A method for preparing silicon dioxide product using fluorosilicic acid and its application

Abstract

The invention discloses a method for preparing a silicon dioxide product from fluorosilicic acid. The method comprises the following steps: mixing fluorosilicic acid and ammonia water by adopting a tubular static mixing reactor or a microchannel reactor, carrying out an amination reaction at 15-50 DEG C, controlling the pH value at the reaction end point to be 2-8, filtering the reaction product to obtain a filtrate and a filter cake containing ammonium fluoride and ammonium fluorosilicate, and washing, dehydrating and drying the filter cake to obtain the silicon dioxide product. The preparation process of the invention is simple, and high-quality silicon dioxide products can be produced stably and continuously. The prepared silicon dioxide product has high specific surface area, large proportion of mesopores and high active silicon content, and can be used as a silicon source for synthesizing a ZSM-5 molecular sieve in an alkali-free metal system, thereby providing a time-saving and efficient method for the comprehensive utilization of fluorosilicic acid.

Description

Technical Field

[0001] The invention relates to a method for preparing a silicon dioxide product from fluorosilicic acid and application of silicon dioxide in preparing a ZSM-5 molecular sieve, belonging to the field of inorganic powder material preparation. Background Art

[0002] In the production process of wet phosphoric acid in the phosphorus chemical industry, a large amount of fluorosilicic acid is produced as a by-product. This by-product is currently mainly used in the production of fluoride salts and anhydrous hydrogen fluoride. The fluoride salt process is long and has low added value. Although the sulfuric acid method for preparing anhydrous hydrogen fluoride has a short process, it produces fluorine-containing silicon slag as a by-product. The specific surface area of ​​the fluorine-containing silicon slag is 8-20m 2 / g, fluorine content ≥10%, not only resulting in a waste of silicon resources, but also polluting the environment. Therefore, it is of great significance to find a utilization route of fluorosilicic acid that has both economic value and environmental protection.

[0003] Process intensification reactors, such as static mixers, utilize a series of identical stationary inserts installed in a pipeline to redistribute the fluid in radial and tangential directions to achieve good mixing. As an attractive alternative to traditional stirred vessels, static mixers are widely used in various homogenization processes in industrial operations such as petrochemicals, pharmaceuticals, food processing, water treatment, polymer synthesis, and papermaking.

[0004] At present, there is a route to produce white carbon black and ammonium fluoride using fluorosilicic acid and ammonia water. In the process of preparing white carbon black with fluorosilicic acid, because white carbon black is easy to agglomerate, it is mostly prepared by intermittent method, which makes it difficult to continuously mass-produce white carbon black products with excellent performance. Preliminary exploration has been carried out on the continuous preparation of white carbon black using the reaction of fluorosilicic acid and ammonia water, such as the method of connecting multi-stage reactors in series and the step-by-step amination of pipeline reactors, but these methods require the addition of ammonia water in multiple times, and there are problems such as long reaction process and cumbersome steps. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for preparing silicon dioxide products from fluorosilicic acid. The method of the present invention uses a process intensification mixer to produce silicon dioxide products with fluorosilicic acid and ammonia water as raw materials, and adopts a one-step ammoniation method to obtain a silicon dioxide product with a nitrogen adsorption specific surface area of ​​400m 2 / g or more, a silica product with a large mesopore ratio and a high active silicon content, which is used as a silicon source for ZSM-5 molecular sieve synthesis, and TPAOH is used as a template agent. A high-crystallinity ZSM-5 molecular sieve product can be prepared in an alkali metal-free system, avoiding the problem of generating a large amount of ammonia nitrogen wastewater due to the ammonium ion exchange step. The process of the invention is environmentally friendly and reduces environmental pollution.

[0006] In order to achieve the above-mentioned purpose, the method for preparing a silicon dioxide product from fluorosilicic acid of the present invention comprises the following steps: mixing fluorosilicic acid and aqueous ammonia in a tubular static mixing reactor or a microchannel reactor, carrying out an amination reaction at 15-50° C., controlling the reaction end point pH to 2-8, filtering the reaction product to obtain a filtrate and a filter cake containing ammonium fluoride and ammonium fluorosilicate, and washing, dehydrating and drying the filter cake to obtain a silicon dioxide product.

[0007] The number of mixing elements of the static mixer in the tubular static mixing reactor is not less than 8; the amount of ammonia water added is 20% to 80% of the theoretical reaction molar amount, the mass concentration of fluorosilicic acid is 5% to 20%, and the mass concentration of ammonia water is 5% to 25%.

[0008] The filter cake is washed by re-slurry washing and / or spray washing; dried at 105-140°C to obtain a filter cake with a specific surface area of ​​400-600m 2 / g, mesopore specific surface area 300~600m 2 / g of silica product.

[0009] Another object of the present invention is to use the silicon dioxide prepared by the above method in the preparation of ZSM-5 molecular sieve in an alkali metal-free system.

[0010] The method for rapidly preparing a silicon dioxide product provided by the present invention can fully mix fluorosilicic acid and ammonia water to avoid the occurrence of uneven amination reaction, and produce a silicon dioxide product with high specific surface area, large mesopore ratio and high active silicon content. The method is applied to synthesizing silicon sources for ZSM-5 molecular sieves, and a high-crystallinity ZSM-5 molecular sieve product can be obtained in an alkali metal-free system. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a nitrogen adsorption-desorption curve diagram of the silicon dioxide product of Example 1;

[0012] Figure 2 This is a nitrogen adsorption-desorption curve diagram of the silicon dioxide product of Example 2;

[0013] Figure 3 This is a nitrogen adsorption-desorption curve diagram of the silicon dioxide product of Example 3;

[0014] Figure 4 This is a nitrogen adsorption-desorption curve diagram of the silicon dioxide product of Example 4;

[0015] Figure 5 This is a nitrogen adsorption-desorption curve diagram of the product of Comparative Example 1;

[0016] Figure 6 This is a nitrogen adsorption-desorption curve diagram of the product of Comparative Example 2;

[0017] Figure 7 This is the XRD pattern of ZSM-5 molecular sieve. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below through the accompanying drawings and embodiments, but the protection scope of the present invention is not limited to the contents described above; Example 1

[0019] At room temperature, a 15% mass concentration of fluorosilicic acid solution and a 15% mass concentration of ammonia water are simultaneously introduced into a tubular static mixing reactor, and the flow rate of fluorosilicic acid is controlled to be 16L / min, and the flow rate of ammonia water is 6L / min. The two react while flowing in the tubular static mixing reactor. After the reaction is completed, a slurry with a pH of 7.30 is obtained, which is filtered to obtain a filtrate containing ammonium fluorosilicate and ammonium fluoride and a transparent gel filter cake. The filter cake is broken and washed with clean water for the first time. The amount of water used is 3 times the mass of the filter cake. The second and third washings are spray washings, and the amount of water used is 3 times the mass of the filter cake. The wet gel after washing is dried at 140°C to obtain a silicon dioxide product. The silicon dioxide product obtained in this embodiment has a specific surface area of ​​456m 2 / g, mesopore specific surface area 456m 2 / g, pore volume 1.131cm 3 / g, hydrogen content 1.419%, its nitrogen adsorption and desorption curve is shown in Figure 1 . Example 2

[0020] At room temperature, a 15% mass concentration of fluorosilicic acid solution and a 15% mass concentration of ammonia water are simultaneously introduced into a tubular static mixing reactor, and the flow rate of fluorosilicic acid is controlled to be 16L / min, and the flow rate of ammonia water is controlled to be 8L / min. The two react while flowing in the tubular static mixing reactor. After the reaction is completed, a slurry with a pH value of 7.92 is obtained, and filtrate containing ammonium fluorosilicate and ammonium fluoride and a transparent gel filter cake are obtained. The filter cake is broken and the first re-slurry washing (using the washing water after the second washing in Example 1) is performed, and the amount of water used is 4 times the mass of the filter cake. The second washing is spray washing (using the washing water after the third washing in Example 1), and the third washing is spray washing (clean water washing), and the amount of water for spray washing is 4 times the mass of the filter cake. The wet gel after washing is dried at 110°C to obtain a silicon dioxide product. The silicon dioxide product obtained in this embodiment has a specific surface area of ​​401m 2 / g, mesopore specific surface area 384m 2 / g, micropore specific surface area 17m 2 / g, pore volume 0.891cm 3 / g, and its nitrogen adsorption and desorption curve is shown in Figure 2 . Example 3

[0021] At room temperature, a 10% mass concentration of fluorosilicic acid solution and a 10% mass concentration of ammonia water are simultaneously introduced into a tubular static mixing reactor, and the flow rate of fluorosilicic acid is controlled to be 14L / min, and the flow rate of ammonia water is 8L / min. The two react while flowing in the tubular static mixing reactor. After the reaction is completed, a slurry with a pH of 7.63 is obtained, and filtered to obtain a filtrate containing ammonium fluorosilicate and ammonium fluoride, and a transparent gel filter cake. The filter cake is broken for the first re-slurry washing (using the washing water after the second washing in Example 2), and the amount of water used is 4 times the mass of the filter cake. The second washing is spray washing (using the washing water after the third washing in Example 2), and the third washing is spray washing (clean water washing), and the amount of water for spray washing is 4 times the mass of the filter cake; the wet gel after washing is dried at 120°C to obtain a silica product. The silica product obtained in this example has a specific surface area of ​​441m 2 / g, mesopore specific surface area 441m 2 / g, pore volume 0.856cm 3 / g, and its nitrogen adsorption and desorption curve is shown in Figure 3 . Example 4

[0022] The preparation process of this embodiment is the same as that of embodiment 1, except that the mass concentration of the fluorosilicic acid solution is 20%, the flow rate of the fluorosilicic acid is 20 L / min, the mass concentration of the ammonia water is 25%, the flow rate of the ammonia water is 10 L / min, and a slurry with a pH of 7.89 is obtained. The specific surface area of ​​the obtained silicon dioxide product is 519 m 2 / g, mesopore specific surface area 519m 2 / g, pore volume 0.897cm 3 / g, and its nitrogen adsorption and desorption curve is shown in Figure 4 .

[0023] Comparative Example 1

[0024] This comparative example has the same preparation process as Example 1, except that the reaction end point pH is adjusted to be greater than 8 by controlling the flow rate of ammonia water, as follows:

[0025] At room temperature, a 15% fluorosilicic acid solution and a 15% ammonia solution were simultaneously introduced into a tubular static mixing reactor, and the flow rate of fluorosilicic acid was controlled to be 16L / h and the flow rate of ammonia solution was 13.60L / h. The two reacted while flowing in the tubular static mixing reactor. After the reaction was completed, a slurry with a pH value of 9.63 was obtained. The slurry was filtered to obtain a filtrate and a white carbon black filter cake. The obtained filter cake was washed three times, the first washing was a re-slurry washing, and the remaining two washings were spray washings. After washing, the filter cake was dried at 140°C to obtain a white carbon black solid product. The specific surface area of ​​the white carbon black product was 422m 2 / g, mesopore specific surface area 84m 2 / g, micropore specific surface area 338 m 2 / g, pore volume 0.264 cm 3 / g, hydrogen content 1.514%, its nitrogen adsorption and desorption curve is shown in Figure 5 .

[0026] Comparative Example 2

[0027] This comparative example uses commercially available white carbon black products and analyzes its various parameters. The specific surface area of ​​the white carbon black product is 138m 2 / g, mesopore specific surface area 105 m 2 / g, micropore specific surface area 33 m 2 / g, pore volume 0.585cm 3 / g, hydrogen content 1.051%, its nitrogen adsorption and desorption curve is shown in Figure 6 .

[0028] Example 5: Preparation of ZSM-5 molecular sieve in an alkali metal-free system

[0029] The silica in Example 1, the white carbon black in Comparative Example 1, and the white carbon black in Comparative Example 2 are used to prepare ZSM-5 molecular sieves. Specifically, TPAOH and pseudo-boehmite are mixed evenly and added to a reactor. The temperature is raised to 140°C and maintained for 2 hours. After cooling to room temperature, the silica in Example 1 or the white carbon black in Comparative Example and water are added to the reactor. The mixture is hydrothermally crystallized at 170°C in a high-pressure reactor for 48 hours, filtered, and the filter residue is dried at 120°C for 20 hours and calcined at 550°C for 3 hours.

[0030] The results show that the silicon dioxide product of Example 1 can produce a high-crystallinity ZSM-5 molecular sieve in the above-mentioned alkali-free metal system. The XRD pattern of the ZSM-5 molecular sieve is shown in Figure 7 It can be seen from the figure that the XRD graph shows characteristic diffraction peaks of ZSM-5 molecular sieve near 2θ=7.9°, 8.7°, 23.0°, 23.9°, and 24.3°, and there are no other impurity peaks, indicating that ZSM-5 molecular sieve is successfully synthesized; while the white carbon black in Comparative Examples 1 and 2 cannot synthesize ZSM-5 molecular sieve in an alkali metal-free system.

Claims

1. Application of silicon dioxide prepared from fluorosilicic acid in the preparation of ZSM-5 molecular sieve in an alkali metal-free system; The silicon dioxide prepared from fluorosilicic acid is prepared by mixing fluorosilicic acid and ammonia water at 15-50° C. in a tubular static mixing reactor or a microchannel reactor, and performing an amination reaction. The pH value at the reaction end point is controlled to be 7.3-8. The reaction product is filtered to obtain a filtrate and a filter cake containing ammonium fluoride and ammonium fluorosilicate. The filter cake is washed, dehydrated and dried to obtain the silicon dioxide. The specific surface area of ​​the silicon dioxide is greater than 400 m 2 / g, mesopore specific surface area is greater than 300m 2 / g.

2. The use according to claim 1, characterized in that: The mass concentration of fluorosilicic acid is 5%~20%, the mass concentration of ammonia water is 5%~25%, and the amount of ammonia water added is 20%~80% of the theoretical reaction molar amount.

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