Hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid and method thereof

Abstract

The invention relates to the technical field of photovoltaic fluorine-containing waste acid reutilization, and discloses hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid and a method thereof, and the method comprises the following steps: S1, heating the photovoltaic fluorine-containing waste acid to 90-120 DEG C, cooling and refluxing to obtain a first mixed acid solution, and absorbing escaped gas by an aqueous solution to obtain a second mixed acid solution; s2, mixing the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution; s3, purifying the third mixed acid solution by using fluorine-resistant anion resin; and S4, distilling and concentrating the purified solution to obtain the industrial-grade hydrofluoric acid. According to the method disclosed by the invention, various methods of ion exchange and extraction are combined, and the anion impurities in the photovoltaic fluorine-containing waste acid are separated from fluorine ions according to the boiling points, the acting force with a solvent and the adsorption acting force with anion resin caused by different existing forms of different anions in a solution, so that the hydrofluoric acid in the fluorine-containing waste acid is extracted; and the extracted hydrofluoric acid solution accords with industrial I-class hydrofluoric acid products.

Description

Technical Field

[0001] The present invention relates to the technical field of recycling of photovoltaic fluorine-containing waste acid, and particularly to hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid and a method therefor. Background Art

[0002] With the rapid development of the photovoltaic industry, the consumption of hydrofluoric acid has increased, and the amount of high-concentration fluorine-containing waste acid generated has been increasing day by day. The resulting environmental pollution problems have become increasingly prominent, and a large amount of fluorine resources have been lost. Therefore, studying the recovery and utilization of fluorine resources in photovoltaic fluorine-containing waste acid is of great significance for the effective regeneration and utilization of a large amount of fluorine resources.

[0003] Currently, the more commonly used process is lime / potassium (sodium) salt / ammonia neutralization-flocculation precipitation process. Patent CN117776218A discloses adding a sodium source to fluorine-containing waste acid at a constant temperature, and obtaining sodium fluoride by a precipitation method. The content of F ions in the fluorine-containing waste acid in the method is 0.1% - 20%, and the content of NO 3- is 1% - 20%, and the purity of the synthesized sodium fluoride is above 99%. Patent CN118026434A discloses converting the fluorine resources in fluorine-containing waste acid into ammonium bifluoride. The precipitation method has simple operation, low cost, and rapid reaction, but has a large amount of sludge and is difficult to dehydrate. The resulting fluorine-containing sludge not only cannot recycle fluorine resources, but also forms fluorine-containing solid waste pollution. Patent CN105753211A discloses a method for preparing potassium fluorosilicate and calcium fluoride from photovoltaic fluorine-containing waste acid. In this method, the fluoride ions in photovoltaic waste acid are first converted into potassium fluorosilicate, and then calcium salt is used to convert potassium fluorosilicate into calcium fluoride. The obtained products are mainly fluorosilicic acid (17.6 wt%) and hydrofluoric acid (4.94 wt%). Since a large amount of other anions in the waste acid are wrapped during the coprecipitation process, there are still a large number of impurities in the products: the sulfuric acid content is 3.34 wt%, the hydrochloric acid content is 0.03 wt%, and the nitric acid content is 2.23 wt%. Although this method realizes the recovery of fluorine resources to a certain extent, due to the high impurity content, the purity and application range of the products are limited. In addition, the treatment of the chlorine-containing mixed waste generated during the coprecipitation process is difficult, and the risk of secondary pollution is high, further increasing the environmental protection pressure.

[0004] During the silicon wafer etching process, according to different production processes, different acids are selected for compounding in a specific ratio, so the composition of the generated fluorine-containing waste acid will also be different. The anions generally include fluoride ions, nitrate ions, fluorosilicate ions, sulfate ions, and chloride ions. In addition, the content of cation impurities in photovoltaic fluorine-containing waste acid is low. Using the above-mentioned fluorine-containing waste acid treatment process, not only the advantage of low content of cation impurities in itself cannot be utilized, but also the anion impurities cannot be effectively separated, resulting in the influence of anion impurities on the product purity.

[0005] Currently, the prior art has recovered hydrofluoric acid from etching waste acid. Patent CN112957758A discloses a method for recovering high-purity nitric acid and hydrofluoric acid from etching waste acid. The waste acid is heated, and the fluorosilicic acid in the waste acid boils, and the SiF4 component in the gas phase significantly increases. It reacts with condensed water in the condenser tube to generate SiO2 and HF. The SiO2 precipitates in the container, and the HF produced by hydrolysis and the HF originally evaporated are collected in the hydrofluoric acid storage tank after condensation to form a hydrofluoric acid solution with relatively high purity; the HF component in the original waste acid is recovered. Then, the temperature is heated to the boiling point of nitric acid to make nitric acid boil, and it is condensed in the nitric acid condenser tube and collected in the nitric acid storage tank after condensation, and the nitric acid component in the original waste acid is recovered. However, in the existing process for recovering hydrofluoric acid from waste acid, other anionic impurities are not effectively separated. For example, the removal of sulfate and chloride ions is not involved, resulting in the product purity not meeting the requirements of hydrofluoric acid purity. In addition, for different etching processes, the fluoride ion content and anionic impurity content in different waste acids are different, and the recovery of fluoride ions by a single process route is not complete, and still a part of fluoride ions are wasted. Summary of the Invention

[0006] The purpose of the present invention is to provide a hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid and its method, to solve the problem of unsatisfactory recovery rate of fluoride ions in the prior art.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A method for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid, comprising the following steps:

[0009] S1. Heat the photovoltaic fluorine-containing waste acid to 90-120 °C, cool and reflux to obtain a first mixed acid solution, and the escaped gas is absorbed by an aqueous solution to obtain a second mixed acid solution. In this step, as the temperature rises, the original hydrofluoric acid in the photovoltaic fluorine-containing waste acid gradually escapes, and the second mixed acid solution obtained after absorption by the aqueous solution contains HF, NO 3- 、Cl - . Setting up an aqueous solution absorption device can not only ensure safety, but also reduce the waste of hydrofluoric acid and improve the recovery and utilization efficiency. When heated to 90-120 °C, the fluorosilicic acid in the photovoltaic fluorine-containing waste acid undergoes a decomposition reaction to generate SiO2 precipitate and hydrogen fluoride gas. The gas is cooled and refluxed to obtain a first mixed acid solution, which contains SO4 2- 、HF、NO3 - 、Cl - . There may be incompletely decomposed fluorosilicic acid in the first mixed acid solution and the second mixed acid solution, but when subsequent purification is carried out using a fluoride-resistant anion resin, the fluorosilicate ion can also be separated.

[0010] S2. Mix the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution, and adjust the pH of the third mixed acid solution to be less than 3;

[0011] S3. Purify the third mixed acid solution using a fluoride-resistant anion resin;

[0012] S4. Distill and concentrate the solution purified in step S3 to obtain industrial-grade hydrofluoric acid.

[0013] Under the condition of pH less than 3, fluoride ions mainly exist in the forms of HF molecules and hydrofluoric acid positive ions (H2F + ). Under this condition, the weakly basic anion resin can preferentially adsorb other impurity ions in the solution, allowing hydrofluoric acid to flow out of the solution, realizing the resource recovery of hydrofluoric acid. This process utilizes the weakly basic groups contained in the weakly basic anion resin, which can maintain the pH stability of the solution, avoiding the influence of the strong basic environment on the existence form of fluorine, and thus more effectively realizing the extraction and purification of fluorine. After distillation and concentration, the concentration of the purified hydrofluoric acid is increased to obtain industrial-grade hydrofluoric acid.

[0014] As a preferred technical solution, before the purification in step S3, the fluoride-resistant anion resin is pretreated as follows: Add the fluoride-resistant anion resin to the photovoltaic fluorine-containing waste acid for reaction. After the resin is fully swollen, filter to remove the solution, add a regeneration reagent for resin regeneration, filter and then add the regeneration reagent again for full reaction. After filtration, rinse with deionized water until neutral, and soak in water for standby.

[0015] The resin needs to be pretreated because during the synthesis of the resin, low molecules, some inorganic impurities (such as copper, iron, etc.), molecular monomer substances, and pore-forming agents are mixed in the resin surface and pores. Therefore, before the resin is officially put into operation, these impurities must be removed, otherwise the resin will be polluted in various ways during use. The pretreatment refers to GB / T5476-2013 "Ion Exchange Resin Pretreatment Method". In the present invention, the HCl treatment link in the pretreatment is replaced with pretreatment using photovoltaic fluorine-containing waste acid. Compared with the HCl waste acid pretreatment, the residual chlorine content and nitrate content in the solution remain unchanged, and the retention of fluorine content increases. Therefore, directly using photovoltaic fluorine-containing waste acid for pretreatment is more conducive to the subsequent filtration and recovery of fluoride ions.

[0016] As a preferred technical solution, when using a fluorine-resistant anion resin for purification in step S3, the fluorine-resistant anion resin is filled into a filter column, and the third mixed acid solution is passed through the fluorine-resistant anion resin at a controlled flow rate of 2-4 mL / min. The solution flowing out in the time period of 30-50 min is collected, and the loading amount of the fluorine-resistant anion resin is ≥ two-thirds of the total volume of the liquid in the filter column. Controlling the flow rate at 2-4 mL / min is beneficial for the full ion exchange reaction between impurity anions and the fluorine-resistant anion resin. If the flow rate is too slow, fluoride ions will be lost; if the flow rate is too fast, the removal rate of impurity anions will be low. By analyzing the residence time of ions in the resin filter column and collecting the filtrate for 30-50 min, the maximum preservation rate of fluoride ions and the maximum removal rate of other impurity ions can be obtained.

[0017] As a preferred technical solution, before step S1, the anion content of the photovoltaic fluorine-containing waste acid is detected first. When the fluoride ion concentration in the photovoltaic fluorine-containing waste acid ≤ 12%, after step S3 is completed, re-purification is carried out, and then it enters step S4. The re-purification is to pre-adsorb the regenerated fluorine-resistant anion resin with a cleaning agent, and after water washing, it is used for re-purification; the cleaning agent is one of a hydrofluoric acid solution or a sodium fluoride solution. After the fluorine-resistant anion resin is activated, pre-treatment of the resin with HF or NaF can ensure that the fluorine adsorption sites on the resin can be occupied. After the photovoltaic fluorine-containing waste acid is re-purified twice, the chloride ion content and nitrate ion content can be further reduced, and it has a good impurity removal effect. Also, according to the requirements for impurity ions, if the requirements are still not met after two filtrations, resin columns pre-treated with HF or NaF can be connected in series for multiple filtrations.

[0018] As a preferred technical solution, before step S1, the anion content of the photovoltaic fluorine-containing waste acid is detected first. When the fluoride ion concentration in the photovoltaic fluorine-containing waste acid > 12%, the solution after purification in step S3 is subjected to extraction treatment. The extraction treatment includes the following steps:

[0019] 1) Extract the fluoride ions in the purified solution with an extractant to obtain an extraction solution enriched with fluoride ions;

[0020] 2) Distill the extraction solution, and after absorption with an aqueous solution, obtain a treated hydrofluoric acid solution;

[0021] 3) Purify the treated hydrofluoric acid solution with a polypyrrole-modified fluorine-resistant anion resin to obtain photovoltaic-grade hydrofluoric acid.

[0022] As a preferred technical solution, the extractant is carbon tetrachloride; in the step (2), the rectification temperature is 110-130°C and the rectification time is 12-18 h; in the step (3), the polypyrrole-modified fluoride-resistant anion resin is filled into a filter column, the extraction solution is passed through the resin, the flow rate is controlled at 2-4 mL / min, and the solution flowing out in the time period of 20-60 min is collected. The loading amount of the polypyrrole-modified fluoride-resistant anion resin is ≥ two-thirds of the total volume of the liquid in the filter column. During the rectification process, the contents of F - and NO3 - in the collected ammonia water receiving solution gradually increase, while SiF6 2- is not detected. In particular, the content of F - significantly increases after 12 hours and 18 hours, indicating that distillation helps the transfer of fluoride to the receiving flask, thereby collecting hydrofluoric acid in the fluoride-containing waste acid. In the step (3), the polypyrrole-modified fluoride-resistant anion resin is filled into a filter column, the extraction solution is passed through the resin, the flow rate is controlled at 2-4 mL / min, and the solution flowing out in the time period of 20-60 min is collected. The loading amount of the polypyrrole-modified fluoride-resistant anion resin is not less than two-thirds of the total volume of the liquid in the filter column. Selecting an appropriate resin dosage can not only achieve the removal rate of nitrate but also avoid waste of the adsorbent.

[0023] As a preferred technical solution, the polypyrrole-modified fluoride-resistant anion resin is prepared by the following steps:

[0024] The fluoride-resistant anion resin is added to the pyrrole solution, stirred and mixed evenly, then an oxidant is added, and the pyrrole monomer undergoes oxidative polymerization under acidic conditions and continues to be stirred at room temperature; after the reaction ends, it is repeatedly rinsed with deionized water, and then rinsed with an ethanol solution until the washing solution is colorless, and dried to obtain the polypyrrole-modified fluoride-resistant anion resin.

[0025] The resin modified by polypyrrole is very different from the original fluoride-resistant anion resin. After polypyrrole modification, it is beneficial to build more adsorption sites, thereby promoting the adsorption process. In addition, polypyrrole adheres to the surface of the activated carbon in a flocculent shape, which indicates that the unmodified resin can serve as a good modification carrier to make polypyrrole adhere to its surface, thereby improving the adsorption performance of the adsorbent for nitrate. In this step, the fluoride-resistant anion resin added to the pyrrole solution is the fluoride-resistant anion resin that has been pretreated.

[0026] As a preferred technical solution, the oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 1.5-2.5:1; and the oxidative polymerization time is 11.5-12.5h. If the dosage of different oxidants is too low, it is easy to cause the production of oligomers and waste pyrrole, while if the dosage of the oxidant is too high, the polypyrrole product will change from the originally stacked granular spheres to a sheet structure that is bonded together. This morphological change is due to the fact that the increase in the proportion of the oxidant significantly affects the number of pyrrole defect groups, and the increase in defect groups interferes with the growth law of polypyrrole. From the perspective of adsorption efficiency, the molar mass ratio of the oxidant to the pyrrole monomer is selected to be 1.5-2.5:1. Different polymerization times have a significant effect on the PPy adsorbent. When the time is short, the polymer has not been fully polymerized, and it is easy to form smaller granular polymers. As the polymerization time increases, the particle size of the polymer particles gradually increases, which effectively increases the polymerization degree of polypyrrole. Prolonging the polymerization time can obtain granular spheres with uniform particle size structure, but the longer the polymerization time is, the better it is. Excessive polymerization will also destroy its stable structure. Therefore, the oxidative polymerization time is selected to be 11.5 to 12.5 h.

[0027] Due to the high electronegativity of fluorine, hydrofluoric acid maintains a relatively high polarity even in non-polar solvents. According to the principle of like dissolves like, extraction is used to try to reduce the content of nitrate and sulfuric acid. Most of the fluoride ions are extracted into the non-polar extractant, which can achieve further separation of fluoride ions from other acid ions. The non-polar solution containing hydrofluoric acid is vaporized using distillation technology. Since the solubility of hydrofluoric acid in water is significantly higher than that of non-polar solvents, hydrofluoric acid and the extract are effectively separated by passing the gaseous substance into water. At the same time, distillation can further separate hydrofluoric acid from other acid substances by relying on the difference in boiling points of each component. After absorption by aqueous solution (leaching method is used in industry), more than 99% of hydrogen fluoride gas can be absorbed.

[0028] At this time, the anion impurity content of the hydrofluoric acid after extraction and distillation has been further reduced. At this time, the adsorption performance of the original fluorine-resistant anion resin for purification can no longer meet the requirements, and can no longer meet the requirements of the impurity content of photovoltaic-grade hydrofluoric acid. The fluorine-resistant anion resin is modified by polypyrrole, and the surface of the synthesized polypyrrole-coated resin is made rougher, with a large number of pores and denser pores, which makes the polypyrrole-modified resin very different from the original resin. After polypyrrole modification, it is beneficial to improve the polarity of the functional groups of the fluorine-resistant anion resin, and to build more adsorption sites, thereby promoting the adsorption process and further improving the adsorption performance of the resin. Even if the anion impurity content is low, it can be effectively separated from the hydrofluoric acid, thereby meeting the anion impurity requirements of photovoltaic-grade hydrofluoric acid.

[0029] As a preferred technical solution, the regeneration reagent of the fluoride-resistant anion resin is one of sodium hydroxide solution or ammonia water solution. In the present invention, the fluoride-resistant anion resin can continuously and effectively remove nitrate and chloride ions in multiple cycles, maintain the fluoride ion content, and exhibit good cycle stability. The adsorption capacity of the resin is limited under acidic conditions. To ensure the utilization efficiency of the resin, the resin can be regenerated multiple times to ensure the utilization efficiency of the resin.

[0030] Advantages of the present invention:

[0031] 1. The present invention combines multiple methods of ion exchange, distillation, and extraction. By virtue of the different forms of existence of different anions in the solution, which result in differences in their boiling points, the interaction forces with the solvent, and the adsorption interaction forces with the anion resin, the anion impurities in the photovoltaic fluorine-containing waste acid are separated from the fluoride ions, realizing the extraction of hydrofluoric acid in the fluorine-containing waste acid. In the extracted hydrofluoric acid solution, the content of anion impurities is significantly reduced, meeting the requirements of hydrofluoric acid products of industrial grade I.

[0032] 2. The present invention can utilize the characteristic of low cation impurity content in the photovoltaic fluorine-containing waste acid to plan and design the process route, collect fluoride ions from the photovoltaic fluorine-containing waste acid with different fluoride ion concentrations, and through further purification treatment, it is possible to prepare photovoltaic-grade hydrofluoric acid, which can be reused in the photovoltaic industry to reduce the production cost of photovoltaic solar cells.

[0033] 3. The present invention uses a fluoride-resistant anion resin to treat the anion impurities in the fluorine-containing waste acid. When the pH < 3, fluoride ions exist in the form of HF molecules or H2F + , and will not be adsorbed by the fluoride-resistant anion resin, thus realizing the effective separation from other anion impurities. At the same time, when the content of anion impurities is further reduced, by modifying the fluoride-resistant anion resin, the separation efficiency and adsorption capacity of the fluoride-resistant anion resin for fluoride ions can be further improved, realizing the effective extraction and separation of hydrofluoric acid in high-concentration fluorine-containing waste acid, and obtaining a hydrofluoric acid product that meets the photovoltaic E1 grade. Description of the drawings

[0034] Figure 1 It is a process flow chart for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid in the present invention;

[0035] Figure 2 It is a schematic diagram of the synthesis of polypyrrole-modified fluoride-resistant anion resin in Example 9 of the present invention;

[0036] Figure 3 It is an SEM image of the fluoride-resistant anion resin in Example 5 of the present invention;

[0037] Figure 4 It is the nitrogen adsorption-desorption isotherm and pore size distribution diagram of the fluoride-resistant anion resin in Example 5 of the present invention, where a is the isotherm and b is the pore size distribution;

[0038] Figure 5 For the poly(fluoride-resistant anion resin) modified with polypyrrole prepared by different process parameters in Example 9 of the present invention, the adsorption efficiency diagram of NO3 - is shown; where a is the molar ratio of oxidant to py; b is the polymerization time; c is the polymerization temperature;

[0039] Figure 6 SEM diagram of the poly(fluoride-resistant anion resin) modified with polypyrrole synthesized at different polymerization times in Example 9 of the present invention;

[0040] Figure 7 XRD pattern of the poly(fluoride-resistant anion resin) modified with polypyrrole in Example 9 of the present invention;

[0041] Figure 8 Particle size distribution diagram of the poly(fluoride-resistant anion resin) modified with polypyrrole in Example 9 of the present invention;

[0042] Figure 9 SEM diagram of the poly(fluoride-resistant anion resin) modified with polypyrrole, where a is multi-particles; b is the whole sphere; c is the unadsorbed surface; d is the adsorbed surface;

[0043] Figure 10 SEM-EDS surface scanning element content before and after adsorption of the poly(fluoride-resistant anion resin) modified with polypyrrole in Example 9 of the present invention;

[0044] Figure 11 FT-IR diagram of the poly(fluoride-resistant anion resin) modified with polypyrrole in Example 9 of the present invention. Detailed implementation manners

[0045] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and illustrated herein can be arranged and designed in various different configurations.

[0046] To make the technical means, creative features, achieved objectives and functions of the present invention easy to understand, the present invention will be further described below with reference to specific embodiments.

[0047] In the embodiments of the present invention, the photovoltaic waste acid provided by photovoltaic manufacturers is used as the treatment raw material to extract the fluorine resources in the waste liquid. The basic information of the waste acid provided by the photovoltaic manufacturers is shown in Table 1 below.

[0048] Table 1 Waste acid content table of photovoltaic manufacturers

[0049] fluoride-containing waste acid <![CDATA[F - (%)]]> <![CDATA[Cl - (%)]]> <![CDATA[NO3 - (%)]]> <![CDATA[SO4 2- (%)]]> <![CDATA[SiF6 2- (%)]]> 1# 9.86 3.02 0.08 0.01 - 2# 21.96 - 14.52 19.38 16.39

[0050] In the embodiments of the present invention, a commercial fluoride-resistant anion resin is used, which is purchased from Dow Chemical Company in the United States.

[0051] Example 1

[0052] A method for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid includes the following steps:

[0053] (1) Heating the photovoltaic fluorine-containing waste acid to 90 °C, cooling and refluxing to obtain a first mixed acid solution, and absorbing the escaped gas through an aqueous solution to obtain a second mixed acid solution;

[0054] (2) Mixing the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution, and adjusting the pH of the third mixed acid solution to be approximately 1 (pH value = 1 ± 0.1);

[0055] (3) Purifying the third mixed acid solution using a fluoride-resistant anion resin;

[0056] (4) Distilling and concentrating the solution purified in step (3) to obtain industrial-grade hydrofluoric acid;

[0057] Before step (3), the fluoride-resistant anion resin also undergoes pretreatment: adding the fluoride-resistant anion resin to the photovoltaic fluorine-containing waste acid for reaction, filtering to remove the solution after the resin is fully swollen, adding a regeneration reagent for resin regeneration, filtering and then adding the regeneration reagent again for reaction, filtering and rinsing with deionized water until neutral, and soaking in water for standby;

[0058] When using the fluoride-resistant anion resin for purification in step (3), the fluoride-resistant anion resin is filled into a filter column, the third mixed acid solution is passed through the resin, the flow rate is controlled at 2 mL / min, and the solution flowing out in the time period of 30 - 50 min is collected. The loading amount of the fluoride-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0059] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 1# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 9.86%, a re-purification step is also carried out between step (3) and step (4): pre-adsorbing the regenerated fluoride-resistant anion resin with a hydrofluoric acid solution or a sodium fluoride solution, and rinsing with water for use in re-purification;

[0060] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 2# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 21.96%, after step (3), the following steps are continued:

[0061] 1), Extracting the fluoride ions in the solution after the purification treatment in step (3) with an extractant to obtain an extraction liquid enriched with fluoride ions;

[0062] 2) The extract obtained from the rectification step 1) is absorbed by an aqueous solution to obtain a hydrofluoric acid solution;

[0063] 3) The hydrofluoric acid solution obtained in step 2) is purified using polypyrrole-modified fluoride-resistant anion resin to obtain photovoltaic-grade hydrofluoric acid;

[0064] In step 1), the extractant is carbon tetrachloride; in step 2), the rectification temperature is 110 - 130 °C, and the rectification time is 12 - 18 h; in step 3), the polypyrrole-modified fluoride-resistant anion resin is filled into a filter column, the extract is passed through the resin, the flow rate is controlled at 2 - 4 mL / min, and the solution flowing out in the time period of 20 - 60 min is collected. The loading amount of the polypyrrole-modified fluoride-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0065] The polypyrrole-modified fluoride-resistant anion resin is prepared through the following steps: adding the fluoride-resistant anion resin into a pyrrole solution, stirring and mixing evenly, then adding an oxidant, and performing oxidative polymerization of pyrrole monomers under acidic conditions, and continuing to stir at room temperature; after the reaction ends, it is repeatedly rinsed with deionized water, rinsed with an ethanol solution after the washing liquid is colorless, and dried to obtain the polypyrrole-modified fluoride-resistant anion resin;

[0066] The oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 2:1; the oxidative polymerization time is 11.5 h;

[0067] The regeneration reagent for the fluoride-resistant anion resin is sodium hydroxide solution or ammonia water solution.

[0068] Example 2:

[0069] A method for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid, comprising the following steps:

[0070] (1) Heating the photovoltaic fluorine-containing waste acid to 90 °C, cooling and refluxing to obtain a first mixed acid solution, and the escaping gas is absorbed by an aqueous solution to obtain a second mixed acid solution;

[0071] (2) Mixing the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution, and adjusting the ratio of the first mixed acid solution and the second mixed acid solution to make the pH of the third mixed acid solution ≈ 1 (pH value = 1 ± 0.1);

[0072] (3) Purifying the third mixed acid solution using fluoride-resistant anion resin;

[0073] (4) Distilling and concentrating the solution purified in step (3) to obtain industrial-grade hydrofluoric acid;

[0074] Before use, the fluorine-resistant anion resin in step (3) also undergoes pretreatment: adding the fluorine-resistant anion resin to the photovoltaic fluorine-containing waste acid for reaction, filtering to remove the solution after the resin is fully swollen, adding a regeneration reagent for resin regeneration, adding the regeneration reagent again for reaction after filtration, rinsing with deionized water until neutral, and soaking in water for standby; the regeneration reagent for the fluorine-resistant anion resin is sodium hydroxide solution.

[0075] When using the fluorine-resistant anion resin for purification in step (3), the fluorine-resistant anion resin is filled into a filter column, the third mixed acid solution is passed through the resin, the flow rate is controlled at 2 mL / min, and the solution flowing out in the time period of 30 - 50 min is collected. The loading amount of the fluorine-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column.

[0076] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 1# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 9.86%, a re-purification step is also carried out between step (3) and step (4): pre-adsorbing the regenerated fluorine-resistant anion resin with hydrofluoric acid solution or sodium fluoride solution, and using it for re-purification after rinsing with water.

[0077] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 2# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 21.96%, the following steps are continued after step (3):

[0078] 1), Extracting the fluoride ions in the solution after the purification treatment in step (3) with an extractant to obtain an extract enriched in fluoride ions;

[0079] 2), Rectifying the extract obtained in step 1), and obtaining hydrofluoric acid solution after absorption with an aqueous solution;

[0080] 3), Using the polypyrrole-modified fluorine-resistant anion resin to purify the hydrofluoric acid solution obtained in step 2) to obtain photovoltaic-grade hydrofluoric acid;

[0081] In step 1), the extractant is carbon tetrachloride; in step 2), the rectifying temperature is 110 °C and the rectifying time is 12 h; in step 3), the polypyrrole-modified fluorine-resistant anion resin is filled into a filter column, the extract is passed through the resin, the flow rate is controlled at 2 mL / min, and the solution flowing out in the time period of 20 - 60 min is collected. The loading amount of the polypyrrole-modified fluorine-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column.

[0082] The polypyrrole-modified fluoride-resistant anion resin is prepared through the following steps: adding the fluoride-resistant anion resin into a pyrrole solution, stirring and mixing evenly, then adding an oxidant, and performing oxidative polymerization on the pyrrole monomer under acidic conditions, and continuously stirring at room temperature; after the reaction ends, repeatedly rinsing with deionized water, and then rinsing with an ethanol solution until the washing liquid is colorless, and drying to obtain the polypyrrole-modified fluoride-resistant anion resin.

[0083] The oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 1.5:1; the oxidative polymerization time is 11.5 h.

[0084] Example 3:

[0085] A method for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid includes the following steps:

[0086] (1) Heating the photovoltaic fluorine-containing waste acid to 120 °C, cooling and refluxing to obtain a first mixed acid solution, and absorbing the evolved gas through an aqueous solution to obtain a second mixed acid solution;

[0087] (2) Mixing the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution, and the pH of the third mixed acid solution is approximately 1 (pH value = 1 ± 0.1);

[0088] (3) Using a fluoride-resistant anion resin to purify the third mixed acid solution;

[0089] (4) Distilling and concentrating the solution purified in step (3) to obtain industrial-grade hydrofluoric acid;

[0090] Before step (3), the fluoride-resistant anion resin also undergoes pretreatment: adding the fluoride-resistant anion resin into the photovoltaic fluorine-containing waste acid for reaction, filtering to remove the solution after the resin swells sufficiently, adding a regeneration reagent for resin regeneration, filtering, adding the regeneration reagent again for reaction, filtering, rinsing with deionized water until neutral, and soaking in water for standby;

[0091] When using the fluoride-resistant anion resin for purification in step (3), filling the fluoride-resistant anion resin into a filter column, passing the third mixed acid solution through the resin, controlling the flow rate at 4 mL / min, and collecting the solution flowing out in the time period of 30 - 50 min. The loading amount of the fluoride-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0092] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 1# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 9.86%, a re-purification step is also carried out between step (3) and step (4): pre-adsorbing the regenerated fluoride-resistant anion resin with a hydrofluoric acid solution or a sodium fluoride solution, rinsing with water, and then using it for re-purification;

[0093] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 2# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 21.96%, after step (3), the following steps are continued:

[0094] 1), Extract the fluoride ions in the solution after the purification treatment in step (3) with an extractant to obtain an extraction solution enriched with fluoride ions;

[0095] 2), Distill the extraction solution obtained in step 1), and obtain a hydrofluoric acid solution after absorption by an aqueous solution;

[0096] 3), Purify the hydrofluoric acid solution obtained in step 2) with polypyrrole-modified fluoride-resistant anion resin to obtain photovoltaic-grade hydrofluoric acid;

[0097] In step 1), the extractant is carbon tetrachloride; in step 2), the distillation temperature is 130 °C and the distillation time is 18 h; in step 3), fill the polypyrrole-modified fluoride-resistant anion resin into a filter column, pass the extraction solution through the resin, control the flow rate at 4 mL / min, collect the solution flowing out in the time period of 20 - 60 min, and the loading amount of the polypyrrole-modified fluoride-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0098] The polypyrrole-modified fluoride-resistant anion resin is prepared through the following steps: Add the fluoride-resistant anion resin to a pyrrole solution, stir and mix evenly, then add an oxidant, and the pyrrole monomer undergoes oxidative polymerization under acidic conditions, and continue to stir at room temperature; after the reaction ends, rinse repeatedly with deionized water, and then rinse with an ethanol solution after the washing solution is colorless, and dry to obtain the polypyrrole-modified fluoride-resistant anion resin;

[0099] The oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 2.5:1; the oxidative polymerization time is 12.5 h;

[0100] The regeneration reagent for the fluoride-resistant anion resin is sodium hydroxide solution or ammonia water solution.

[0101] Example 4:

[0102] A method for preparing hydrofluoric acid from photovoltaic fluorine-containing waste acid, including the following steps:

[0103] (1) Heat the photovoltaic fluorine-containing waste acid to 120 °C, cool and reflux to obtain a first mixed acid solution, and the escaped gas is absorbed by an aqueous solution to obtain a second mixed acid solution;

[0104] (2) Mix the first mixed acid solution and the second mixed acid solution to obtain a third mixed acid solution, and adjust the pH of the third mixed acid solution to approximately 1 (pH value = 1 ± 0.1);

[0105] (3) Purify the third mixed acid solution with fluoride-resistant anion resin;

[0106] (4) Distill and concentrate the solution purified in step (3) to obtain industrial-grade hydrofluoric acid;

[0107] Before use, the fluorine-resistant anion resin in step (3) also undergoes pretreatment: add the fluorine-resistant anion resin to the photovoltaic fluorine-containing waste acid for reaction. After the resin is fully swollen, filter to remove the solution, add a regeneration reagent for resin regeneration, filter and then add the regeneration reagent again for reaction. After filtration, rinse with deionized water until neutral, and soak in water for standby;

[0108] When using the fluorine-resistant anion resin for purification in step (3), fill the fluorine-resistant anion resin into a filter column, pass the third mixed acid solution through the resin, control the flow rate at 3 mL / min, and collect the solution flowing out in the time period of 30 - 50 min. The loading amount of the fluorine-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0109] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 1# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 9.86%, a re-purification step is also carried out between step (3) and step (4): pre-adsorb the regenerated fluorine-resistant anion resin with a hydrofluoric acid solution or a sodium fluoride solution, and after rinsing with water, use it for re-purification;

[0110] Before step (1), there is also an anion content detection step. When the photovoltaic fluorine-containing waste acid is the 2# fluorine-containing waste acid in Table 1 and the fluoride ion concentration = 21.96%, continue with the following steps after step (3):

[0111] 1), Extract the fluoride ions in the solution purified in step (3) with an extractant to obtain an extract enriched in fluoride ions;

[0112] 2), Rectify the extract obtained in step S1, and obtain a hydrofluoric acid solution after absorption with an aqueous solution;

[0113] 3), Purify the hydrofluoric acid solution obtained in step S2 with a polypyrrole-modified fluorine-resistant anion resin to obtain photovoltaic-grade hydrofluoric acid;

[0114] In step 1), the extractant is carbon tetrachloride; in step 2), the rectification temperature is 120 °C and the rectification time is 17 h; in step 3), fill the polypyrrole-modified fluorine-resistant anion resin into a filter column, pass the extract through the resin, control the flow rate at 3 mL / min, and collect the solution flowing out in the time period of 20 - 60 min. The loading amount of the polypyrrole-modified fluorine-resistant anion resin is not less than 2 / 3 of the total volume of the liquid in the filter column;

[0115] The polypyrrole-modified fluoride-resistant anion resin is prepared through the following steps: adding the fluoride-resistant anion resin into a pyrrole solution, stirring and mixing evenly, then adding an oxidant, and carrying out oxidative polymerization of pyrrole monomers under acidic conditions, and continuously stirring at room temperature; after the reaction is completed, rinsing repeatedly with deionized water, and then rinsing with an ethanol solution until the cleaning solution is colorless, and drying to obtain the polypyrrole-modified fluoride-resistant anion resin;

[0116] The oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 1.5:1; the oxidative polymerization time is 12 h; the regeneration reagent of the fluoride-resistant anion resin is a sodium hydroxide solution or an ammonia water solution.

[0117] The data of hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid (Example 4) are shown in Table 2 below.

[0118] Table 2 Data of hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid

[0119]

[0120] Example 5: Comparative experiment on the pretreatment of the fluoride-resistant anion resin before use

[0121] The surface morphology of the resin was tested by SEM. As Figure 3 shown, it can be observed that the surface of the spherical fluoride-resistant anion resin is relatively smooth, with many small pores. Before pretreatment, some impurities can be found on the resin surface, so it is necessary to carry out pretreatment experiments. At the cracks, it can be seen that the resin contains a large number of non-cylindrical pores that are interconnected, irregular in shape, and uneven in structure inside. This porous structure can serve as the diffusion channel and the adsorption site for the adsorbate.

[0122] The N2 adsorption-desorption isotherm, pore size distribution curve, and BET characterization results of the resin are shown in the figure. The specific surface area refers to the sum of the total outer surface areas of all particles in each gram of the substance. The specific surface area is closely related to the particle size, shape, and pore structure. For macroporous resins, the pore diameter is generally above 25 nm. As Figure 4 can be seen, the isotherm belongs to the IUPAC type Ⅳ isotherm. The type Ⅳ isotherm belongs to the adsorption of mesoporous solids, which is consistent with macroporous resins. A relatively obvious adsorption hysteresis loop appears between the relative pressures (P / P0) of 0.8 and 1. The reason for this is that when the mesoporous capillary condensation is filled, if the adsorbent still has pores with larger pore diameters or the intermolecular forces between the adsorbate molecules are stronger, multi-layer adsorption will continue to occur, and the adsorption isotherm will continue to rise. This shows that the commercial resin is a porous adsorbent with both mesopores and macropores. From the pore size distribution curve, it can be seen that the pore diameter is mainly distributed between 2 and 50 nm, which is a mesoporous material, consistent with the isotherm results. The BET specific surface area of the MP62 resin is 35.368 m 2 / g, and the total pore volume is 0.412 cm3 / g, where the proportion of the mesoporous pore volume (Vmes) in the total pore volume is the highest, which is 0.270 cm 3 / g, and the average pore diameter is 46.556 nm.

[0123] According to GB / T 5476 - 2013 "Ion Exchange Resin Pretreatment Method", the resin is pretreated with 1 mol / L HCl to ensure the removal of metal impurity ions in the resin. At the same time, in multiple experiments, we found that there is a swelling phenomenon in the untreated resin during actual use. In order to avoid the blockage of the solution flow path due to swelling of the resin particles during use, the photovoltaic fluorine-containing waste acid is used to pretreat the resin instead of HCl, and the treatment effects are compared as shown in Table 3.

[0124] Table 3 Influence of Different Pretreatments on Filtration

[0125] fluoride-containing waste acid <![CDATA[F - (%)]]> <![CDATA[Cl - (%)]]> <![CDATA[NO3 - (%)]]> HCL pretreatment - 1 0.30 1.71 0.07 HCL pretreatment - 2 3.89 1.95 0.07 waste acid pretreatment - 1 1.87 1.84 0.07 waste acid pretreatment - 2 3.27 1.89 0.07

[0126] After replacing the HCl treatment link in the pretreatment with the fluorine-containing waste acid treatment, after normal filtration of the fluorine-containing waste acid, the components of the collected filtrate are detected. It can be seen that compared with the pretreatment with HCl waste acid, after the pretreatment with the fluorine-containing waste acid, the residual chlorine content and nitrate content in the solution remain unchanged, and the retention of fluorine content increases.

[0127] Example 6: Comparative experiment on the effect of re-purification and impurity removal and the effect of primary purification and impurity removal.

[0128] The photovoltaic fluorine-containing waste acid that has undergone primary purification treatment with a fluorine-resistant anion resin is re-purified. In order to ensure the maximum retention of fluoride ions, before the secondary filtration, an additional pretreatment process of ion resin is added. After activating the resin with NaOH, the resin is pretreated with 0.5% HF and 0.5% NaF respectively to ensure that the fluorine adsorption sites on the resin can be occupied. The results are shown in Table 4 below.

[0129] Table 4 Re-purification Data Table

[0130] sample <![CDATA[F - (%)]]> <![CDATA[Cl - (%)]]> <![CDATA[NO3 - (%)]]> primary purification 4.88 1.24 0.021 0.5% HF 5.81 0.07 0.007 0.5% NaF 5.9-6 0.02 0.005 NaOH regeneration 3.11 1.22 0.019

[0131] For the waste acid after secondary filtration, the chloride ion content and nitrate content are further reduced, showing a good impurity removal effect. In the subsequent process design, according to the requirements for impurity ions, if the primary filtration does not meet the requirements, the resin pretreated with 0.5% HF or 0.5% NaF can be connected in series for secondary or multiple filtrations. If only NaOH regeneration is used and the secondary filtration is not carried out through the resin pretreated with 0.5% HF or 0.5% NaF, the fluoride ion content decreases due to participating in the adsorption occupancy, and the chloride ion content and nitrate content only decrease slightly.

[0132] Example 7: Comparative experiment on the purification effects of using polypyrrole-modified fluoride-resistant anion resin and the original fluoride-resistant anion resin after extraction and rectification.

[0133] In this example, photovoltaic and hydrofluoric acid were prepared using the same method as in Example 4, except that in step S3, the original fluoride-resistant anion resin was used to purify the hydrofluoric acid solution obtained in step S2, and the data of the obtained hydrofluoric acid are shown in Table 4 below.

[0134] Table 5 Data of the hydrofluoric acid solution obtained in step S2 after purification using different resins

[0135]

[0136] The experimental results in Table 5 show that for the hydrofluoric acid that has been purified, extracted, and rectified by the previous original fluoride-resistant anion resin, the content of anionic impurities in it has been further reduced. When using the original fluoride-resistant anion resin for secondary purification again, its adsorption performance can no longer meet the requirements and cannot further reduce NO3 - , SO4 2- to the required concentration of photovoltaic-grade hydrofluoric acid. At this time, the polypyrrole-modified fluoride-resistant anion resin has stronger adsorption performance. After secondary purification with the polypyrrole-modified fluoride-resistant anion resin, the concentrations of NO3 - , SO4 2- are significantly lower than those after treatment with the original fluoride-resistant anion resin.

[0137] Example 8: Recycling performance experiment of commercial fluoride-resistant anion resin.

[0138] In the process design, for the filtrate of 30 - 50 minutes, the maximum retention rate of fluoride ions and the maximum removal rate of other impurity ions can be obtained. To achieve the reusable of the resin, the resin regenerated with 4% NaOH was reused. As can be seen from Table 6, the resin has good stability when treating fluoride-containing waste acid. In the untreated sample, the fluorine content was 9.86%, the chlorine content was 3.42%, and the nitrate content was 0.08%. After 1 cycle, the chlorine content dropped significantly to 0.71%, the fluorine content decreased slightly to 8.92%, while the nitrate content remained basically unchanged. In subsequent multiple cycles, the chlorine content was stable between 0.7% and 0.9%, the fluorine content was maintained in the range of 8.77% to 8.95%, and the nitrate content hardly changed. This shows that the resin can continuously and effectively remove nitrate and chloride ions and maintain the fluorine ion content during multiple cycles, demonstrating good cycle stability. This ion resin can be recycled multiple times. Because the adsorption capacity of the resin is limited under acidic conditions, to ensure the utilization efficiency of the resin, the resin can be regenerated multiple times to ensure the utilization efficiency of the resin.

[0139] Table 6 Experimental data on the recycling performance of fluoride-resistant anion resin

[0140]

[0141] Example 9: Preparation experiment of polypyrrole-modified fluoride-resistant anion resin.

[0142] As Figure 2 shown, the polypyrrole-modified fluoride-resistant anion resin is prepared through the following steps: adding the fluoride-resistant anion resin into the pyrrole solution, stirring and mixing evenly, then adding an oxidant, and performing oxidative polymerization of pyrrole monomers under acidic conditions, and continuing to stir at room temperature; after the reaction is completed, rinsing repeatedly with deionized water, and then rinsing with ethanol solution until the cleaning liquid is colorless, and drying to obtain the polypyrrole-modified fluoride-resistant anion resin. Different process parameters are designed and their effects are compared.

[0143] Table 7 Composition ratios and preparation process parameters for preparing polypyrrole-modified fluoride-resistant anion resin with different oxidant dosages

[0144]

[0145]

[0146] Table 8 Process parameters for preparing polypyrrole-modified fluoride-resistant anion resin with different polymerization times

[0147]

[0148] Table 9 Process parameters for preparing polypyrrole-modified fluoride-resistant anion resin with different polymerization temperatures

[0149]

[0150] As Figure 5 shown, a series of polypyrrole products were synthesized under the conditions that the molar mass ratios of the oxidant to pyrrole monomers were 0.5:1, 1:1, 1.5:1, 2:1, and 2.5:1 respectively. According to the reaction equation of pyrrole polymerized with ferric chloride as the oxidant, the dosage of ferric chloride was 2.33:1. If the dosage of different oxidants was too low, it was easy to cause the generation of oligomers and waste of pyrrole, while if the dosage of the oxidant was too much, the polypyrrole product would change from the original stacked granular small balls to the plate-like structure agglomerated together. This morphological transformation was because the increase in the oxidant ratio significantly affected the number of pyrrole defect groups, and the increase in defect groups interfered with the growth law of polypyrrole. From the perspective of adsorption efficiency, it was better to select the molar mass ratio of the oxidant to pyrrole monomer of 2:1.

[0151] Different polymerization times have a significant effect on the polypyrrole modified fluoride-resistant anion resin. If the time is short, the polymer has not yet been fully polymerized, and it is easy to form smaller granular polymers. As the polymerization time increases, the particle size of the polymer particles gradually increases, which effectively increases the polymerization degree of polypyrrole. When the polymerization time is extended to 5h, 8h, and 12h (such as Figure 6 ), the polymer is in the form of granules with uniform particle size and regular structure. When the polymerization time is extended to 24h, the polymer becomes hardened, which is caused by excessive polymerization of pyrrole monomer due to excessive polymerization time. The above results show that the polymerization time has a significant effect on the morphology and structure of polypyrrole. Prolonging the polymerization time can obtain granular spheres with uniform particle size and structure, but the longer the polymerization time is, the better it is. Excessive polymerization will also destroy its stable structure. Therefore, the best choice for the oxidative polymerization time of Py on the resin surface is 12h( Figure 5 (b)).

[0152] Figure 5 (c) shows the effect of different polymerization temperatures on nitrate adsorption. As the reaction temperature increases from 4°C to 45°C, the efficiency is maintained at around 91.0% to 92.5%, indicating that within this temperature range, the synthesis temperature has little effect on the adsorption performance of the material. Considering the material ratio, reaction time and reaction temperature, the molar mass ratio of the oxidant and pyrrole monomer is selected as 2:1, the oxidative polymerization time is selected as 12h, and room temperature (25°C) is selected as the optimal synthesis temperature.

[0153] The XRD spectrum of polypyrrole modified fluoride-resistant anion resin is as follows: Figure 7 As shown. The adsorbent has characteristic fluctuations between 2θ=10° and 30°, indicating that the obtained material is essentially amorphous. The XRD of polypyrrole is attributed to the repeating unit of the pyrrole ring, that is, there are three polymerization modes between pyrrole monomers, namely α-α, α-β, and β-β. This diversified polymerization mode leads to poor symmetry and regularity of the polypyrrole molecular chain, resulting in the prepared polypyrrole being an amorphous non-crystalline substance.

[0154] from Figure 8 It can be seen that the particle size distribution of the modified resin coated with polypyrrole is reduced. This is because pyrrole is polymerized in the surface layer of the resin, which weakens the expansion performance of the resin and is beneficial to the water stability and thermal stability of the adsorbent.

[0155] For materials used for adsorption and separation, morphology, surface structure and porosity are very important. Figure 9The SEM images of the polypyrrole-modified fluoride-resistant anion resin adsorbent are shown. It can be seen from the figure that the overall shape is a uniform sphere, with an overall size of about 500 μm. Different from the original resin with pores and tiny cracks on the surface and dense pores on the surface, the surface of the synthesized polypyrrole-coated material is rougher, has a large number of pores and the pores are denser. This shows that the resin modified by polypyrrole is very different from the original resin, which is beneficial to building more adsorption sites and thus promoting the adsorption process. In addition, after the resin is modified by polypyrrole, the polypyrrole adheres to the surface of the activated carbon in a flocculent state, which indicates that the unmodified resin can serve as a good modification carrier for the polypyrrole to adhere to its surface, thereby improving the adsorption performance of the adsorbent for nitrates. Figure 9 (d) is the scanning test of the PPy-modified resin adsorbent after drying and sample preparation after adsorption saturation. Compared with Figure 9 (c) before adsorption, there is obviously a layer of impurity-like particles attached to the surface.

[0156] The results of EDS energy spectrum analysis are as Figure 10 shown. The nitrogen element content after adsorption increases significantly, from 5.91% to 10.91%, while the chlorine element decreases. The active sites on the material surface adsorb NO3 from the solution through ion exchange - , and release the chloride ions (Cl - ) originally bound to the material surface into the solution. The nitrogen element content on the material surface increases, while the chlorine element content decreases.

[0157] The polypyrrole-modified fluoride-resistant anion resin and the commercial original resin are ground to be particle-free respectively when sent for infrared detection. From the FTIR ( Figure 11 ) of the adsorbent, it can be seen that the samples show similar spectral peaks. The strong and broad peak at 3437 cm -1 corresponds to the O-H stretching vibration that may be due to water absorption, and there is an association phenomenon between groups due to hydrogen bonds. The peaks at 2929 cm -1 ~2765 cm -1 belong to the C-H stretching vibration. The peak at 1625 cm -1 corresponds to the C═C stretching vibration and deformation of the benzene ring, and the peak at 1453 cm -1 corresponds to the C-C characteristic absorption peak. PPy shows peaks at 1552 cm -1 and 790 cm -1 , which are related to the deformation vibration of the N-H bond and the out-of-plane bending vibration of the C-H bond on the polypyrrole. The peak at 1364.08 cm -1 shows the C-N stretching vibration of the tertiary amine group in the resin. It can be seen that the polymerization of PPy in the commercial resin has little effect on the groups and no new groups are generated. At 1186 cm -1 , 1025 cm -1The peak at this position is the in-plane bending vibration peak of the C-H bond; 921 cm -1 , 701 cm -1 The peak at this position is the out-of-plane bending vibration peak of the C-H bond.

[0158] Example 10: Regeneration experiment of polypyrrole-modified fluoride-resistant anion resin.

[0159] The used polypyrrole-modified fluoride-resistant anion resin was soaked in sodium hydroxide solution and oscillated for 12 h for desorption. As shown in Table 10, with the increase in the number of regeneration experiments, the adsorption capacity and removal rate of the polypyrrole-modified fluoride-resistant anion resin for nitrate gradually decreased. After 6 cycles, the removal rate was still as high as 88.55%. The continuous decrease in the adsorption capacity of the polypyrrole-modified fluoride-resistant anion resin for nitrate ions was due to the partial blockage of some pores of the polypyrrole-modified fluoride-resistant anion resin and the reduction in the weight of the adsorbent during the washing process after multiple adsorption and desorption.

[0160] Table 10 Reusability data table of polypyrrole-modified fluoride-resistant anion resin for adsorbing NO3 -

[0161] Cycle Qe η mg / g ％ 1 57.8885 96.6419 2 57.3395 95.72538 3 55.857 93.25042 4 55.9275 93.36811 5 54.51 91.00167 6 53.04 88.54758

[0162] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid, characterized in that: The following steps are involved: S1, heating photovoltaic fluorine-containing waste acid to 90-120° C., cooling and refluxing to obtain a first mixed acid solution, and absorbing the escaping gas with an aqueous solution to obtain a second mixed acid solution; S2, mixing the first mixed acid solution with the second mixed acid solution to obtain a third mixed acid solution, and adjusting the pH of the third mixed acid solution to less than 3; S3, purifying the third mixed acid solution using a fluorine-resistant anion resin; S4, distilling and concentrating the solution purified in step S3 to obtain industrial grade hydrofluoric acid.

2. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 1, characterized in that: In the step S3, the fluorine-resistant anion resin is pretreated before purification: the fluorine-resistant anion resin is added to photovoltaic fluorine-containing waste acid for reaction, the solution is filtered to remove the resin after the resin is fully swollen, a regeneration reagent is added to regenerate the resin, the regeneration reagent is added again after filtering to fully react, and after filtering, it is rinsed with deionized water to neutrality and soaked in water for use.

3. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 1, characterized in that: When the fluorine-resistant anion resin is used for purification in step S3, the fluorine-resistant anion resin is filled into the filter column, the third mixed acid solution passes through the fluorine-resistant anion resin, the flow rate is controlled to be 2-4 mL / min, and the solution flowing out over a period of 30-50 min is collected, and the filling amount of the fluorine-resistant anion resin is ≥ two-thirds of the total volume of the liquid in the filter column.

4. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 1, characterized in that: Prior to step S1, the photovoltaic fluorine-containing waste acid is tested for anion content. When the fluorine ion concentration in the photovoltaic fluorine-containing waste acid is ≤12%, re-purification is performed after step S3 is completed, and then step S4 is entered. The re-purification is to use a cleaning agent to pre-adsorb the regenerated fluorine-resistant anion resin, and then rinse with water for re-purification; the cleaning agent is one of a hydrofluoric acid solution or a sodium fluoride solution.

5. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 1, characterized in that: Before step S1, the photovoltaic fluorine-containing waste acid is first tested for anion content. When the fluorine ion concentration in the photovoltaic fluorine-containing waste acid is greater than 12%, after step S3 is completed, extraction treatment is performed and then step S4 is entered. The extraction treatment includes the following steps: 1) extracting fluoride ions in the purified solution with an extractant to obtain an extract rich in fluoride ions; 2) distilling the extract to obtain a treated hydrofluoric acid solution after absorption by an aqueous solution; 3) Purifying the treated hydrofluoric acid solution using a polypyrrole-modified fluoride-resistant anion resin to obtain photovoltaic-grade hydrofluoric acid.

6. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 5, characterized in that: The extractant is carbon tetrachloride; the distillation temperature in the step 2) is 110-130° C., and the distillation time is 12-18 hours; in the step 3), the polypyrrole-modified fluorine-resistant anion resin is filled into the filter column, the extract passes through the resin, the flow rate is controlled to be 2-4 mL / min, and the solution flowing out in a time period of 20-60 minutes is collected, and the filling amount of the polypyrrole-modified fluorine-resistant anion resin is ≥ two-thirds of the total volume of the liquid in the filter column.

7. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 5, characterized in that: The polypyrrole modified fluoride-resistant anion resin is prepared by the following steps: The fluorine-resistant anion resin is added to the pyrrole solution, wherein the mass ratio of the resin to the pyrrole monomer is 15-25:1, and after stirring and mixing evenly, an oxidant is added, and the pyrrole monomer is oxidatively polymerized under acidic conditions, and stirring is continued at room temperature; after the reaction is completed, it is repeatedly rinsed with deionized water, and then rinsed with ethanol solution after the cleaning liquid is colorless, and after drying, a polypyrrole-modified fluorine-resistant anion resin is obtained.

8. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 7, characterized in that: The oxidant is ferric chloride; the molar mass ratio of the oxidant to the pyrrole monomer is 1.5-2.5:1; and the oxidative polymerization time is 11.5-12.5 hours.

9. A method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid as claimed in claim 2, characterized in that: The regeneration agent of the fluoride-resistant anion resin is a sodium hydroxide solution or an ammonia solution.

10. A hydrofluoric acid prepared from photovoltaic fluorine-containing waste acid, characterized in that: The hydrofluoric acid prepared by the method for preparing hydrofluoric acid using photovoltaic fluorine-containing waste acid according to any one of claims 1 to 9.

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