Method for synthesizing mordenite using coal gangue as raw material

Abstract

The present invention relates to a method for synthesizing mordenite using coal gangue as a raw material. In this method, the coal gangue is first acid-treated and activated, and the silicon source and aluminum source in the coal gangue are extracted as raw materials by precisely adjusting the amount of sodium hydroxide used. Finally, the preparation of pure-phase mordenite is achieved by precisely adjusting the acetic acid content. The experiment for synthesizing mordenite from coal gangue in the present invention has low cost, a simple process, good repeatability, and is convenient for large-scale promotion to realize the high-value utilization of coal gangue.

Description

Technical Field

[0001] The present invention uses coal gangue as a raw material to prepare mordenite, which can be applied to the fields of gas adsorption and industrial catalysis. Background Art

[0002] As the main by-product of coal mining, coal gangue accounts for about 10% - 30% of the total solid waste. Affected by different regions and mining technologies, etc., sometimes the proportion can reach about 30% - 40%, which cannot be ignored. The large-scale stacking of coal gangue will not only occupy land resources, but also the leaching water will penetrate into the ground, polluting the soil and groundwater; when it burns spontaneously, it will also release a large amount of harmful gases, seriously polluting the environment and harming human health. Coal gangue mainly contains kaolinite and other clay minerals, so the silicon (Si) source and aluminum (Al) source in it can be extracted to synthesize other zeolite molecular sieves, realizing the conversion and utilization of waste. Its main preparation process is to first acid-treat the crushed coal gangue to remove the iron and other metal oxides in it to improve the performance of the zeolite, and then add a certain proportion of alkali and other chemical structure agents for secondary treatment. For molecular sieves with a high silicon-aluminum ratio, the target molecular sieve will be synthesized by adding an additional silicon source. The methods for synthesizing zeolite using coal gangue mainly include hydrothermal crystallization method, alkali dissolution activation method, microwave-assisted hydrothermal crystallization method and solvent-free method, etc. Among them, the hydrothermal crystallization method is widely used because the synthesized molecular sieve has high purity, good crystallinity, strong structural controllability and is easy to operate. At present, there are a large number of literature reports on using coal gangue to prepare NaX zeolite, NaA zeolite and ZSM-5 molecular sieve, etc. Due to its good acidity, adsorption and thermal stability, mordenite has been widely studied and applied in industrial catalysis. Zhou Tongxiao (Microporous and Mesoporous Materials, 2021, 314: 110872.) synthesized mordenite using fly ash as a raw material, but other impurity phases such as mullite and quartz exist in the synthesized zeolite; Shen Xiang (CN201010149287.6) synthesized mordenite using mineral raw materials, but an additional silicon source is required, resulting in an increase in raw material costs. Therefore, in order to achieve the efficient and low-cost utilization of coal-based solid waste and be able to stably synthesize mordenite, it is urgent to develop a preparation technology with a simple preparation method, cheap raw materials and capable of stably obtaining a pure-phase mordenite molecular sieve. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for synthesizing mordenite from coal gangue in view of the limitations in the current technology. In this method, coal gangue is first acid-treated and activated, and the silicon source and aluminum source in the coal gangue are extracted as raw materials by precisely adjusting the amount of sodium hydroxide used. Finally, pure-phase mordenite is prepared by precisely adjusting the acetic acid content. The experiment of synthesizing mordenite from coal gangue in the present invention has low cost, simple process, good repeatability, and is convenient for large-scale promotion to realize the high-value utilization of coal gangue.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] A method for synthesizing mordenite using coal gangue as a raw material, the method comprising the following steps:

[0006] (1) Crushing the coal gangue into powder, transferring it to a muffle furnace, and calcining it at 800 - 1000 °C for 3 - 6 hours to obtain activated coal gangue;

[0007] (2) Mixing the activated coal gangue with hydrochloric acid solution, stirring at 70 - 90 °C for 5 - 8 h, filtering, washing until neutral, and drying to obtain acid-treated coal gangue powder;

[0008] Among them, the solid-liquid ratio of the activated coal gangue to the hydrochloric acid solution is 1:5 - 10 by mass (g);

[0009] The concentration of hydrochloric acid is 4 - 8 mol / L;

[0010] (3) Mixing the acid-treated coal gangue powder with solid sodium hydroxide, then adding deionized water, and stirring at 60 - 90 °C for 4 - 8 h to obtain a solution rich in silicon and aluminum;

[0011] Among them, the mass ratio is acid-treated coal gangue:sodium hydroxide = 1:0.05 - 0.3; 0.6 g of sodium hydroxide is added per 30 - 50 mL of water;

[0012] (4) First adding acetic acid to the solution obtained in step (3) to adjust the pH to 7 - 12, then adding tetraethylammonium hydroxide solution, and finally adding mordenite seeds, and aging for 1 - 24 h;

[0013] Among them, the mass of the mordenite seeds is 0.5% - 10% of the mass of the acid-treated coal gangue; the mass of tetraethylammonium hydroxide is 0.5 - 1.5% of the mass of the acid-treated coal gangue;

[0014] (5) Hydrothermally treating the sol-gel obtained in step (4) at 170 - 190 °C for 48 h - 144 h, filtering, washing, and drying the obtained solid, and then performing high-temperature calcination to obtain mordenite.

[0015] Preferably, the particle size of the crushed coal gangue in step (1) is between 100 and 200 mesh.

[0016] Preferably, the calcination in step (5) is as follows: calcination at 550-600 °C for 4-6 hours in an air atmosphere.

[0017] The substantial features of the present invention are as follows:

[0018] In the existing methods for synthesizing mordenite from coal-based solid waste, there are problems such as impurity crystal phases in the product, low crystallinity of the synthesized product, and complex synthesis processes. It is necessary to additionally add a silicon source or an aluminum source to adjust the pH, composition, etc. of the solution in order to obtain mordenite with high purity and a good crystal structure.

[0019] All the raw materials of the present invention are derived from coal gangue. By adjusting the addition ratio of solid sodium hydroxide in coal gangue, the extraction of a silicon-rich aluminum solution is achieved, and a gel solution is prepared by adjusting the content of acetic acid to obtain a mother liquor with a more optimal concentration and alkalinity suitable for synthesizing mordenite. Mordenite without impurity phases is synthesized by a one-step hydrothermal method. The present invention realizes the conversion of coal gangue into high-value-added products, and the preparation conditions are simple and easy to achieve, and it is easy to carry out large-scale batch production.

[0020] The present invention has the following beneficial effects:

[0021] 1. Coal gangue belongs to coal-based solid waste. The present invention utilizes coal gangue waste and converts it into high-value-added products, realizing the recycling of resources.

[0022] 2. The raw materials used in the synthesis of the present invention are cheap and easily available. By finely controlling the content of acetic acid, the crystal phase of the generated product is adjusted, and by finely controlling the content of sodium hydroxide, the crystallinity of the synthesized product is controlled. Therefore, the preparation process of the present invention is simple, the experimental cost is low, the conditions are easy to meet, and the repeatability is high, and it can achieve batch and large-scale production.

[0023] 3. The amount of sodium hydroxide used in the extraction of silicon source from coal gangue in the present invention is low, and the environmental pollution is small. In the present invention, only 0.6 g of sodium hydroxide is required for every 10 g of coal gangue to extract the silicon source in coal gangue and synthesize mordenite, reducing environmental pollution. Description of the Drawings

[0024] Figure 1 XRD analysis spectrum of the mordenite sample synthesized in Example 2 and the standard card;

[0025] Figure 2 SEM image of the mordenite synthesized in Example 2; Detailed Embodiments

[0026] The present invention provides a test scheme for preparing mordenite from coal gangue, including the following steps:

[0027] (1)Take 25 g of coal gangue that has been pulverized and passed through a 200-mesh sieve and place it in a muffle furnace. Under an air atmosphere, heat it at a heating rate of 10 °C / min to 900 °C and hold for 6 h to obtain high-temperature activated coal gangue powder.

[0028] (2)Take 25 g of high-temperature activated coal gangue, 62.5 ml of hydrochloric acid (12 mol / L), and 62.5 ml of pure water and mix them. Stir at 80 °C for 6 h. Take the solid after acid treatment, wash it three times with pure water and then three times with ethanol, wash the coal gangue after acid treatment to neutrality, and dry it in an oven at 100 °C.

[0029] (3)Take 10 g of the acid-treated coal gangue dried in step (2) and mix it with 0.6 g of solid sodium hydroxide, add 38 - 40 ml of pure water, stir at 80 °C for 6 h, centrifuge to separate and take the supernatant to obtain a silicon-rich alkali solution. Add 1 ml of acetic acid to the silicon-rich alkali solution to adjust the pH to 12 to obtain a gel solution. Add 1.8 - 1.9 g of tetraethylammonium hydroxide solution to the gel solution, stir for about 3 min, and then add 0.1 g of mordenite seed crystals and age for 4 - 5 h.

[0030] (4)Preheat the oven to 170 °C, pour the aged gel solution in step (3) into a reaction kettle and transfer it to the oven for crystallization for 72 h. After hydrothermal treatment, filter to separate the solid and liquid, wash with pure water 3 times, dry the separated solid, place it in a muffle furnace, and heat it at a rate of 1 °C / min in an air atmosphere to 550 °C and calcine for 4 h to remove the template agent to obtain mordenite.

[0031] The technical solutions provided by the present invention will be described in detail below in conjunction with the embodiments and the accompanying drawings, but they should not be construed as limiting the protection scope of the present invention.

[0032] In this embodiment, the XRD analysis of mordenite is carried out using an X-ray diffractometer produced by Anton Paar. The measurement uses Cu Kα radiation (λ = 1.5406 Å), the scanning speed is 2θ = 10 °C / min, and the scanning range is 5° - 60°. The sum of the XRD peak areas of the (200), (300), (150), (202), and (350) crystal planes of the product is selected for calculation. Taking the crystallinity of Example 2 as 100%, the relative crystallinity of other samples to be measured is obtained by comparing with it.

[0033] In this embodiment, the SEM morphology analysis of mordenite is carried out using a Phenom-type scanning electron microscope produced by Funan Scientific Instruments Co., Ltd.

[0034] The mordenite seed crystals described above are well-known materials and are purchased from Raodong (Liaoning) New Materials Co., Ltd. Tetraethylammonium hydroxide (with a content of not less than 25%) is purchased from Tianjin Guangfu Technology Development Co., Ltd.;

[0035] Example 1: 25 g of coal gangue was placed in a porcelain boat and transferred to a muffle furnace. In an air atmosphere, it was heated from room temperature to 900 °C at a heating rate of 10 °C / min and calcined for 6 h. After cooling, 25 g of coal gangue was weighed and mixed with 62.5 ml of hydrochloric acid (12 mol / L) and 62.5 ml of distilled water, and stirred at 80 °C for 6 h to obtain acid-treated activated coal gangue. The acid-treated coal gangue can not only remove the metal oxide impurities therein, making silicon and aluminum more enriched, but also corrode the surface of the coal gangue to form more defects and pores and activate the silicon and aluminum components, which is beneficial to the subsequent alkali treatment process to reduce the addition amount of sodium hydroxide and extract the required silicon and aluminum elements, making the utilization of coal gangue more sufficient.

[0036] Example 2: 10 g of the acid-treated coal gangue obtained in Example 1 was weighed and mixed with 0.6 g of solid sodium hydroxide, and then 39 ml of pure water was added. It was stirred at 80 °C for 6 h, and the solid and liquid were centrifuged. The liquid was placed in a polytetrafluoroethylene inner liner, and 2 ml of analytical pure acetic acid was added dropwise to adjust the pH to 12 to make the solution become a gel state. Then, 1.84 g of a 25% tetraethylammonium hydroxide solution was added to the gel and stirred at room temperature for 5 min. Then, 0.1 g of mordenite seed crystals, which is 1% of the mass of the coal gangue, was added to the gel and stirred at room temperature for 6 h (500 rpm). After stirring, the reaction kettle was locked and immediately placed in an oven preheated to 170 °C and reacted for 72 h. After the oven cooled to room temperature, a white powder was obtained through suction filtration, washing, and drying overnight at 100 °C. The white powder was transferred to a muffle furnace. In an air atmosphere, it was heated from room temperature to 550 °C at a heating rate of 1 °C / min and held for 2 h to remove tetraethylammonium hydroxide to obtain mordenite, and XRD and SEM tests were performed on it.

[0037] Figure 1 The XRD test results corresponded well to the crystal planes of the mordenite PDF standard card (PDF#04-011-3643), and the crystallinity was relatively high. From Figure 2 the SEM image, it can be observed that its morphology is nanoparticles.

[0038] Example 3: Other steps are the same as those in Example 2. The difference is that 4 ml of acetic acid was added dropwise during adjustment to make the pH = 7, and the solution became a gel state. 1.84 g of tetraethylammonium hydroxide solution was added to the gel and stirred at room temperature for 5 min. Then, 0.1 g of mordenite seeds was added to the gel and stirred at room temperature for 6 h (500 rpm). After stirring, the reaction kettle was locked and immediately placed in an oven preheated to 170 °C for reaction for 72 h. After the oven cooled to room temperature, a white powder was obtained by suction filtration, washing, and drying overnight at 100 °C. The white powder was transferred to a muffle furnace, and in an air atmosphere, it was heated from room temperature to 550 °C at a heating rate of 1 °C / min and held for 2 h to remove the template agent to obtain mordenite.

[0039] Example 4: Other steps are the same as those in Example 2. The difference is that no acetic acid was added dropwise to the solution. Instead, 1.84 g of tetraethylammonium hydroxide solution was directly added and stirred at room temperature for 5 min. Then, 0.1 g of mordenite seeds was added to the gel and stirred at room temperature for 6 h (500 rpm). After stirring, the reaction kettle was locked and immediately placed in an oven preheated to 170 °C for reaction for 72 h. After the oven cooled to room temperature, a white powder was obtained by suction filtration, washing, and drying overnight at 100 °C. The white powder was transferred to a muffle furnace, and in an air atmosphere, it was heated from room temperature to 550 °C at a heating rate of 1 °C / min and held for 2 h to remove the template agent. After measurement, the obtained substance was analcime.

[0040] Example 5: Other steps are the same as those in Example 2. The difference is that after stirring at room temperature for 6 h (500 rpm), the oven was preheated to 190 °C and the reaction kettle was placed in it for crystallization for 72 h. After the oven cooled to room temperature, a white powder was obtained by suction filtration, washing, and drying overnight at 100 °C. The white powder was transferred to a muffle furnace, and in an air atmosphere, it was heated from room temperature to 550 °C at a heating rate of 1 °C / min and held for 2 h to remove the template agent to obtain mordenite.

[0041] Example 6: Take 10 g of the gangue treated in Example 1 and mix it with 0.8 g of sodium hydroxide solid, and then add 39 ml of pure water. Stir at 80°C for 6 h, centrifuge to separate the solid and liquid, and take the liquid and place it in a polytetrafluoroethylene liner. After dissolution, add 1 ml of acetic acid to the solution to adjust the pH to 12, and the solution becomes a gel state. Then add 1.84 g of tetraethylammonium hydroxide to the gel and stir at room temperature for 5 min, continue to add 0.1 g of mordenite seeds to the gel, and stir at room temperature for 6 h (500 rpm). After the stirring is completed, lock the reactor and immediately put it into an oven preheated to 170°C for 72 h. After the oven is cooled to room temperature, filter, wash and dry at 100°C overnight to obtain a white powder. Transfer the white powder to a muffle furnace, and in an air atmosphere, heat it from room temperature to 550°C at a rate of 1°C / min and keep it for 2 h to remove the template to obtain mordenite.

[0042] Example 7: Take 10 g of the gangue treated in Example 1 and mix it with 1.2 g of sodium hydroxide solid, and then add 39 ml of pure water. Stir at 80°C for 6 h, centrifuge to separate the solid and liquid, and take the liquid and place it in a polytetrafluoroethylene liner. Then add 4 ml of acetic acid to the solution to adjust the pH to 12, and the solution becomes a gel state. Then add 1.84 g of tetraethylammonium hydroxide to the gel and stir at room temperature for 5 min, continue to add 0.1 g of mordenite seeds to the gel, and stir at room temperature for 6 h (500 rpm). After the stirring is completed, lock the reactor and immediately put it into an oven preheated to 170°C for 72 h. After the oven is cooled to room temperature, filter, wash and dry at 100°C overnight to obtain a white powder. The white powder is transferred to a muffle furnace, and in an air atmosphere, the temperature is increased from room temperature to 550°C at a rate of 1°C / min and maintained for 2 h to remove the template to obtain mordenite.

[0043] The concentration of sodium hydroxide and the pH value of the solution have an important influence on the crystallinity of mordenite. The synthesis of mordenite requires a specific pH value. Too high or too low a concentration will affect the crystallinity, and its crystal structure is very sensitive to the silicon-aluminum ratio during the synthesis process. An appropriate silicon-aluminum ratio is conducive to the formation of high-crystallinity mordenite. If the concentration of sodium hydroxide is too high, it may cause excessive dissolution of silicates, destroy the balance of silicon-aluminum ratio, and thus reduce crystallinity; if the concentration is too low, it may cause insufficient dissolution of aluminosilicate precursors, affecting crystal growth.

[0044]

[0045] The crystal phases and relative crystallinities of the synthesized samples in Examples 2-7 are summarized in Table 1. The relative crystallinity is calculated by selecting the sum of the XRD peak areas of the (200), (300), (150), (202), and (350) crystal planes of the product, with the crystallinity of Example 2 taken as 100% and compared with other samples to be measured.

[0046] Compared with Example 2, Example 3 has a lower alkalinity, and the crystallinity of mordenite is significantly affected by different pH values. Therefore, the crystal phase of the mordenite synthesized in Example 3 is lower than that in Example 2. For Example 4, hydrothermal crystallization is carried out using a silica-rich solution without adjusting the pH, and the obtained crystals are analcime. Because a strong alkaline environment is very conducive to the formation of analcime, even if a structure-directing agent is added, it will not induce its growth into the mordenite crystal phase. Example 5 studied the effect of increasing temperature on the crystal phase. From the calculated crystallinity values, it can be seen that temperature does not affect the synthesized crystal phase, but it will slightly reduce its crystallinity. This is because the solubility of molecular sieve crystals increases with increasing temperature, and the formed crystals partially dissolve. The released silicon-aluminum species may re-stack disorderly, forming an amorphous phase or mixed crystals, resulting in a decrease in crystallinity. Examples 6 and 7 investigated the effects of different sodium contents and pH values on the synthesized products. From the crystallinity, it can be seen that when the sodium hydroxide content exceeds or is lower than a certain threshold, even if the pH value is adjusted to make its alkalinity consistent with the optimal synthesis pH, the crystallinity will also decrease significantly. Because aluminum in the mordenite framework needs to be coordinated with 4 oxygen atoms, and the charge is balanced by sodium. When sodium is insufficient, the aluminum coordination environment is unstable, resulting in local charge imbalance in the framework and lattice defects. Excessive sodium will preferentially adsorb on the surface of the negatively charged precursor, hindering the template-induced directed assembly and leading to distortion or collapse of the pore structure, so the crystallinity decreases.

[0047] Mordenite with high crystallinity is of great significance in the fields of catalysis, adsorption, and ion exchange. Its stable crystal structure endows it with high thermal stability and excellent performance in high-temperature catalytic reactions such as petroleum cracking; its complete crystal structure makes its chemical stability excellent, and it can maintain its performance in strong acid and strong alkali environments and has a longer service life; its regular and uniform pore structure not only provides a large specific surface area and high adsorption capacity, but also can effectively improve the selectivity of gas separation and selective catalysis. At the same time, high crystallinity can bring more active sites and ion exchange sites, making its catalytic activity better and more efficient in ion exchange applications such as wastewater treatment.

[0048] As can be seen from the above embodiments, the present invention provides a method for synthesizing mordenite using coal gangue as a raw material through a simple one-step hydrothermal method. The coal gangue activated at high temperature is first mixed with hydrochloric acid and stirred at 80 °C for 6 h to obtain acid-treated coal gangue. Then, the acid-treated coal gangue is mixed with a small amount of solid sodium hydroxide and dissolved in deionized water, stirred at 80 °C for 6 h to obtain a silicon-rich solution, which is added to a polytetrafluoroethylene inner lining. Sodium aluminate is added to the solution and the pH is adjusted with acetic acid to obtain a gel solution. Then, tetraethylammonium hydroxide solution and mordenite seeds are added to the solution. Finally, the reaction kettle is locked and immediately placed in an oven preheated to a certain temperature for reaction for a certain time. When the oven cools to room temperature, sodium mordenite is obtained by centrifugation, washing and drying. Finally, the mordenite obtained hydrothermally is transferred to a muffle furnace and calcined in an air atmosphere to remove the template agent.

[0049] The above-described embodiments are cases explored and realizable for the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made without departing from the core of the present invention shall be included within the protection scope of the present invention.

[0050] Matters not covered in the present invention are well-known technologies.

Claims

1. A method for synthesizing mordenite using coal gangue as raw material, characterized in that the method comprises the following steps: (1) Crushing the coal gangue into powder, transferring it to a muffle furnace, calcining at 800 - 1000 °C for 3 - 6 hours to obtain activated coal gangue; (2) Mixing the activated coal gangue with hydrochloric acid solution, stirring at 70 - 90 °C for 5 - 8 h, filtering, washing until neutral and drying to obtain acid-treated coal gangue powder; Among them, The mass ratio of the activated coal gangue to the hydrochloric acid solution is 1:5 - 10; (3) Mixing the acid-treated coal gangue powder with sodium hydroxide solid, then adding deionized water, stirring at 60 - 90 °C for 4 - 8 h to obtain a solution rich in silicon and aluminum; Among them, the mass ratio is: acid-treated coal gangue:sodium hydroxide = 1:0.05 - 0.3; 0.6 g of sodium hydroxide is added per 30 - 50 mL of water; (4) Adjusting the pH of the solution obtained in (3) to 7 - 12 by adding acetic acid first, then adding tetraethylammonium hydroxide solution, finally adding mordenite seed crystals, and aging for 1 - 24 h; Among them, the mass of the mordenite seed crystals is 0.5% - 10% of the mass of the acid-treated coal gangue; the mass of tetraethylammonium hydroxide is 0.5 - 1.5% of the acid-treated coal gangue; (5) Hydrothermally treating the sol-gel obtained in (4) at 170 - 190 °C for 48 h - 144 h, filtering, washing and drying the obtained solid, and then performing high-temperature calcination to obtain mordenite; In step (1), the particle size of the crushed coal gangue is between 100 - 200 mesh; The calcination in step (5) is: calcining at 550 - 600 °C for 4 - 6 hours in an air atmosphere; In step (2), the concentration of hydrochloric acid is 4 - 8 mol / L.

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