Method for separating and preparing high-orientation purified montmorillonite film from bentonite

Abstract

The invention discloses a method for separating and preparing a high-orientation purified montmorillonite film from bentonite, which comprises the following steps: firstly, removing large-particle impurities in the bentonite by physical screening; then, washing the bentonite in a flowing state by adopting a saturated salt solution so as to replace original positive ions between layers of the bentonite; thirdly, adding a small amount of dispersing agent into the purified bentonite suspension, adjusting the pH value of the suspension, weakening the agglomeration effect of montmorillonite and impurity minerals, and dispersing montmorillonite lamellas through the ultrasonic effect; and finally, finishing directional arrangement of the crystal layer structure on the nanoscale silicon wafer in a natural settling mode. According to the method, impurity minerals symbiotic with montmorillonite in bentonite can be effectively removed, a montmorillonite laminated structure with highly purified interlayer cations and directionally arranged in the d001 direction is obtained, and the problem of directional micromechanical characterization of montmorillonite in bentonite can be solved.

Description

Technical Field

[0001] The present invention relates to civil engineering (geotechnical) and geological engineering technical methods, especially sample preparation techniques related to micro-mechanical property testing, and particularly relates to a method for separating and preparing highly oriented purified montmorillonite films from bentonite. Background Art

[0002] Due to its clean, low-carbon, stable and efficient characteristics, nuclear energy has become a major energy option in many developed countries. According to the statistics of the International Atomic Energy Agency, the global nuclear power generation in 2022 was 2545 billion kWh, accounting for 11% of the global total power generation. It is expected that by 2050, the proportion of nuclear power will increase to 14%. The current proportion of nuclear power in China is 4.9%, and it is expected to increase to 22.1% by 2050. However, with the acceleration of nuclear energy development, the generation of high-level radioactive waste (hereinafter referred to as "HLW") will also increase significantly. How to effectively dispose of these wastes has become the core challenge restricting the sustainable development of nuclear energy.

[0003] Deep geological disposal is widely regarded as a safe disposal solution for HLW. This solution buries HLW in mine shafts or tunnels 250 to 1000 meters deep underground, and uses the natural barrier of the surrounding rock and the engineering barrier to achieve effective isolation from the biosphere. In this process, bentonite is considered the preferred buffer material due to its high expansibility, low permeability and strong radionuclide adsorption capacity. Montmorillonite is the main mineral component of bentonite, which plays a decisive role in its hydro-mechanical properties and directly affects the macroscopic properties of the buffer layer. Research shows that the type and content of montmorillonite have a significant impact on the compressibility and mechanical strength of bentonite; and the interlayer cation exchange reaction of montmorillonite under the action of groundwater chemistry may have an irreversible effect on the long-term performance of the buffer layer. Therefore, in-depth study of the micro-mechanical properties of montmorillonite under chemical action is of great significance for optimizing the design of buffer materials and evaluating the long-term performance of engineering barriers.

[0004] At present, techniques such as nanoindentation and atomic force microscopy have been widely used in the micro-mechanical property testing of clay minerals, but these techniques have high requirements for the surface flatness of samples. However, montmorillonite in bentonite is usually agglomerated with other minerals, resulting in an uneven surface; at the same time, montmorillonite has a flaky crystal structure, and its mechanical properties show strong anisotropy. In addition, the interlayer cation exchange reaction of montmorillonite under conventional conditions is a long process, especially slower in the groundwater chemical environment. Therefore, there is an urgent need for an efficient, simple and low-cost technical method that can separate highly pure and oriented montmorillonite films from bentonite to meet the needs of micro-mechanical testing and related theoretical research.

[0005] In bentonite, montmorillonite often aggregates with other minerals, resulting in a poor flatness of its crystal layer surface and anisotropic mechanical properties, which makes it difficult for conventional micro-mechanical testing techniques to accurately characterize. In addition, the interlayer cation exchange process of montmorillonite is slow, posing challenges to sample purification. Existing technologies have not effectively solved the problems of montmorillonite separation and orientation arrangement. Therefore, there is an urgent need for an efficient and low-cost technical method to meet the requirements of montmorillonite micro-mechanical characterization. Summary of the Invention

[0006] Object of the Invention: To overcome the deficiencies of the prior art, the present invention provides a method for separating and preparing a highly oriented purified montmorillonite film from bentonite. This method can effectively overcome the influence of uneven sample surface and insufficient interlayer cation exchange of montmorillonite on test results, and provide a highly pure and oriented purified montmorillonite film that meets the requirements of micro-mechanical testing. This technology not only provides a new technical path for the micro-mechanical property characterization of montmorillonite, but also provides a theoretical basis and technical support for the optimization design of buffer materials for nuclear waste disposal repositories.

[0007] Technical Solution: The method for separating and preparing a highly oriented purified montmorillonite film from bentonite includes the following steps:

[0008] (1) Physically screen bentonite powder, dissolve the screened bentonite powder in a saturated salt solution, and perform interlayer cation purification in a flowing state. The purified bentonite is washed with deionized water to obtain an aqueous suspension of purified bentonite.

[0009] (2) Add a small amount of dispersant to the bentonite aqueous suspension and adjust the pH value. The dosage of the dispersant is 0.7-1.0% of the mass of bentonite, and then it is fully stirred and ultrasonically treated to obtain a montmorillonite supernatant with high purity.

[0010] (3) Drop the supernatant onto a nanoscale silicon wafer, cover a petri dish above the silicon wafer to prevent dust contamination, and raise one side of the petri dish to ensure the smooth drainage of water. Wait for the droplet to dry naturally to obtain a highly oriented purified bentonite film.

[0011] Further, the screening device is a vibrating sieve, the sieve mesh used is between 200 and 300 meshes, and the screening time is 30-60 minutes.

[0012] Further, the mass fraction of the aqueous suspension of purified bentonite is 1-4%.

[0013] Further, the saturated salt solution is sodium chloride, potassium chloride, lithium chloride or calcium chloride.

[0014] Further, a mixer is used to keep the solution in a flowing state during the purification process. The volume of the solution is between 1 / 3 and 1 / 2 of the maximum capacity of the centrifuge tube, and the rotation speed is set at 60 - 80 rpm.

[0015] Further, the dispersant is sodium chloride, sodium carbonate, potassium carbonate, or sodium hexametaphosphate.

[0016] Further, the pH value of the suspension after adding the dispersant is controlled between 10 and 10.5.

[0017] Further, the stirring time after adding the dispersant is 3 - 12 hours, and the ultrasonic dispersion time is 30 - 60 minutes.

[0018] Further, the nanoscale silicon wafer is a single-sided polished silicon wafer with a flatness less than 3 μm and a roughness less than 0.5 nm.

[0019] The key steps of the present invention:

[0020] 1. Physical screening and purification treatment of bentonite powder:

[0021] First, a vibrating sieve is used to remove large particle impurities in bentonite through physical screening; the screened bentonite powder is dissolved in a saturated salt solution, and fluid washing is carried out for interlayer cation purification. This process accelerates the cation exchange reaction in bentonite through a continuously flowing solution. The purified bentonite is washed multiple times with deionized water to obtain an aqueous suspension of purified bentonite.

[0022] 2. Dispersant addition and suspension treatment:

[0023] A small amount of dispersant is added to the aqueous suspension of purified bentonite, and the pH value of the suspension is adjusted, followed by sufficient stirring and ultrasonic treatment. Through these steps, the agglomeration effect between montmorillonite and other minerals is weakened, and montmorillonite is dispersed to obtain a high-purity montmorillonite supernatant.

[0024] 3. Preparation of highly oriented montmorillonite film:

[0025] The obtained high-purity montmorillonite supernatant is dropped onto a nanoscale silicon wafer with a surface flatness less than 3 μm and a roughness less than 0.5 nm to ensure the quality of the film. To avoid dust contamination, a petri dish is covered above the silicon wafer, and one side of the petri dish is raised to ensure the smooth drainage of water. Wait for the liquid drop to air-dry naturally, and finally obtain a highly oriented purified montmorillonite film. This film has an excellent oriented arrangement structure and can meet the requirements of micro-mechanical property testing.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] Through multiple steps of physical screening, washing with saturated salt solution, regulation by dispersant, and ultrasonic dispersion, the present invention can not only effectively extract high-purity montmorillonite sheets, but also achieve the oriented arrangement of montmorillonite crystal layers through the natural sedimentation method. This method has a simple technical route, low cost, and can prepare high-purity and highly oriented montmorillonite films, meeting the requirements for testing the micro-mechanical properties of montmorillonite, and having broad engineering application prospects, especially being of great significance in the optimal design of buffer materials for nuclear waste repositories and the long-term performance evaluation thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of the principle for preparing a highly oriented montmorillonite film by the natural sedimentation method;

[0029] Figure 2 is an electron microscope image of a highly oriented purified montmorillonite film. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described below.

[0031] Step 1: Physical screening and purification of interlayer cations

[0032] Preparation of bentonite raw material: Select bentonite raw material from natural bentonite ore, and perform crushing and pulverization treatment. The powder should be screened by a screening device to remove large particle impurities. A vibrating screen is selected as the screening device, and a screen with 200 to 300 meshes is selected. The screening time is set to 30 to 60 minutes.

[0033] Physical screening: Dissolve the screened bentonite powder in a saturated salt solution (optional sodium chloride, potassium chloride, lithium chloride, or calcium chloride). The temperature of the saturated salt solution is kept at room temperature to ensure that the solution is completely saturated. Use a mixer to keep the solution in a flowing state, with the rotation speed set to 60 to 80 rpm and the washing time of 24 hours to ensure sufficient cation exchange in the bentonite.

[0034] Water washing and precipitation separation: After the interlayer cation exchange is completed, wash with deionized water multiple times to remove impurities in the solution. Through centrifugal separation, use a centrifuge to set the centrifugal speed to 12000 r / min and the centrifugal time to 10 minutes to ensure that all impurities are effectively removed. Drop the supernatant after centrifugation into silver nitrate solution to detect whether white precipitate forms until no obvious white precipitate appears.

[0035] Obtain a purified bentonite aqueous suspension: After centrifugation of the washed bentonite solution, a purified bentonite aqueous suspension is obtained, with the mass fraction controlled at 1% to 4%.

[0036] Step 2: Dispersion and ultrasonic treatment

[0037] Dispersant addition: A dispersant is added to the aqueous suspension of purified bentonite. Optional dispersants include sodium chloride, sodium carbonate, potassium carbonate, sodium hexametaphosphate, etc., with sodium hexametaphosphate being preferred. The dosage of the dispersant is 0.7% to 1.0% of the mass of the bentonite. After adding the dispersant, the pH value of the suspension is adjusted to between 10 and 10.5.

[0038] Stirring and sonication: The suspension after adding the dispersant needs to be stirred under constant temperature conditions for 3 to 12 hours to ensure that the dispersant is fully combined with the bentonite. Then, sonication is carried out, and the sonication dispersion time is 30 to 60 minutes to ensure that the montmorillonite crystals are fully dispersed and do not agglomerate with other minerals.

[0039] Obtaining montmorillonite supernatant: After stirring and sonication, the suspension is left standing for a period of time to separate the supernatant. The montmorillonite in the supernatant will form a high-purity dispersion, ready for the next step of treatment.

[0040] Step 3: Preparation of montmorillonite film

[0041] Dropwise addition onto silicon wafer: The obtained montmorillonite supernatant is gently dropped onto a high-quality nanoscale silicon wafer, ensuring that the surface of the silicon wafer is smooth and free of contamination. The silicon wafer used is a single-sided polished silicon wafer with a flatness less than 3 μm and a roughness less than 0.5 nm. When dropping the liquid droplet, ensure that the droplet size is appropriate to avoid being too much or too little.

[0042] Petri dish protection and natural air drying: A Petri dish is covered above the silicon wafer to prevent dust contamination, and at the same time, one side of the Petri dish is elevated to ensure that the liquid can drain smoothly. By controlling the air flow, the liquid droplet is naturally air-dried to form a stable film. The orientation principle of the montmorillonite lamellae is as Figure 1 shown.

[0043] Obtaining a highly oriented purified montmorillonite film: As Figure 1 shown, after the liquid droplet is air-dried, a highly oriented and purified montmorillonite film is obtained. Since the size of the basal plane direction of the montmorillonite lamellae is much larger than the size of the thickness direction, the crystal layer structure of the film has significant oriented arrangement characteristics, meeting the requirements of micro-mechanical testing.

[0044] Example 1:

[0045] This example provides a method for preparing a high-purity bentonite film using the natural bentonite ore raw material from the Gaomiaozi area in Inner Mongolia.

[0046] First, natural bentonite ore from the Gaomiaozi area in Inner Mongolia is selected. After being crushed and pulverized, bentonite powder is obtained. The bentonite powder is sieved through a 200-mesh sieve to obtain bentonite powder with an appropriate particle size.

[0047] Dissolve the sieved bentonite powder in a saturated sodium chloride solution, and use a mixer to mix the solution, keeping it in a flowing state. Set the rotation speed to 80 rpm and the mixing time to 24 hours for purification.

[0048] After purification, wash the bentonite 5 times by centrifugation with deionized water. The centrifugation is carried out at a speed of 12,000 r / min, and the centrifugation time for each time is 10 minutes. After centrifugation, the supernatant is dropped into a silver nitrate solution, and no obvious white precipitate is observed, indicating that the impurities in the bentonite have been basically removed.

[0049] Then, prepare a 4% water suspension of the purified bentonite powder with deionized water.

[0050] Add the dispersant sodium carbonate to the bentonite water suspension. The addition amount of sodium carbonate is 1.0% of the mass of the bentonite. After adding the dispersant, adjust the pH value of the suspension to 10, and stir the suspension at room temperature for 3 hours. Subsequently, perform ultrasonic dispersion treatment for 30 minutes to ensure that the bentonite particles are fully dispersed.

[0051] Let the suspension stand for 12 hours until it is fully stratified. Then, use a pipette to drop the supernatant onto a nanoscale silicon wafer, and cover a petri dish above the silicon wafer, while ensuring that one side of the petri dish is raised to ensure the smooth drainage of the liquid.

[0052] Finally, place the silicon wafer in a room temperature environment and let it air dry naturally for 7 days until the liquid drops are completely evaporated, so as to form a stable bentonite film on the surface of the silicon wafer.

[0053] After testing, the montmorillonite content in the prepared film is 98.2%, and the film thickness is 71.4 nm.

[0054] The preparation method provided in this example can effectively improve the purity and stability of the bentonite film, and has high application value.

[0055] Comparative Example 1:

[0056] Prepare the sample in the same method as in Example 1, with the only difference being that no dispersant is added and the pH value is not adjusted when preparing the bentonite suspension. The purity of the obtained montmorillonite film is 95.9%, and the film thickness is 65.6 nm.

[0057] Comparative Example 2:

[0058] Prepare the sample in the same method as in Example 1, with the only difference being that the mass fraction of bentonite in the prepared bentonite suspension is 1%. The purity of the obtained montmorillonite film is 98.4%, and the film thickness is 6.4 nm.

[0059] Comparative Example 3:

[0060] The sample preparation was carried out in the same manner as in Example 1, except that the mass fraction of bentonite in the prepared bentonite suspension was 2%. The purity of the obtained montmorillonite film was 98.4%, and the film thickness was 32.6 nm.

[0061] Comparative Example 4:

[0062] The sample preparation was carried out in the same manner as in Example 1, except that the mass fraction of bentonite in the prepared bentonite suspension was 1%. The purity of the obtained montmorillonite film was 98.2%, and the film thickness was 56.1 nm.

[0063] Comparative Example 5:

[0064] The sample preparation was carried out in the same manner as in Example 1, except that saturated sodium chloride solution was used for interlayer cation purification. The purity of the obtained montmorillonite film was 98.8%, and the film thickness was 79.3 nm.

[0065] The above are only the preferred embodiments of the present invention and do not impose any limitation on the present invention. Any person skilled in the art, without departing from the technical solution of the present invention, makes any form of equivalent substitution or modification and other changes to the technical solution and technical content disclosed by the present invention, which are all within the content of the technical solution of the present invention and still fall within the protection scope of the present invention.

Claims

1. A method for separating and preparing a highly oriented purified montmorillonite film from bentonite, characterized in that: First, physical screening is used to remove large particle impurities in bentonite; then, saturated salt solution is used to wash the bentonite in a flowing state to replace the original cations between its layers; next, a dispersant is added to the purified bentonite suspension, and the pH value of the suspension is adjusted to weaken the agglomeration effect between montmorillonite and impurity minerals, and the exfoliation of montmorillonite sheets is achieved through ultrasonic action; finally, the oriented arrangement of the crystal layer structure is completed on the nanoscale silicon wafer by natural sedimentation method.

2. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 1, characterized in that: It includes the following steps: (1) Physically screen the bentonite powder, dissolve the screened bentonite powder in saturated salt solution, conduct interlayer cation purification in a flowing state, and wash the purified bentonite with deionized water to obtain an aqueous suspension of purified bentonite; (2) Add a small amount of dispersant to the aqueous suspension of bentonite and adjust the pH value, where the dosage of the dispersant is 0.7 - 1.0% of the mass of bentonite, and then conduct sufficient stirring and ultrasonic treatment on it to obtain a supernatant of montmorillonite with high purity; (3) Drop the supernatant onto the nanoscale silicon wafer, cover a petri dish above the silicon wafer to prevent dust pollution, and prop up one side of the petri dish to ensure the smooth drainage of water, and wait for the droplet to air-dry naturally to obtain a highly oriented purified bentonite film.

3. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, wherein: The screening device is a vibrating screen, and the sieve mesh used is between 200 and 300 meshes, and the screening time is 30 - 60 minutes.

4. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, wherein: The mass fraction of the aqueous suspension of the purified bentonite is 1 - 4%.

5. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 1, wherein: The saturated salt solution is sodium chloride, potassium chloride, lithium chloride or calcium chloride.

6. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, characterized in that: A mixer is used to keep the solution in a flowing state during the purification process, the volume of the solution is between 1 / 3 and 1 / 2 of the maximum capacity of the centrifuge tube, and the rotation speed is set at 60 - 80 rpm.

7. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, characterized in that: The dispersant is sodium chloride, sodium carbonate, potassium carbonate or sodium hexametaphosphate.

8. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, characterized in that: The pH value of the suspension after adding the dispersant is controlled between 10 and 10.

5.

9. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, characterized in that: The stirring time after adding the dispersant is 3 - 12 hours, and the ultrasonic dispersion time is 30 - 60 minutes.

10. The method for separating and preparing a highly oriented purified montmorillonite film from bentonite according to claim 2, characterized in that: The nanoscale silicon wafer is a single-sided polished silicon wafer with a flatness less than 3 μm and a roughness less than 0.5 nm.

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