Fluorosilicic acid purification method and application

Abstract

The invention provides a fluosilicic acid purification method and application, and belongs to the technical field of comprehensive utilization of fluosilicic acid. The fluosilicic acid purification method is mainly used for removing silica gel in a fluosilicic acid raw material and comprises the following steps that a coagulant is added into a to-be-treated fluosilicic acid aqueous solution to be mixed, a mixed solution is obtained, solid-liquid separation is conducted after silica gel sediment is separated out of the mixed solution, and the coagulant is perfluoroalkyl carboxylic acid. The fluosilicic acid purification method can be applied to preparation of fluorine-containing products from fluosilicic acid. According to the method, the silica gel in the fluosilicic acid is removed by using the low-dosage coagulant, the method is simple and easy to implement, the coagulant is quantitatively added after the concentration of the fluosilicic acid aqueous solution and the silica gel content are calculated in detail, and the silica gel is more efficiently and thoroughly removed under the condition of combining a low-temperature measure, and new impurities are not introduced.

Description

Technical Field

[0001] The invention belongs to the technical field of comprehensive utilization of fluorosilicic acid, and specifically relates to a fluorosilicic acid purification method and application. Background Art

[0002] Fluorosilicic acid (H2SiF6), as one of the important fluorine resources produced as a by-product of phosphate rock or phosphate fertilizer, can be produced by circulating and washing silicon tetrafluoride gas in the production of wet phosphoric acid (SiF4+H2O=H2SiF6+SiO2). The comprehensive application of fluosilicic acid not only saves fluorite resources, but also conforms to the green development model of resource recycling advocated by the country. For example, the preparation of hydrogen fluoride by decomposition reaction of concentrated fluosilicic acid aqueous solution with concentrated sulfuric acid is an important way to utilize fluosilicic acid.

[0003] However, fluorosilicic acid raw materials (fluorosilicic acid aqueous solution) often contain a small amount of silica gel, generally 0.5-2wt%, and are dispersed in the solution in the form of nanoparticles. During the transportation of fluorosilicic acid, silica gel is prone to scaling and clogging the pipeline, and it will also induce the precipitation of more silica gel during storage. Therefore, the fluorosilicic acid raw materials must first undergo a silica gel removal step before use.

[0004] Chinese patent CN221933170U discloses a silica gel recycling device in fluosilicic acid, and the working process is: the concentration of about 18% fluosilicic acid solution of wet-process phosphoric acid byproduct is sent to a silica filter through a transfer pump, and a filter with a filter cloth of more than 200 mesh is selected to remove the silica particles in the fluosilicic acid solution, and the silica filter cake is filtered and washed with water by a transfer pump three, and then enters the silica slurry tank. This purification system and method utilizes a filter device to remove silica gel by filtering. The filter cloth used by the filter device is more than 200 mesh, and the pore size of 200 mesh is 0.075mm, and the size of nano silica gel is between 1-100nm, so this method has limited effect on removing silica gel. Because the silica gel particles are extremely small, it is difficult to completely remove them using a general solid-liquid separation method. If ultrafiltration or nanofiltration materials are used for filtration, the efficiency is very low. If conventional flocculants such as polyacrylamide are used, the flocculation effect is poor under strong acid conditions. Summary of the invention

[0005] In order to solve the problem that the removal efficiency by ultrafiltration or nanofiltration in the prior art is low and the flocculation effect of polyacrylamide flocculants is poor under strong acid conditions, the first object of the present invention is to provide a method for purifying fluorosilicic acid.

[0006] The second object of the present invention is to provide a method for purifying fluorosilicic acid for use in preparing fluorine-containing products using fluorosilicic acid.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is: A fluosilicic acid purification method comprises the following steps: adding a coagulant to a fluosilicic acid aqueous solution to be treated and mixing to obtain a mixed solution, and performing solid-liquid separation after silica gel precipitation is precipitated from the mixed solution, wherein the coagulant is a perfluoroalkyl carboxylic acid.

[0008] The present invention is a pioneering invention, which uses perfluoroalkyl carboxylic acid as a coagulant, the coagulant is soluble in water, and its larger negatively charged anions contact with silica gel particles, destroying the charge balance of the original silica gel particles, so that the silica gel particles gather together with the coagulant anions as the center to form larger agglomerates, and the agglomerates have poor suspension properties and gradually precipitate under the action of gravity. The fluosilicic acid purification method proposed by the present invention removes silica gel more efficiently and thoroughly without introducing new impurities, creating conditions for the convenient application of subsequent fluosilicic acid.

[0009] Preferably, the perfluoroalkyl carboxylic acid is one or both of perfluoroheptanoic acid and perfluorooctanoic acid.

[0010] Preferably, the process of precipitating silica gel is carried out under static conditions, which are convenient for the precipitation of silica gel.

[0011] More preferably, the standing temperature is 0-15°C. Low temperature can make the fluosilicic acid aqueous solution more stable, inhibit the reaction of fluosilicic acid and water to continue to generate silica gel, reduce the consumption of fluosilicic acid, and effectively improve the efficiency of silica gel removal.

[0012] More preferably, the standing time is 30-60 min, so that the silica gel precipitation can be fully separated out.

[0013] Preferably, the molar ratio of the coagulant to the silica gel in the fluosilicic acid aqueous solution is 1:50-300. Adding too little coagulant will result in poor silica gel removal, while adding too much coagulant will not precipitate by itself, forming new impurities.

[0014] More preferably, the concentration of the fluosilicic acid aqueous solution is 15-30wt%, and the content of silica gel in the fluosilicic acid aqueous solution is 0.5-2wt%. Application within this range can achieve a suitable silica gel purification effect.

[0015] Preferably, the solid-liquid separation method includes one of filtration, centrifugal separation and gravity sedimentation, and the settled silica gel solids are removed from the fluosilicic acid aqueous solution.

[0016] To achieve the above-mentioned purpose, the present invention provides an application of a fluorosilicic acid purification method in the preparation of fluorine-containing products using fluorosilicic acid. Whether fluorosilicic acid is reacted with concentrated sulfuric acid to prepare hydrogen fluoride, or fluorosilicic acid is reacted with aqueous ammonia to prepare ammonium fluoride, the technical solution of the present invention can be used to purify fluorosilicic acid to reduce the problem of pipeline blockage during the preparation process. DETAILED DESCRIPTION

[0017] The present invention mainly aims at the problem that the removal efficiency of silica gel by ultrafiltration or nanofiltration in the prior art is low, and the flocculation effect of polyacrylamide flocculants is poor under strong acid conditions. Perfluoroalkyl carboxylic acid is used as a flocculant to destroy the charge balance of silica gel particles, so that silica gel aggregates and precipitates, thereby achieving the purpose of removing silica gel. At the same time, low-temperature standing can make the fluosilicic acid aqueous solution more stable, reduce the rightward movement of the following equation (1), and further reduce the total amount of silica gel precipitation. If the temperature is not lowered, new silica gel will be generated after the silica gel settles, which will reduce the overall removal efficiency.

[0018] H2SiF6+ 2H2O ←→ SiO2+ 6HF (1) The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0019] 1. Specific embodiments of the fluorosilicic acid purification method of the present invention Example 1 The fluorosilicic acid purification method of this embodiment includes the following steps: adding 0.5 g of perfluoroheptanoic acid to 500 g of an aqueous solution containing 1.5 wt % of silica gel and 18 wt % of fluorosilicic acid, with a coagulant to silica gel molar ratio of 1:91, mixing evenly, standing for 60 minutes, filtering to remove the precipitate, and the resulting filtrate is the purified fluorosilicic acid.

[0020] Example 2 The fluorosilicic acid purification method of this embodiment includes the following steps: adding 0.5 g of perfluoroheptanoic acid to 500 g of an aqueous solution containing 1.5 wt % of silica gel and 18 wt % of fluorosilicic acid, with a coagulant to silica gel molar ratio of 1:91, mixing evenly, cooling to 5° C. and standing for 30 minutes, filtering to remove the precipitate, and the resulting filtrate is the purified fluorosilicic acid.

[0021] Example 3 The fluorosilicic acid purification method of this embodiment includes the following steps: adding 0.65 g of perfluorooctanoic acid to 500 g of an aqueous solution containing 1.0 wt% silica gel and 15 wt% fluorosilicic acid, with a coagulant to silica gel molar ratio of 1:53, mixing evenly, cooling to 10°C and standing for 40 minutes, filtering to remove the precipitate, and the resulting filtrate is the purified fluorosilicic acid.

[0022] Example 4 The fluorosilicic acid purification method of this embodiment includes the following steps: adding 0.23 g of perfluorooctanoic acid to 500 g of an aqueous solution containing 2.0 wt% of silica gel and 30 wt% of fluorosilicic acid, with a coagulant to silica gel molar ratio of 1:300, mixing evenly, cooling to 15°C and standing for 30 minutes, centrifuging to remove the precipitate, and the resulting clear liquid is the purified fluorosilicic acid.

[0023] Comparative Example 1 The fluorosilicic acid purification method of this comparative example comprises the following steps: adding 0.5 g of conventional flocculant polyacrylamide to 500 g of an aqueous solution containing 1.5 wt % of silica gel and 18 wt % of fluorosilicic acid, mixing evenly, standing for 60 min, filtering to remove precipitates, and the obtained filtrate is the purified fluorosilicic acid.

[0024] 2. Experimental Examples This experimental example evaluates the silica gel content and coagulant residue of each embodiment, and the results are shown in Table 1. 200 g of the purified fluorosilicic acid obtained in Examples 1-4 were taken to detect the silica gel content and coagulant residue. The silica gel detection method is to filter the test liquid with a nanofiltration membrane, collect the filtrate and weigh it, and calculate the residual silica gel content (%) in the form of filtrate / the amount of fluorosilicic acid sampled after purification * 100%. High performance liquid chromatography tandem mass spectrometry was used for coagulant residue detection.

[0025] Table 1 Experimental results of various embodiments Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Residual silica content (%) 0.016 0.011 0.007 0.013 0.22 Coagulant concentration (ppm) 13 12 32 6 / From the results in Table 1, it can be seen that the residual content of fluorosilicic acid silica gel after purification in the embodiments of the present invention is between 0.007-0.016%, which is significantly lower than the 0.22% residual glue content in Comparative Example 1; the residual concentration of the coagulant is between 6-32ppm, which is only at the ppm level and does not affect subsequent use, indicating that the technical solution of the present invention has a good glue removal effect and almost no impurities are introduced.

[0026] 3. Application of Fluorosilicic Acid Purification Method in the Preparation of Fluorine-Containing Products Using Fluorosilicic Acid The above experimental examples prove that the technical solution of the present invention has a good effect on removing silica gel in fluosilicic acid. When fluosilicic acid is used to prepare fluorine-containing products (for example, fluosilicic acid is reacted with ammonia water to prepare ammonium fluoride, fluosilicic acid is reacted with concentrated sulfuric acid to prepare hydrogen fluoride, etc.), the fluosilicic acid raw material is first purified using the method of the above embodiment, and then the purified fluosilicic acid is used to participate in the preparation of fluorine-containing products. This can reduce pipeline blockage during storage or transportation, ensure the smooth progress of the reaction and avoid the introduction of a small amount of silica gel into the reaction system.

Claims

1. A method for purifying fluorosilicic acid, characterized in that: The following steps are involved: A coagulant is added to the hydrofluorosilicic acid aqueous solution to be treated and mixed to obtain a mixed solution. After silica gel precipitation is precipitated from the mixed solution, solid-liquid separation is performed, and the coagulant is a perfluoroalkyl carboxylic acid.

2. The method for purifying fluorosilicic acid according to claim 1, characterized in that: The perfluoroalkyl carboxylic acid is one or both of perfluoroheptanoic acid and perfluorooctanoic acid.

3. The method for purifying fluorosilicic acid according to claim 1, characterized in that: The process of separating out the silica gel precipitation is carried out under static conditions.

4. The method for purifying fluorosilicic acid according to claim 3, characterized in that: The standing temperature is 0-15°C.

5. The method for purifying fluorosilicic acid according to claim 3 or 4, characterized in that: The standing time is 30-60 min.

6. The method for purifying fluorosilicic acid according to claim 1, characterized in that: The molar ratio of the coagulant to the silica gel in the fluosilicic acid aqueous solution is 1:50-300.

7. The method for purifying fluorosilicic acid according to claim 6, characterized in that: The concentration of the fluosilicic acid aqueous solution is 15-30wt%, and the content of silica gel in the fluosilicic acid aqueous solution is 0.5-2wt%.

8. The method for purifying fluorosilicic acid according to claim 1, characterized in that: The solid-liquid separation method includes one of filtration, centrifugal separation and gravity sedimentation.

9. Use of the fluorosilicic acid purification method according to any one of claims 1 to 8 in the preparation of fluorine-containing products using fluorosilicic acid.