Process for the selective recovery of fluorine from wet-process phosphoric acid and the preparation of lithium difluorooxalato borate

Abstract

The present application relates to the field of comprehensive utilization of wet-process phosphoric acid resources and preparation of new energy materials, and discloses a method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorophosphate. By adding boric acid to the wet-process phosphoric acid, the fluorine species is converted, and tri-octylamine organic extraction system is used to selectively separate fluorine and phosphorus, and then lithium fluoroborate is prepared by back extraction, and further reacts with anhydrous oxalic acid to generate lithium difluorophosphate, and after recrystallization, battery-grade lithium difluorophosphate with a purity of 99.7% or above is obtained. The method realizes efficient recovery and high-value conversion of by-product fluorine resources in wet-process phosphoric acid, has the advantages of simplified process, high extraction selectivity, high resource utilization rate and significantly improved product added value, and can be used for preparation of lithium ion battery electrolyte.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization of wet-process phosphoric acid resources and preparation of new energy materials, specifically to a method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate through fluorine speciation regulation, selective extraction and multi-step synergistic conversion. Background Technology

[0002] Wet-process phosphoric acid, a core intermediate in the phosphorus chemical industry, generates a large amount of fluorine-containing byproducts during its production. This system is complex, mainly consisting of phosphoric acid, free sulfuric acid, and various fluorine species (such as F). - (e.g., HF, H2SiF6, etc.) and Fe 3+ Al 3+ Ca 2+ These include metallic impurity ions. If fluorine resources are not recycled, it not only wastes strategic resources but also poses a risk of fluoride pollution, creating a potential threat to the ecological environment.

[0003] In existing technologies, wet-process fluorine recovery from phosphate mainly relies on high-temperature stripping: the feed liquid is heated to above 100-120°C, and air or steam is introduced as a carrier gas, causing volatile fluorine components such as HF to escape. These fluorine components are then absorbed by alkaline solution or water to form low-value products such as sodium fluorosilicate or calcium fluoride. Although this process can achieve preliminary fluorine transfer, it suffers from problems such as high energy consumption, severe equipment corrosion, large fluctuations in fluorine recovery rate (typically 50-65%), low product purity, and difficulty in directly using it in high-end applications. Summary of the Invention

[0004] In existing technologies, the recovery of fluorine resources in wet-process phosphoric acid systems generally suffers from problems such as low recovery efficiency, uncontrollable morphology, poor separation selectivity, and lack of high-value-added conversion pathways. As a result, fluorine resources are difficult to meet the stringent requirements of emerging applications such as lithium battery electrolyte additives for high-purity and high-stability fluorinated lithium salts.

[0005] This invention relates to a method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate. By constructing a four-stage synergistic mechanism—in-situ conversion of fluoroboric acid, selective complexation extraction with trioctylamine, lithium source back-extraction and enrichment, and Lewis acid-catalyzed oxalic acid substitution—it achieves the directional migration, efficient separation, and precise conversion of fluorine from the wet-process phosphoric acid system. Furthermore, it addresses key technical issues at the mechanistic level, such as the instability of fluorine speciation, difficulty in separating phosphorus and fluorine, insufficient lithium salt purity, and uncontrollable battery-grade quality. The specific solution is as follows: This invention provides a method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate, comprising the following steps: (1) Premixing: adding boric acid to the wet-process phosphoric acid system in proportion for premixing, so that the fluorine species in it are converted into fluoroboric acid; (2) Selective extraction: using an organic extraction system containing trioctylamine to selectively extract the converted fluoroboric acid, thereby achieving efficient separation of phosphorus and fluorine; (3) Back-extraction: adding water and an alkaline lithium source to the organic phase obtained by extraction in proportion for back-extraction, thereby obtaining solid lithium fluoroborate; (4) Synthesis of lithium difluorooxalate borate: dissolving the purified lithium fluoroborate and anhydrous oxalic acid in a non-protic polar organic solvent, and carrying out a Lewis acid catalytic reaction under inert gas protection to obtain crude lithium difluorooxalate borate; (5) Purification and refining of lithium difluorooxalate borate: recrystallizing the obtained crude lithium difluorooxalate borate 2-3 times in a non-polar organic solvent to obtain lithium difluorooxalate borate with a purity ≥99.7%.

[0006] Furthermore, before adding boric acid in step (1), the fluorine content in the system must be determined, and then added according to the molar ratio of fluorine to boric acid of 1:(0.25~0.35).

[0007] Furthermore, the trioctylamine-containing organic extraction system in step (2) includes trioctylamine, n-octanol and kerosene, wherein trioctylamine accounts for 20% to 50% of the total volume of the extraction system and n-octanol accounts for 5% to 15% of the total volume.

[0008] Further, in step (2), the trioctylamine extractant is added to the premixed liquid-solid mixture at a ratio of O / A = 0.5~2.0, stirred and allowed to stand until the organic phase and acid phase separate into layers.

[0009] Further, in step (3), water is added to the organic phase at a volume ratio of organic phase:water = (1:0.05~0.2), and alkaline lithium source is added at a molar ratio of F:Li = 1:(0.25~0.35). After mixing, the mixture is allowed to stand and separate. The organic phase is circulated to the extraction stage. The aqueous phase turbid liquid is purified by ethanol filtration, and then recrystallized and dried to obtain lithium fluoroborate solid.

[0010] Furthermore, the alkaline lithium source used in step (3) is lithium hydroxide, lithium carbonate, or lithium bicarbonate.

[0011] Further, in step (4), the molar ratio of anhydrous oxalic acid to lithium fluoroborate is (1~1.5):1, and the amount of catalyst added is 1%~5% of the total mass of anhydrous oxalic acid and lithium fluoroborate.

[0012] Furthermore, the aprotic polar organic solvent used in step (4) is dimethyl carbonate, acetonitrile, tetrahydrofuran or dichloromethane, the catalyst is anhydrous aluminum trichloride, tin tetrachloride, ferric trichloride, titanium tetrachloride or zinc dichloride, and the inert gas is nitrogen.

[0013] Furthermore, the nonpolar organic solvent in step (5) is tetrachloromethane, benzene, toluene, or xylene.

[0014] Furthermore, the lithium difluorooxalate borate prepared according to the method described above is used as an electrolyte for lithium-ion batteries.

[0015] Compared with the prior art, this application has at least the following beneficial effects: By converting boric acid in situ, various dispersed, volatile, and highly corrosive fluorine species in wet-process phosphoric acid are uniformly converted into stable, low-toxicity, and complexable fluoroboric acid, fundamentally avoiding the risk of HF release and equipment corrosion problems. Relying on the specific complexing ability of trioctylamine to fluoroborate in acidic media, efficient separation of phosphorus and fluorine is achieved (P / F separation ratio > 1000), significantly improving the fluorine recovery rate (up to 85% or more), breaking through the bottleneck of recovery rate in traditional stripping methods; By using a water-alkaline lithium source dual-factor back-extraction system, lithium fluoroborate is selectively precipitated under mild conditions (room temperature and pressure), and the resulting intermediate has a purity of ≥99.2%, providing a reliable raw material guarantee for the subsequent synthesis of high-purity lithium salts; By using an oxalic acid substitution reaction catalyzed by Lewis acids, two fluorine atoms in lithium fluoroborate are precisely replaced by oxalic acid groups, thus constructing the molecular framework of lithium difluorooxalateborate in one step. After 2-3 recrystallizations in nonpolar solvents, a battery-grade product with a purity ≥99.7% is obtained. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of the method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate according to the present invention. Detailed Implementation

[0017] The first aspect of this invention provides a method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate, the specific steps of which are as follows: (1) First, the fluoride concentration in the crude phosphoric acid solution is determined by a fluoride ion selective electrode, and the total fluoride content in the crude phosphoric acid is calculated. Boric acid solid is added to the solution for premixing for 5-10 minutes to convert the fluoride species into fluoroboric acid. This step gives the fluoride species an anionic form that can be selectively complexed by trioctylamine (Formulas 1 and 2).

[0018] (2) After step (1) is completed, add the trioctylamine extractant to the liquid-solid mixture at an O / A (organic phase / aqueous phase) ratio of 0.5~2.0, and stir for 5~10 minutes. The HBF4 in the solution is transferred to the organic phase by the trioctylamine ligand in the extractant (Formula 3). Allow to stand until the organic and acidic phases separate, then separate the liquid to obtain the phosphate raffinate phase and the extract phase. This step utilizes the reaction of trioctylamine with H+ in an acidic medium.+ -BF 4- The ability to form stable ion-paired compounds enables efficient decoupling and separation of fluorine from high-concentration phosphate and metal impurities. After filtering to remove silica solids from the phosphate raffinate phase, the fluorine concentration in the phosphate solution after extraction is measured, and the total fluorine content and fluorine extraction rate are calculated. The trioctylamine extractant in step (2) consists of trioctylamine (ligand), n-octanol (co-solvent), and kerosene (solvent), wherein trioctylamine accounts for 20%–50% of the total volume of the extractant, and n-octanol accounts for 5%–15% of the total volume of the extractant. In this step, trioctylamine acts as the main extractant, which is protonated under acidic conditions to form R3NH. + and BF4 - Hydrophobic ionic pair R3NH is formed through electrostatic interaction. + ·BF4 - Entering the organic phase; n-octanol, as a co-extractant, enhances the extraction of R3NH through hydrogen bonding. + ·BF4 - On the one hand, it improves the solubility stability in the organic phase, and on the other hand, it reduces interfacial tension, inhibits emulsification, and prevents the formation of a third phase; kerosene, as an inert diluent, adjusts the viscosity and density difference of the system and promotes rapid stratification of the two phases; the synergistic effect of the three, within the ratio of 20% to 50% trioctylamine and 5% to 15% n-octanol, achieves high selectivity, high capacity and good phase behavior for HBF4, and keeps the fluorine extraction rate stably maintained above 85%.

[0019] (3) Add a small amount of water to the extract phase, and add the alkaline lithium source at a ratio of Li:F = 0.25~0.35. Mix for 5-10 min, and let stand to separate the liquids. The organic phase after back-extraction is recycled to the extraction stage, while excess ethanol is added to the remaining aqueous turbidity. The insoluble matter is filtered out, and the mixture is evaporated, concentrated, and recrystallized at 60-80℃. The bound water is then removed by vacuum drying at 100-130℃ to obtain solid lithium fluoroborate. The alkaline lithium source is lithium hydroxide, lithium carbonate, or lithium bicarbonate solid, etc.

[0020] (4) Add the solid to an aprotic polar organic solvent at a molar ratio of anhydrous oxalic acid to lithium fluoroborate = 1~1.5, and add a small amount of Lewis acid catalyst (Formula 4) at 1%~5% of the total mass of oxalic acid and lithium fluoroborate. Use N2 as a protective gas, and absorb the tail gas through sodium hydroxide solution. Recycle at 60~120℃ for 5~8 hours until no acidic gas is generated in the solution. Stop heating and slowly cool down to crystallize and obtain crude lithium difluorooxalate borate. The aprotic polar organic solvent includes dimethyl carbonate, acetonitrile, tetrahydrofuran, dichloromethane, etc., and the catalyst includes anhydrous aluminum trichloride, tin tetrachloride, ferric chloride, titanium tetrachloride, zinc dichloride, etc.

[0021] The crude lithium difluorooxalate borate was redissolved in a non-polar organic solvent and then recrystallized. Step (5) was repeated 2-3 times to obtain battery-grade lithium difluorooxalate borate. The non-polar organic solvents included tetrachloromethane, benzene, toluene, and xylene.

[0022] Unless otherwise specified, all materials, reagents, and instruments used in the embodiments of this invention are commercially available. The wet-process phosphoric acid samples in this invention, as determined by ICP-AES, have a fluorine content of 1.4–1.8 wt.% and a P₂O₅ content of 20.0–55.0 wt.%. To further illustrate the technical means and effects adopted by this invention to achieve its intended purpose, the following detailed description, in conjunction with the accompanying drawings and preferred embodiments, describes the specific implementation methods, structures, features, and effects of this invention.

[0023] Example 1 (Optimal Example) (1) Premixing: Weigh 1000 g of wet phosphoric acid (the content of each element in the wet phosphoric acid is calculated as oxides: P2O5 content is 27 wt.%, Al2O3 content is 1.3 wt.%, MgO content is 0.25 wt.%, Fe2O3 content is 1.2 wt.%, CaO content is 0.3 wt.%, F content is 1.8 wt.%), and determine the fluorine content to be 1.8 wt.% by ion selective electrode method. Calculate the amount of fluorine, and then feed it according to the boric acid:fluorine molar ratio = 0.3. Weigh 17.9 g of boric acid and add it to 1000 g of wet phosphoric acid solution. Stir mechanically at room temperature for 5 min to complete the premixing. Extraction: Trioctylamine extractant was prepared according to the volume ratio of 35% trioctylamine, 10% n-octanol / v, and 55% kerosene; 1000 mL of extractant was added to the premixed solution at O / A (organic phase / aqueous phase) = 1, stirred for 10 min, allowed to stand for 10 min to separate the layers, and then separated to obtain the phosphate raffinate phase and the fluorine-loaded organic phase; Back-extraction: Take 500 mL of organic phase, add 50 mL of deionized water, add 5.7 g of lithium hydroxide solid (LiOH) at a molar ratio of Li:F = 0.25 (based on initial fluorine), stir at room temperature for 5 min, allow to stand and separate into layers, take the aqueous phase, add 3 times the volume of anhydrous ethanol, filter out the insoluble matter, evaporate and concentrate the filtrate at 80 °C and recrystallize, dry under vacuum at 120 °C to remove bound water, and obtain 19.1 g of lithium fluoroborate solid. The fluoroborate-selective electrode analysis showed that the lithium fluoroborate content was 99.2 wt.%, and the fluorine recovery rate was calculated to be 86.1%. Synthesis: 19.1 g of lithium fluoroborate and 22.0 g of anhydrous oxalic acid (molar ratio of oxalic acid:lithium fluoroborate = 1.2:1) were dissolved in 100 mL of dimethyl carbonate. 1.23 g of anhydrous AlCl3 (3% of the total mass) was added, and high-purity nitrogen was introduced as a protective gas. The reaction was refluxed at 80 °C for 6 hours. The tail gas was absorbed by sodium hydroxide solution until no acidic gas was produced in the solution. After the reaction was completed, the temperature was lowered to 25 °C and allowed to stand for crystallization to obtain crude lithium difluorooxalate borate. Recrystallization: Crude lithium difluorooxalate borate was purified by recrystallization. The crude product was added to 50 mL of toluene, heated to reflux under nitrogen protection, and stirred thoroughly. It was then slowly cooled to 25°C and allowed to crystallize. The precipitated solid was separated by vacuum filtration. The obtained solid was recrystallized twice more using the same method to further remove organic impurities and unreacted raw materials. The final product was dried under vacuum at 60°C for 12 h to obtain 28.5 g of white crystalline powder; the purity was determined by HPLC to be 99.7 wt.% (area normalization method), and the yield was 97.3% (based on initial lithium difluorooxalate).

[0024] In this embodiment, the content of each element in the wet-process phosphoric acid solution, calculated as oxides, is as follows: P2O5 content 35 wt.%, Al2O3 content 1.1 wt.%, MgO content 0.20 wt.%, Fe2O3 content 1.0 wt.%, CaO content 0.25 wt.%, and F content 1.6 wt.%. The trioctylamine extractant used includes trioctylamine, n-octanol, and kerosene, with trioctylamine accounting for 30% of the total volume and n-octanol accounting for 10% of the total volume. The method includes the following steps: (1) Premixing: According to the molar ratio of boric acid to fluorine of 0.28, 15.1 g of boric acid solid was added to 1000 g of wet phosphoric acid solution and premixed for 5 minutes; (2) Extraction: Add the trioctylamine extractant to the liquid-solid mixture at a ratio of O / A=1, stir for 10 minutes, let stand for stratification and then separate the liquids; (3) Back-extraction: 60 mL of water was added to the extract phase, and 14.3 g of lithium bicarbonate solid was added at a molar ratio of Li:F = 0.25 (based on initial fluorine). After mixing for 5 minutes, the mixture was allowed to stand and separated. Excess ethanol was added to the aqueous phase, and the insoluble matter was filtered off. The mixture was evaporated and concentrated at 80 °C and recrystallized. It was then dried under vacuum at 120 °C to obtain 17.6 g of lithium fluoroborate solid. The fluorine recovery rate was 89.4%. (4) Synthesis: Anhydrous oxalic acid: lithium fluoroborate = 1.1 was added to 100 mL of acetonitrile with 18.5 g of anhydrous oxalic acid and 17.6 g of lithium fluoroborate. 1.1 g of tin tetrachloride (mass ratio 3%) was added and refluxed at 80 °C for 6 hours. The mixture was then cooled and crystallized to obtain crude lithium difluorooxalate borate.

[0025] (5) Recrystallization: The crude lithium difluorooxalate borate was redissolved in benzene and recrystallized. This process was repeated 2-3 times to obtain 26.28 g of lithium difluorooxalate borate with a purity of 99.8 wt.% and a yield of 97.4%.

[0026] Example 3 In this embodiment, the content of each element in the wet-process phosphoric acid solution, calculated as oxides, is as follows: P2O5 content is 40 wt.%, Al2O3 content is 1.0 wt.%, MgO content is 0.18 wt.%, Fe2O3 content is 0.9 wt.%, CaO content is 0.22 wt.%, and F content is 1.4 wt.%. The trioctylamine extractant used includes trioctylamine, n-octanol, and kerosene, with trioctylamine accounting for 25% of the total volume and n-octanol accounting for 10% of the total volume. The method includes the following steps: (1) Premixing: According to the molar ratio of boric acid to fluorine of 0.28, 15.1 g of boric acid solid was added to 1000 g of wet phosphoric acid solution for premixing for 5 minutes.

[0027] (2) Extraction: Add the trioctylamine extractant to the liquid-solid mixture at a ratio of O / A=1 and stir for 10 minutes. After standing and separating the layers, separate the liquids.

[0028] (3) Back-extraction: 50 mL of water was added to the extract phase, and 7.6 g of lithium carbonate solid was added at a molar ratio of Li:F = 0.25. After mixing for 5 minutes, the mixture was allowed to stand and separated. Excess ethanol was added to the aqueous phase, and the insoluble matter was filtered off. The solution was evaporated and concentrated at 80 °C and recrystallized. The solution was then dried under vacuum at 110 °C to obtain 15.3 g of lithium fluoroborate solid. The fluorine recovery rate was 88.4%.

[0029] (4) Synthesis: Anhydrous oxalic acid: lithium fluoroborate = 1.2. 17.6 g of anhydrous oxalic acid and 15.3 g of lithium fluoroborate were added to 100 mL of dichloromethane, and 0.72 g of zinc dichloride (2% by mass) was added. The mixture was refluxed at 90 °C for 6 hours. Crude lithium difluorooxalate borate was obtained by cooling and crystallization.

[0030] (5) Recrystallization: The crude lithium difluorooxalate borate was redissolved in xylene and recrystallized. This process was repeated 2-3 times to obtain 22.8 g of lithium difluorooxalate borate with a purity of 99.8 wt.% and a yield of 96.8%.

[0031] Example 4 With all other preparation conditions identical to those in Example 1, the only difference was that the volume fraction of trioctylamine in the trioctylamine extractant was 50%, and the n-octanol and kerosene were adjusted to 15% and 35% respectively. The remaining steps were the same as in Example 1. The results showed that under these conditions, the fluorine extraction rate was 85.3%, the lithium fluoroborate yield was 18.6 g / 1000 g phosphoric acid, and the final product purity of lithium difluorooxalate borate was 99.7 wt.%, with a yield of 96.9%.

[0032] Example 5 With all other preparation conditions identical to those in Example 1, the only difference was that the O / A ratio was changed from 1 to 2, i.e., 2000 mL of extractant was added; the remaining steps remained unchanged. The results showed that under these conditions, the fluorine extraction rate was 85.8%, the lithium fluoroborate yield was 18.9 g / 1000 g phosphoric acid, and the final product purity of lithium difluorooxalate borate (LiODFB) was 99.7 wt.%, with a yield of 97.1%.

[0033] Example 6 With all other preparation conditions identical to those in Example 1, the only difference was that the Li:F molar ratio in the back-extraction step was adjusted from 0.25 to 0.35, i.e., 7.96 g of LiOH was added; the remaining steps remained unchanged. The results showed that under these conditions, the yield of lithium fluoroborate was 19.3 g / 1000 g phosphoric acid, but ICP-OES analysis revealed residual Li in the product. + The content increased to 1200 ppm; after subsequent synthesis and recrystallization, the Li content in the final product was reduced. + The residue level remained at 85 ppm; the fluorine recovery rate increased to 86.5%.

[0034] Example 7 With all other preparation conditions identical to those in Example 1, only the amount of oxalic acid added was adjusted from 22.0 g (1.2:1) to 27.43 g (1.5:1), while the other steps remained unchanged. Results showed that under these conditions, after 6 h of reaction, HPLC analysis revealed a residual LiBF4 content of 1.8%, the LiODFB content in the crude product was 94.2%, and the final product purity after recrystallization was 99.7 wt.%, with a yield of 96.5%.

[0035] Example 8 Following the method of Example 1, the difference was that in step (4), dimethyl carbonate was replaced with an equal volume of acetonitrile (100 mL), while the other conditions (oxalic acid: lithium fluoroborate = 1.2:1, AlCl33%, 80 °C, 6 h) remained unchanged. Crude lithium difluorooxalate borate was obtained, which, after recrystallization twice from toluene, yielded 27.9 g of a white powder with a purity of 99.7 wt.% and a yield of 96.8%.

[0036] Example 9 Based on the crude product obtained in Example 1, in step (5), toluene was replaced with an equal volume of xylene, while the other recrystallization conditions (twice, dissolution at 60 °C, crystallization at 25 °C) remained unchanged. The result was 28.1 g of lithium difluorooxalate borate with a purity of 99.8 wt.% and a yield of 97.0%.

[0037] Comparative Example 1 This comparative example illustrates a method for the selective extraction and recovery of fluoride from a wet-process phosphoric acid system. The wet-process phosphoric acid solution used contains the same elemental compositions as in Example 1. However, unlike Example 1, this comparative example does not involve the addition of boric acid solid for fluoride conversion. The method includes the following steps: (1) Extraction: Add trioctylamine extractant (with the same ratio as in Example 1) to 1000g of wet phosphoric acid solution at a ratio of O / A=1 and stir for 10 minutes. Let stand and separate the organic phase and acid phase after separation.

[0038] (2) Back-extraction: 50 mL of water was added to the extract phase, and 22.7 g of lithium hydroxide solid was added according to the molar ratio Li:F=1. The mixture was mixed for 5 minutes and allowed to stand for separation. Excess ethanol was added to the aqueous turbidity, and the mixture was allowed to stand to obtain lithium fluoride precipitate. The precipitate was dried under vacuum at 100 °C to remove water, yielding 8.87 g of lithium fluoride solid. The fluorine recovery rate was 36.1%.

[0039] Comparative Example 2 This comparative example illustrates a traditional stripping recovery method in the wet-process phosphoric acid production process. The elemental content of the wet-process phosphoric acid solution used is the same as in Example 1. However, the method differs from Example 1, and the specific method is as follows: Stripping: 1000g of wet-process phosphoric acid was heated to 120℃ in a closed system. 17.1g of diatomaceous earth solid was added to the solution according to a molar ratio of silica to fluorine of 0.3, and N2 was introduced for stripping. The tail gas was absorbed by water for 2 hours. The fluorine content in the absorbent was determined, and the fluorine recovery rate was 52.5%.

[0040] To better illustrate this solution, the relevant parameters and results of the above embodiments and comparative examples are summarized in a table as follows:

[0041] In the table above, the fluorine recovery rate is calculated as follows: (mass of fluorine in the solution before extraction - mass of fluorine in the solution after extraction) / mass of fluorine in the solution before extraction × 100%.

[0042] Application Experimental Examples The test samples included lithium difluorooxalate borate products prepared in Examples 1, 2, and 3, respectively. The lithium difluorooxalate borate (LiODFB) obtained in Examples 1-3 were dissolved in a carbonate-based mixed solvent composed of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) in a mass ratio of 1:1:3 to prepare a solution with a concentration of 1 mol·L⁻¹. -1Electrolytes were prepared. Button batteries were assembled using lithium iron phosphate as the positive electrode and lithium metal sheets as the negative electrode, and charge-discharge tests were conducted at a 1C rate. The results showed that the initial discharge specific capacity of the batteries prepared with the electrolyte systems in Examples 1, 2, and 3 was 134.5 mAh·g⁻¹. -1 138.8 mAh·g -1 and 136.2 mAh·g -1 After 500 cycles at 1C rate, the battery capacity retention rates reached 99.4%, 98.8%, and 99.2%, respectively.

[0043] In summary, the technical solution of this invention utilizes boric acid to remove free HF, H2SiF6, and SiF6 from wet-process phosphoric acid. 2- All polyfluorinated species are uniformly converted into HBF4, giving it an anionic form that can be selectively complexed with trioctylamine; then, trioctylamine reacts with H+ in an acidic medium... + -BF4 - The ability to form stable ion-paired compounds enables efficient decoupling and separation of fluorine from high-concentration phosphate and metal impurities. Subsequently, an aqueous phase is introduced into the supported organic phase to dilute and disrupt the complexation equilibrium, while an alkaline lithium source is simultaneously added to neutralize H+. + Provide Li + This process promotes the precipitation of LiBF4 in solid form; furthermore, LiBF4 undergoes a Lewis acid-catalyzed fluorine-oxygen exchange and ligand substitution reaction with oxalic acid in an aprotic polar solvent to construct ODFB. - The anionic structure generates the target product LiODFB. Finally, by leveraging its significant temperature-dependent solubility difference in nonpolar solvents, trace amounts of residual catalyst, unreacted substances, and colored impurities are removed through 2-3 recrystallizations. This process ensures a high fluorine recovery rate (≥85%) while obtaining battery-grade purity (≥99.7 wt.%) lithium difluorooxalate borate, solving the technical problems of low fluorine recovery rate, high risk of direct HF treatment, low added value of by-products, and difficulty in meeting the stringent purity requirements of electrolytes in existing technologies.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for selectively recovering fluorine from wet-process phosphoric acid and preparing lithium difluorooxalate borate, characterized in that, The steps include: (1) Premixing: Boric acid is added to the wet phosphoric acid system in proportion for premixing, so that the fluorine species in it are converted into fluoroboric acid; Selective extraction: An organic extraction system containing trioctylamine is used to selectively extract the converted fluoroboric acid, achieving efficient separation of phosphorus and fluorine; Back-extraction: The organic phase obtained by extraction is added to water and an alkaline lithium source in a certain proportion for back-extraction to obtain lithium fluoroborate solid; Synthesis of lithium difluorooxalate borate: Purified lithium fluoroborate and anhydrous oxalic acid were dissolved in an aprotic polar organic solvent and subjected to Lewis acid catalysis under inert gas protection to obtain crude lithium difluorooxalate borate. Purification and refining of lithium difluorooxalate borate: The crude lithium difluorooxalate borate obtained is recrystallized 2-3 times in a non-polar organic solvent to obtain lithium difluorooxalate borate with a purity ≥99.7%.

2. The method according to claim 1, characterized in that, Before adding boric acid in step (1), the fluorine content in the system must be measured first, and then added according to the molar ratio of fluorine to boric acid of 1: (0.25~0.35).

3. The method according to claim 1, characterized in that, The trioctylamine-containing organic extraction system in step (2) includes trioctylamine, n-octanol and kerosene, wherein trioctylamine accounts for 20% to 50% of the total volume of the extraction system and n-octanol accounts for 5% to 20% of the total volume.

4. The method according to claim 1, characterized in that, In step (2), the trioctylamine extractant is added to the premixed liquid-solid mixture at a ratio of O / A = 0.5~2. After stirring, the mixture is allowed to stand until the organic phase and acid phase separate into layers.

5. The method according to claim 1, characterized in that, In step (3), water is added to the organic phase at a volume ratio of organic phase:water = 1:(0.05~0.2), and alkaline lithium source is added at a molar ratio of F:Li = 1:(0.25~0.35). After mixing, the mixture is allowed to stand and separated. The organic phase is circulated to the extraction stage. The aqueous phase turbid liquid is purified by ethanol filtration, and then recrystallized and dried to obtain lithium fluoroborate solid.

6. The method according to claim 1, characterized in that, The alkaline lithium source used in step (3) is lithium hydroxide, lithium carbonate, or lithium bicarbonate.

7. The method according to claim 1, characterized in that, In step (4), the molar ratio of anhydrous oxalic acid to lithium fluoroborate is (1~1.5):1, and the amount of catalyst added is 1%~5% of the total mass of anhydrous oxalic acid and lithium fluoroborate.

8. The method according to claim 1, characterized in that, The aprotic polar organic solvent used in step (4) is dimethyl carbonate, acetonitrile, tetrahydrofuran or dichloromethane, the catalyst is anhydrous aluminum trichloride, tin tetrachloride, ferric trichloride, titanium tetrachloride or zinc dichloride, and the inert gas is nitrogen.

9. The method according to claim 1, characterized in that, The non-polar organic solvent in step (5) is tetrachloromethane, benzene, toluene or xylene.

10. Lithium difluorooxalate borate prepared by the method according to any one of claims 1 to 9 is used as an electrolyte for lithium-ion batteries.

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