydroxide (NaOH) as an alkali activator, and weigh 0.15g of sodium chloride as a structure directing agent. Grind the three together in an agate mortar in a mass ratio of 1:0.5:0.15.

[0044] (2) Slurry preparation: Add 2.8g of deionized water to the above solid mixture, stir evenly to form a slurry, and transfer it to a quartz boat.

[0045] (3) Preparation of reaction apparatus and atmosphere control: The sample boat loaded with slurry is placed into the reaction apparatus. Figure 1 In the heating apparatus (tube furnace) shown by label 9, a mixed atmosphere of nitrogen, steam, and air is introduced, wherein the volume ratio of N2:Steam:Air is approximately 18.3:48:33.7, the total gas flow rate is approximately 300 mL / min, and the reaction is carried out at atmospheric pressure.

[0046] (4) Stepwise heat treatment reaction: Set the heating program of the heating equipment. First, heat from the initial room temperature to the pretreatment temperature of 180°C at a heating rate of 10°C / min, and hold at this temperature for 2 hours. Then, continue heating at a heating rate of 10°C / min to the final reaction temperature of 725°C, and hold at this reaction temperature for 60 minutes.

[0047] (5) Product collection and analysis: After the reaction is completed, the product is cooled and taken out for XRD phase analysis.

[0048] According to XRD semi-quantitative analysis, the product obtained in this embodiment contains 68.5 wt% sodalite, 14.8 wt% nepheline, 2.6 wt% sodium silicate, 2.1 wt% unreacted sodium chloride, and 12.0 wt% other impurity phases.

[0049] Example 2

[0050] Based on Example 1, this embodiment uses the stepwise heat treatment procedure described in this invention, and adds a reducing gas NH3 to the atmosphere to examine the effect of the reducing atmosphere on improving the purity of the synthesized sodalite.

[0051] (1) Raw material preparation and mixing: Same as in Example 1, weigh 1.0g of #1 chiral powder kaolin, 0.5g of sodium hydroxide (NaOH), and 0.15g of sodium chloride (NaCl). Grind the kaolin, sodium hydroxide, and sodium chloride in an agate mortar at a mass ratio of 1:0.5:0.15 until uniform to obtain a solid mixture.

[0052] (2) Slurry preparation: Same as in Example 1, add 2.8g of deionized water to the solid mixture obtained in step (1) (total mass 1.65g, of which 1.0g is kaolin) to make the mass ratio of aqueous medium to kaolin 2.8:1, and stir evenly to form a slurry. Transfer the slurry completely to the sample boat. Record the mass of the material in the boat before the reaction.

[0053] (3) Preparation of reaction apparatus and atmosphere control: The sample boat loaded with slurry is placed into the reaction apparatus. Figure 1 In the heating device (tube furnace) shown by label 9, a specific atmosphere consisting of nitrogen, water vapor, air and ammonia is introduced, wherein the volume fraction of NH3 is controlled at 4%, the volume ratio of NH3:N2:Steam:Air is approximately 4:17:46:33, and the total flow rate is approximately 300 mL / min.

[0054] (4) Stepwise heat treatment reaction: Same as in Example 1. The heating program of the heating equipment was set. First, the temperature was increased from the initial room temperature to the pretreatment temperature of 180°C at a heating rate of 10°C / min, and then held at this temperature for 2 hours to promote pre-mineralization or preliminary conversion of raw materials. Then, the temperature was increased to the final reaction temperature of 725°C at a heating rate of 10°C / min, and held at this reaction temperature for 60 minutes.

[0055] (5) Product collection and analysis: Same as in Example 1.

[0056] According to XRD semi-quantitative analysis, the mass fraction of sodalite in the product obtained in this example was significantly increased to 73.5 wt%, while the nepheline content decreased to about 12.5 wt%.

[0057] Example 3

[0058] Based on the process of Example 2, this embodiment further optimizes the aqueous medium from deionized water to ammonia solution, aiming to demonstrate the technical effect of the optimal solution of the present invention.

[0059] (1) Raw material preparation and mixing: Same as in Example 1, weigh 1.0g of #1 chiral powder kaolin, 0.5g of sodium hydroxide (NaOH), and 0.15g of sodium chloride (NaCl). Grind the kaolin, sodium hydroxide, and sodium chloride in an agate mortar at a mass ratio of 1:0.5:0.15 until uniform to obtain a solid mixture.

[0060] (2) Slurry preparation: Add 2.8 g of 4 wt% ammonia solution as an aqueous medium to the solid mixture obtained in step (1) (total mass 1.65 g, of which 1.0 g is kaolin), so that the mass ratio of ammonia solution to kaolin is 2.8:1 (at this time, the mass ratio of aqueous medium to the total mass of solid mixture is approximately 1.7:1), and stir evenly to form a slurry. Transfer the slurry completely to the sample boat. Record the mass of the material in the boat before the reaction.

[0061] (3) Preparation of the reaction apparatus and atmosphere control: Same as in Example 1. First, the airtightness of the experimental apparatus was checked. The sample boat loaded with slurry was placed into the container. Figure 1 In the heating apparatus (tube furnace) shown by label 9, the furnace atmosphere is adjusted by introducing a specific atmosphere composed of nitrogen, water vapor, air, and ammonia (NH3, a reducing gas). The volume fraction of ammonia is controlled to be 4%; the air flow rate is approximately 97 mL / min, the water vapor flow rate is approximately 138 mL / min, and the nitrogen flow rate is approximately 51 mL / min, such that the volume ratio of NH3:N2:Steam:Air in the mixed atmosphere is approximately 4:17:46:33. The reaction is carried out at atmospheric pressure. The total gas flow rate is approximately 300 mL / min.

[0062] (4) Stepwise heat treatment reaction: Same as in Example 1. Set the heating program of the heating equipment. First, heat from the initial room temperature to the pretreatment temperature of 180°C at a heating rate of 10°C / min, and hold at this temperature for 2 hours. Then, continue heating at a heating rate of 10°C / min to the final reaction temperature of 725°C, and hold at this reaction temperature for 60 minutes.

[0063] (5) Product collection and analysis: Same as in Example 1.

[0064] According to the semi-quantitative analysis results of the XRD pattern, the mass fraction of sodalite in the product obtained in this example was significantly increased to 85.6 wt%. The mass fraction of nepheline was further reduced to 9.6 wt%; the mass fraction of sodium silicate was 2.3 wt%; the mass fraction of unreacted sodium chloride was 1.9 wt%; and the remaining 0.9 wt% consisted of other impurity phases (mainly including a small amount of sodium carbonate that may have been introduced or generated during the reaction, and a small amount of silica that may remain due to incomplete conversion of kaolin). Compared with Example 1, this example further significantly improved the yield and purity of sodalite by changing the aqueous medium from deionized water to a 4 wt% ammonia solution, under essentially the same conditions. This indicates that the introduction of the ammonia solution has a superior effect on promoting the efficient conversion of kaolin to sodalite.

[0065] Comparative Example 1

[0066] This comparative example uses a one-step heating heat treatment method and does not include the pretreatment isothermal stage required in Examples 1-3, in order to compare the effects with the embodiments of the present invention.

[0067] (1) Raw material preparation and mixing: Same as in Example 1, weigh 1.0g of #1 chiral powdered kaolin, weigh 0.5g of powdered sodium hydroxide (NaOH) as an alkali activator, and weigh 0.15g of sodium chloride as a structure directing agent. Grind the three ingredients evenly in an agate mortar at a mass ratio of 1:0.5:0.15.

[0068] (2) Slurry preparation: Same as in Example 1, add 2.8g of deionized water to the above solid mixture, stir evenly to form a slurry, and transfer it to a quartz boat.

[0069] (3) Preparation of reaction apparatus and atmosphere control: The sample boat loaded with slurry is placed into the reaction apparatus. Figure 1 In the heating apparatus (tube furnace) shown by label 9, a mixed atmosphere of nitrogen, steam, and air is introduced, wherein the volume ratio of N2:Steam:Air is approximately 18.3:48:33.7, the total gas flow rate is approximately 300 mL / min, and the reaction is carried out at atmospheric pressure.

[0070] (4) Heat treatment reaction: Set the heating program to raise the temperature directly from room temperature to 725℃ at a rate of 10℃ / min and keep it constant for 60 minutes.

[0071] (5) Product collection and analysis: After the reaction is completed, the product is cooled and taken out for XRD phase analysis.

[0072] According to XRD semi-quantitative analysis, the product obtained in this comparative example contained 59.1 wt% sodalite, 19.8 wt% nepheline, 2.8 wt% sodium silicate, 4.8 wt% unreacted sodium chloride, and the remainder were other phases.

[0073] Comparative Example 2

[0074] This comparative example aims to examine the effect of introducing an NH3 atmosphere on the synthesis of sodalite when the "stepwise heat treatment" procedure is absent.

[0075] (1) Raw material preparation and mixing: Same as in Example 1.

[0076] (2) Slurry preparation: Same as in Example 1.

[0077] (3) Preparation of reaction apparatus and atmosphere control: The sample boat loaded with slurry is placed into the reaction apparatus. Figure 1 In the heating device (tube furnace) shown by label 9, a mixed atmosphere consisting of nitrogen, water vapor, air and ammonia is introduced, with the component volume ratio being basically the same as in Example 2 (the volume ratio of NH3:N2:Steam:Air is approximately 4:17:46:33), and the total flow rate is approximately 300 mL / min.

[0078] (4) Heat treatment reaction: The same one-step heating program as Comparative Example 1 was used. The heating program was set to raise the temperature directly from room temperature to 725°C at a heating rate of 10°C / min, and the temperature was held constant for 60 minutes.

[0079] (5) Product collection and analysis: After the reaction is completed, the product is cooled and taken out for XRD phase analysis.

[0080] According to XRD semi-quantitative analysis, the product obtained in this comparative example contained only 30.1 wt% sodalite, 34.5 wt% nepheline, 10.9 wt% sodium silicate, 6.7 wt% unreacted sodium chloride, and the remainder were other impurity phases.

[0081] Comparative Example 3

[0082] This comparative example uses the same stepwise heat treatment procedure as Example 1, but the reaction is carried out under a different atmosphere than required by the present invention, in order to demonstrate the key role of the specific atmosphere required by the present invention.

[0083] (1) Raw material preparation and mixing: Same as in Example 1, weigh 1.0g of #1 chiral powdered kaolin, weigh 0.5g of powdered sodium hydroxide (NaOH) as an alkali activator, and weigh 0.15g of sodium chloride as a structure directing agent. Grind the three ingredients evenly in an agate mortar at a mass ratio of 1:0.5:0.15.

[0084] (2) Slurry preparation: Same as in Example 1, add 2.8g of deionized water to the above solid mixture, stir evenly to form a slurry, and transfer it to a quartz boat.

[0085] (3) Preparation of reaction apparatus and atmosphere control: The sample boat loaded with slurry is placed into the reaction apparatus. Figure 1 In the heating apparatus shown by label 9, a dry air atmosphere is introduced at a total flow rate of approximately 300 mL / min. Note that this atmosphere does not contain water vapor, the core component required by this invention.

[0086] (4) Stepwise heat treatment reaction: Same as in Example 1, the heating program of the heating equipment is set. First, the temperature is increased from the initial room temperature to the pretreatment temperature of 180°C at a heating rate of 10°C / min, and held at this temperature for 2 hours. Then, the temperature is increased to the final reaction temperature of 725°C at a heating rate of 10°C / min, and held at this reaction temperature for 60 minutes.

[0087] (5) Product collection and analysis: Same as in Example 1. After the reaction is completed, the product is cooled and taken out for XRD phase analysis.

[0088] According to XRD semi-quantitative analysis, the product obtained in this comparative example contained only 16.2 wt% sodalite, and also contained a large amount of nepheline (44.7 wt%) and unreacted raw materials.

[0089] To more clearly demonstrate the beneficial effects of the various technical features of the present invention, the key conditions and main results of the above comparative examples and embodiments are summarized in Table 2 below.

[0090] Table 2 Comparison of key conditions and main phase content in products between comparative examples and embodiments.

[0091]

[0092] To more clearly demonstrate the beneficial effects of the various technical features of this invention, the key conditions and main results of the comparative examples and embodiments are summarized in Table 2. Based on the data in Table 2, the following conclusions can be drawn:

[0093] 1. The core of this invention lies in the synergistic effect of "stepwise heat treatment" and "water vapor atmosphere". Example 1 uses this core scheme to obtain 68.5 wt% sodalite.

[0094] Compared with Comparative Example 1 (one-step heating, yield 59.1 wt%), under the same conditions, simply changing the one-step heating to the stepwise heat treatment of the present invention significantly improved the yield of sodalite, demonstrating the necessity of "stepwise heat treatment".

[0095] Comparing Example 1 and Comparative Example 3 (dry atmosphere, yield only 16.2 wt%), even with stepwise heat treatment, the lack of a water vapor atmosphere alone caused a sharp drop in yield, demonstrating the necessity of a "water vapor atmosphere".

[0096] 2. The "stepwise heat treatment" and the "NH3 atmosphere" of the present invention have a synergistic effect. Under the premise of adopting the "stepwise heat treatment" of the present invention, both Example 1 (without NH3, yield 68.5 wt%) and Example 2 (with NH3, yield 73.5 wt%) have high yields. The introduction of NH3 atmosphere can further improve the purity of sodalite, which plays a positive promoting role.

[0097] However, in the absence of "stepwise heat treatment", compared with Comparative Example 1 (one-step heating, no NH3, yield 59.1 wt%) and Comparative Example 2 (one-step heating, with NH3, yield only 30.1 wt%), the introduction of NH3 atmosphere actually significantly reduced the yield of sodalite and generated more byproducts such as nepheline.

[0098] This comparative result demonstrates that the correct active intermediate must first be formed through the "stepwise heat treatment" of this invention for the NH3 atmosphere to exert its optimizing effect. Without this prerequisite, NH3 will instead interfere with the reaction and inhibit the formation of sodalite.

[0099] 3. Comparing Example 2 (using deionized water, yield 73.5 wt%) with Example 3 (using 4 wt% ammonia solution, yield 85.6 wt%), it is shown that by further optimizing the type of aqueous medium based on the optimal atmosphere and heat treatment procedure, the purity of sodalite can be achieved to the optimal level.

[0100] In summary, this invention achieves the efficient conversion of kaolin into high-purity sodalite through the synergistic effect of multiple technical features, such as stepwise heat treatment, a specific water-containing vapor atmosphere, optional reducing gas, and preferred water-containing medium, thus solving the problems existing in the prior art.

[0101] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for synthesizing sodalite by stepwise heat treatment of kaolin as raw material under a specific atmosphere, characterized in that, Includes the following steps: 1) A solid mixture is obtained by mixing kaolin, alkali activator and structure directing agent; 2) Add an aqueous medium to the solid mixture obtained in step 1) to form a slurry; 3) Place the slurry obtained in step 2) in a heating device and perform heat treatment under a specific atmosphere, wherein the specific atmosphere includes inert gas, water vapor and oxygen; The heat treatment includes: first heating to the pretreatment temperature at a certain heating rate and holding the temperature for a period of time, then continuing to heat to 700-750℃ and holding the temperature at this reaction temperature for 0.5-3 hours, during which the kaolin undergoes aluminosilicate framework structure reconstruction to generate sodalite crystals.

2. The method according to claim 1, characterized in that, The structure directing agent mentioned in step 1) is selected from at least one of sodium chloride, potassium chloride, sodium sulfate, and potassium sulfate; the alkali activator is selected from one or more of sodium hydroxide, potassium hydroxide, and calcium hydroxide; kaolin, alkali activator, and structure directing agent are mixed in a mass ratio of 1:(0.3-0.5):(0.1-0.2) to obtain a solid mixture.

3. The method for processing sodalite according to claim 2, characterized in that, Ammonium chloride is added simultaneously or after mixing kaolin, alkali activator and structure directing agent, wherein the mass ratio of ammonium chloride to kaolin is (0.05-0.15):

1.

4. The method according to claim 1, characterized in that, In step 2), the aqueous medium is deionized water or ammonia solution, and the mass ratio of the aqueous medium to the solid mixture is (1.5-3.0):

1.

5. The method for processing sodalite according to claim 1, characterized in that, Step 3) The specific atmosphere may optionally further comprise one or more reducing gases selected from CO and NH3; When the reducing gas is NH3, its volume fraction is 1-5.5%, the volume fraction of water vapor is 40-60%, the volume fraction of oxygen is 3-10%, and the remainder is inert gas; When the reducing gas is CO, its volume fraction is 0.5-5%; the volume fraction of water vapor is 40-60%; the volume fraction of oxygen is 3-10%; and the remainder is inert gas.

6. The method according to claim 1, characterized in that, During the heat treatment process, the pretreatment temperature is 80-250℃, and the holding time after reaching the pretreatment temperature is 0.5-3.0 hours; the heating rate before reaching the pretreatment temperature is 10-25℃ / min; the heating rate from the pretreatment temperature to 700-750℃ is 10-25℃ / min.

7. The method for processing sodalite according to claim 1, characterized in that: The specific atmosphere contains 40-60% water vapor by volume, 3-10% oxygen by volume, and the remaining components are inert gases.

8. The method for processing sodalite according to claim 1, characterized in that: Step 3) involves loading the slurry into a container and reacting at atmospheric pressure up to 1 MPa.

9. A sodalite synthesized using the method according to any one of claims 1-8, characterized in that: in, The mass fraction of sodalite phase is greater than 85%.

10. The use of sodalite as a stabilizing and solidifying carrier according to claim 9 in the sealing of radionuclides, characterized in that, The sodalite contains the radioactive nuclides through ion exchange and lattice solidification, and the nuclides include Cs. + 、Sr 2+ Co 2+ At least one of a cation or a chlorine-containing radioactive anion.

Images