A process for wet-process phosphoric acid refining

Abstract

The present application belongs to the technical field of wet-process phosphoric acid refining, and particularly relates to a method for wet-process phosphoric acid refining, comprising: removing sulfate ions in wet-process phosphoric acid by step-by-step precipitation of calcium-containing compounds and barium salt; removing arsenic in wet-process phosphoric acid by sulfide; adding active SiO2 to the system and removing fluoride ions by combination of stripping distillation method; removing precipitated slag by composite filtration method; adding N-containing organic precipitator to the filtered clear liquid to form a double salt precipitate with phosphate ions; separating the double salt precipitate by filtration, washing with deionized water, alkali dissolution treatment, and obtaining a phosphoric acid salt-containing solution after filtration; treating the phosphoric acid salt-containing solution by bipolar membrane electrodialysis to generate an alkali solution and a phosphoric acid solution; evaporating and concentrating to obtain refined phosphoric acid. The present application has the core advantages of complete impurity removal, high phosphoric acid recovery rate, low waste slag discharge, and low production cost, effectively avoiding the problems of resource waste, serious pollution, high cost, etc. existing in traditional refining methods.

Description

Technical Field

[0001] This invention belongs to the field of wet phosphoric acid refining technology, and particularly relates to a method for wet phosphoric acid refining. Background Technology

[0002] Phosphate rock is an important guarantee for maintaining food and energy security. The main way to develop and utilize phosphate rock is to produce phosphoric acid, which can be divided into thermal phosphoric acid, kiln phosphoric acid, and wet phosphoric acid according to its production process. The wet phosphoric acid process uses medium and low grade phosphate rock as raw material and obtains phosphoric acid by decomposing the phosphate rock with sulfuric acid and phosphoric acid. It does not have high requirements for the grade of phosphate rock, and the production cost is low and the pollution is minimal. Therefore, the wet phosphoric acid process has gradually become the main process for producing phosphoric acid.

[0003] Wet-process phosphoric acid contains various impurities, including Mg. 2+ Al 3+ Fe 3+ Ca 2+ SO4 2- and F - The high impurity content makes it difficult to meet the requirements for use as a raw material in phosphate production, or for industrial, food-grade, and electronic-grade phosphoric acid. To improve the utilization value of phosphoric acid and produce high-quality phosphate chemical products, further purification and impurity removal are necessary to refine the wet-process phosphoric acid.

[0004] With the electronics industry continuously raising its standards for soluble impurities, solid particles, and ion content in phosphoric acid, in-depth separation and purification of wet-process phosphoric acid has become crucial. To meet these high standards, the industry has developed various purification technologies, including ion exchange, electrodialysis, solvent extraction, chemical precipitation, membrane separation, and crystallization. Currently, the main methods for large-scale phosphoric acid purification in China and abroad are chemical precipitation and solvent extraction. The former is primarily used for the production of feed-grade dicalcium phosphate and sodium phosphate, while the latter can produce general-purpose industrial-grade phosphoric acid. Other methods are still in the research stage and have not yet been industrialized.

[0005] Although solvent extraction yields high-purity phosphoric acid, several problems exist in actual production: Corrosion occurs during extraction due to the physicochemical properties of the extractant and the materials used in the equipment; extractants are typically flammable, explosive, and highly volatile substances, posing safety hazards, and direct discharge can pollute the environment; single-stage extraction is usually insufficient to achieve the desired purification effect, requiring combination with other methods or multi-stage extraction, which complicates the process, increases purification difficulty, and results in low phosphoric acid recovery rates; solvent extraction produces residual raffinate, which, if not recycled, wastes phosphoric acid resources and increases processing costs.

[0006] To upgrade wet-process phosphoric acid to higher purity phosphoric acid, many researchers have proposed using a combination of purification technologies to achieve efficient purification of electronic-grade phosphoric acid. In 2025, researchers summarized a phosphoric acid cooling crystallization method using medium-to-high concentration food-grade phosphoric acid as raw material. This method involves adding seed crystals and strictly controlling the cooling process to induce crystallization and multi-stage crystallization. This process promotes the formation of hemihydrate phosphoric acid crystals. After centrifugation and washing to effectively separate impurity ions, the resulting crystals are heated and melted to obtain electronic-grade phosphoric acid. Because impurities in the mother liquor may enter the crystals through surface adhesion or intergranular inclusion during industrial crystallization, specific solvents are typically used to wash the crystals after filtration to improve product purity. This method is only suitable for using commercial phosphoric acid as raw material, requiring multiple crystallizations combined with chemical precipitation to remove impurities, in order to achieve the electronic-grade phosphoric acid standard.

[0007] In recent years, there have been many research reports on the deep removal of impurities and resource utilization of phosphate raffinate. For example, Shi Tongshan (Ecological Industry Science and Phosphorus Fluorine Engineering. 2025, 40(02): 44-48) reported that reacting the magnesium removal slag of raffinate with concentrated sulfuric acid at high temperature will produce a mixed gas containing water vapor, SiF4 and HF. The mixed gas can react directly with the raffinate or be absorbed first and then used for magnesium removal of the raffinate. The remaining reaction residue is a solid magnesium fertilizer mainly composed of magnesium sulfate. After two-stage absorption, most of the fluorine can be absorbed, with an absorption rate of 92.24%. At the same time, fluorine-containing acid with a mass fraction of 32.42% can be obtained for magnesium removal. The process is feasible overall. Lu Qun (Ecological Industry Science and Phosphorus Fluorine Engineering. 2024, 39(10): 18-20+54) proposed that the raffinate should be pretreated first, and then a defluorination process should be carried out to enrich the impurity ions of the raffinate in the fertilizer, so that the supernatant meets the requirements for producing feed-grade dicalcium phosphate. Neutralizing the phosphorus-containing solution after defluorination with lime slurry can produce a product that meets the requirements of Type I of GB22549-2017 "Feed Additives - Dicalcium Phosphate", achieving a P2O5 recovery rate of 81.21% in the residual acid. However, this method is not applicable to wet-process phosphoric acid refining.

[0008] Numerous patents have been reported on wet-process phosphoric acid purification. For example, patent (CN202510866148.1) discloses a removal agent and method for cadmium in phosphoric acid. It uses a composition of dithiophosphoric acid trapping agents as the cadmium removal agent in wet-process phosphoric acid. After the reaction, a precipitate that is poorly soluble in a strongly acidic aqueous phase is formed, and cadmium removal is achieved after filtration. Compared with conventional sulfide precipitation methods, this method results in less cadmium residue and less pollution in phosphoric acid. However, this method can only remove cadmium ion impurities from crude phosphoric acid.

[0009] Patent (CN202510453212.3) discloses a method for purifying phosphoric acid by crystallization. This invention involves heating and concentrating wet-process phosphoric acid, causing metal ions in the phosphoric acid to precipitate as pyrophosphate. The concentrated phosphoric acid is then diluted with water and filtered to separate the precipitated metaphosphate. The resulting diluted acid exhibits a significantly reduced viscosity, making it suitable for purification by crystallization. The purified crystalline phosphoric acid shows a substantial reduction in iron, aluminum, and magnesium content. However, the method still suffers from high energy consumption and phosphoric acid loss.

[0010] Patent (CN202510568259.4) discloses a method for primary membrane separation of wet-process phosphoric acid using an acid-resistant nanofiltration membrane to obtain a primary clear phase; the primary clear phase is then subjected to secondary membrane separation using an acid-resistant nanofiltration membrane to obtain a secondary concentrated phase and purified acid; the purified acid, filter cake, and water are mixed and subjected to a crystallization reaction; the slurry is filtered and dried, and the solid phase is then calcined to obtain anhydrous ferric phosphate. However, this membrane separation method cannot achieve deep purification of wet-process phosphoric acid, and the purified acid still contains heavy metal impurity ions, making it unsuitable as a raw material for the production of battery-grade ferric phosphate.

[0011] In general, existing wet-process phosphoric acid refining methods mostly remove only some impurities, lacking the ability to synergistically and deeply remove different impurities such as heavy metals and anions. Furthermore, mainstream solvent extraction methods suffer from low phosphoric acid yield, difficulty in handling residual acid, high cost, and pollution and safety hazards. Other methods either result in incomplete purification, significant phosphorus loss, or high costs. Therefore, there is an urgent need for a new wet-process phosphoric acid refining method that thoroughly removes impurities, achieves high phosphoric acid recovery, produces less waste residue, and is cost-effective, thus overcoming the shortcomings of existing technologies. Summary of the Invention

[0012] The purpose of this invention is to provide a method for wet-process phosphoric acid purification to solve the above-mentioned problems.

[0013] To achieve the above objectives, the present invention provides the following solution: A method for wet-process phosphoric acid purification includes the following steps: S1: Sulfate ions in wet-process phosphoric acid are removed by a stepwise compound precipitation reaction involving calcium-containing compounds and barium salts. S2: Arsenic is removed from wet-process phosphoric acid using sulfides; S3: Add active SiO2 to the system and remove fluoride ions by stripping distillation, and then remove the corresponding precipitates of sulfate, arsenic and fluorine by composite filtration. S4: Add an N-containing organic precipitant to the filtered liquid to form a double salt precipitate with phosphate ions. After filtration, separate the double salt precipitate and wash it with deionized water. Then, treat the washed double salt precipitate with alkaline solution and filter it to obtain a phosphate-containing solution. S5: The phosphate-containing solution is treated with bipolar membrane electrodialysis to generate an alkaline solution and a phosphoric acid solution; S6: The phosphoric acid solution is evaporated and concentrated to obtain refined phosphoric acid.

[0014] Preferably, the molar ratio of the total amount of calcium-containing compounds and barium salts added in step S1 to the S content in wet-process phosphoric acid is 1:1–3:1; The molar ratio of the added calcium-containing compound to the barium salt is 1:1–10:1.

[0015] Preferably, the calcium-containing compound in step S1 is one or a combination of several of CaO, Ca(OH)2, CaCO3, Ca(HCO3)2, Ca3(PO3)2, CaHPO4, and Ca(H2PO4)2; The barium salt is one or a combination of BaCl2, BaCO3, and Ba(NO3)2.

[0016] Preferably, the chemical precipitation method used in step S1 has a reaction temperature of 50–90°C and a reaction time of 1–10 h.

[0017] Preferably, the sulfide in step S2 is one or a combination of sodium sulfide, sodium persulfide, and phosphorus pentasulfide; The amount of sulfide added is 1-2 times the stoichiometry. In the chemical precipitation method, "stoichiometry" refers to the theoretical amount of feed required to completely convert the target ion into a precipitate (here, it refers to the complete conversion of arsenic into As2S3 precipitate) according to the main reaction equation.

[0018] Preferably, in step S2, the reaction temperature for adding sulfide to remove arsenic is 50–90°C, the reaction time is 1–2 h, and the tail gas from adding sulfide to remove arsenic is treated by absorption with 2%–5% NaOH solution.

[0019] Preferably, in step S3, the amount of active SiO2 added is equal to the amount of F in the wet-process phosphoric acid. - The molar ratio of ions is 1:1–5:1.

[0020] Preferably, in step S3, the temperature of the stripping distillation is controlled at 80–120°C, and the activated SiO2+ stripping distillation method removes F from the filtered liquid. - The ionization time is 1–5 h, and the fluorine-containing tail gas is treated by absorption with 2%–5% NaOH solution.

[0021] Preferably, in step S3, the composite filtration includes any two or more combinations of multi-media filtration, activated carbon filtration, precision filtration, and membrane filtration, and the precision of the composite filtration is 1–10 μm.

[0022] Preferably, the N-organic precipitant in step S4 is any one or a combination of urea, melamine, 8-hydroxyquinoline, dimethylglyoxime, quinomolybdate and their derivatives; The molar ratio of the N-containing organic precipitant added in step S4 to the phosphate ions in wet-process phosphoric acid is 1:1–3:1.

[0023] Preferably, in step S4, the reaction time for adding an N-containing organic precipitant to form a double salt precipitate with phosphate ions is 1–3 hours, the filtration accuracy of the filtration method is 1–10 μm, the double salt precipitate is washed with deionized water 1–5 times, and the amount of deionized water used for washing is 1–3 times the mass of the double salt precipitate per wash.

[0024] Preferably, the alkaline solution used for alkali dissolution of the double salt precipitation in step S4 is any one or a combination of ammonia, NaOH, KOH, Na2CO3 and NaHCO3.

[0025] Preferably, in step S4, the concentration of the alkali solution used for the alkaline dissolution of the double salt precipitation is 5%–25%, the reaction time is 30–180 min, and the reaction temperature is 30–90℃.

[0026] Preferably, the N-containing organic precipitant precipitated during the alkali dissolution process in step S4 is separated, recovered, and then returned to the double salt precipitation step for recycling.

[0027] Preferably, the phosphate solution obtained in step S4 is converted into an alkaline solution and a phosphoric acid solution by the bipolar membrane electrodialysis system in step S5, and the alkaline solution generated by the bipolar membrane electrodialysis system in step S5 is returned to the alkaline dissolution process of the double salt precipitation in step S4 for recycling.

[0028] The phosphoric acid solution generated by the bipolar membrane electrodialysis system in step S5 is then evaporated and concentrated to obtain refined phosphoric acid.

[0029] Compared with the prior art, the present invention has the following advantages and technical effects: This invention utilizes a synergistic pretreatment process of chemical precipitation, stripping distillation, and filtration to deeply remove anionic impurities such as sulfate, arsenic, and fluorine, as well as some heavy metal ions, from wet-process phosphoric acid. This lays a high-purity foundation for subsequent refining, enabling the generated phosphate complex salt precipitate to achieve a purity of over 95.5%. By utilizing the specific double salt reaction between N-containing organic precipitants and phosphate ions, combined with filtration separation and deionized water washing processes, phosphate and Mg ions can be separated. 2+ Al³ + Fe³ + Ca 2+The efficient and deep separation of metal ion impurities reduces the impurity ion content in the phosphate complex salt precipitate to less than 1 mg / Kg. After alkali dissolution treatment, the double salt precipitate can simultaneously generate phosphate solution and precipitate N-containing organic precipitant. Through separation and recovery, the N-containing organic precipitant can be efficiently recovered and recycled (the recovery rate can reach more than 98%), which significantly reduces reagent consumption and reduces waste generation (emission reduction of more than 50%). The bipolar membrane electrodialysis technology is used to convert phosphate solution into phosphoric acid and alkaline solution. The generated alkaline solution can be reused in the alkaline dissolution process of double salt precipitation, forming a resource cycle. Finally, the purified phosphoric acid with a concentration of 85% is obtained by evaporation and concentration. The total phosphoric acid recovery rate can reach more than 90%, which greatly improves the utilization rate of phosphorus resources and economic benefits compared with the traditional solvent extraction method (about 60% recovery rate). In summary, the process of this invention has the core advantages of thorough impurity removal, high phosphoric acid recovery rate, low waste discharge and low production cost, effectively avoiding the problems of resource waste, serious pollution and high cost of traditional refining methods. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly described below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a schematic diagram of the wet-process phosphoric acid purification process in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram illustrating the principle of bipolar membrane electrodialysis for phosphate conversion. Figure 3 These are photographs taken under different reaction conditions for the precipitation of crude phosphoric acid using urea. Figure 4 Photos of the desulfurization precipitation reaction operation and the filtration operation of the precipitate residue; Figure 5 XRD analysis of the precipitate of phosphoric acid and melamine complex salt. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Raw material specifications The following examples and experimental cases all used 46% wet-process crude phosphoric acid (obtained by evaporating and concentrating 24% crude phosphoric acid and filtering to remove calcium sulfate), and its chemical composition is shown below: The content of calcium (Ca) is 0.0524 g / L, sodium (Na) is 0.8881 g / L, iron (Fe) is 3.0195 g / L, copper (Cu) is 0.0072 g / L, zinc (Zn) is 0.1672 g / L, sulfur (S) is 20.3113 g / L, magnesium (Mg) is 15.3446 g / L, and potassium (K) is 2.6776 g / L. The aluminum (Al) content is 9.1938 g / L, the silicon (Si) content is 0.0181 g / L, the lead (Pb) content is 0.0001 g / L, and the chloride ion (Cl) content is... - The content of total fluorine (F) was 0.2227 g / L, the content of arsenic (As) was 0.0244 g / L, the content of total fluorine (F) was 0.4140 g / L, the content of phosphorus (P-gravimetric method) was 334.54 g / L, and the content of total organic carbon (TOC) was 45.38 mg / L.

[0034] Example 1 Reference Figures 1 to 2 This embodiment discloses a method for wet-process phosphoric acid purification, comprising: S1, Pretreatment for desulfurization (1) Primary desulfurization: Calcium carbonate reacts with sulfuric acid and some phosphoric acid in phosphoric acid to form calcium sulfate precipitate, thereby removing a large amount of sulfate ions. The amount of calcium carbonate added is controlled to avoid adding excessive Ca. 2+ Losses occur due to reaction with phosphate ions. Reaction process: CaCO3+H2SO4→CaSO4↓+CO2↑+H2O CaCO3+2H3PO4→Ca(H2PO4)2+CO2↑+H2O Reaction conditions: Temperature controlled at 80°C, pH controlled at 3.0, calcium carbonate powder slowly added in batches with vigorous stirring, and after the addition is complete, continue to maintain the temperature and stir for aging for 60 minutes. This ensures a sulfate removal rate >90%.

[0035] (2) Deep desulfurization: Barium chloride reacts with residual sulfate ions to generate barium sulfate precipitate with extremely low solubility, which can achieve deep purification of sulfate ions.

[0036] BaCl2+SO4 2-→BaSO4↓+2Cl - Reaction conditions: Temperature 75℃, slowly add 20% barium chloride solution under good stirring, the amount of barium chloride added is 110% of the stoichiometry, continue stirring for 30 minutes after addition to ensure complete reaction, so that the sulfate removal rate is >99.9%.

[0037] S2, Pretreatment for Arsenic Removal (1) Hydrolysis of sodium sulfide and generation of H2S: Sodium sulfide will rapidly hydrolyze in acidic phosphoric acid solution to generate hydrogen sulfide (H2S).

[0038] Na₂S + 2H₃PO₄ → H₂S↑ + 2NaH₂PO₄ (2) Reduction and precipitation of arsenic: In wet-process phosphoric acid, arsenic usually exists as arsenic acid (H3AsO4) and arsenous acid (H3AsO3). H2S is a strong reducing agent, which first reduces the high-valence As(V) to the low-valence As(III).

[0039] H3AsO4 + H2S → H3AsO3 + S↓ + H2O The generated As(III) reacts with excess H2S to form a bright yellow precipitate of arsenic trisulfide (As2S3).

[0040] 2H3AsO3 + 3H2S → As2S3↓ + 6H2O (3) Coprecipitation and adsorption: During the formation of As2S3 colloidal precipitate, it adsorbs and coprecipitates other trace amounts of heavy metal sulfides in the solution, playing a certain synergistic purification role.

[0041] Reaction conditions: pH between 1.5 and 2.5, reaction temperature between 70 and 90℃, sodium sulfide addition of 1.5 times the stoichiometric amount, reaction time of 60 min under stirring, and aging time of 120 min. This achieves an arsenic removal rate of >99%, reducing the arsenic content in phosphoric acid from tens or even hundreds of ppm to below 10 ppm.

[0042] S3, Deep removal of fluoride ions Adding activated silica to wet-process phosphoric acid causes HF and H₂SiF₆ in the phosphoric acid to react with SiO₂ and be converted into SiF₄, thus achieving defluorination. The relevant reaction equation is as follows: 6HF + SiO2 = H2SiF6 + 2H2O H2SiF6 = SiF4↑ + 2HF↑.

[0043] Meanwhile, activated silica has a large specific surface area and porous structure, which can adsorb fluorides in phosphoric acid and further promote the removal of fluoride.

[0044] In the stripping distillation process, phosphoric acid is heated to cause its fluorides to volatilize in gaseous form. The fluorides are then carried away by a carrier gas such as air or steam, thus separating the fluoride from the phosphoric acid. Increasing the temperature and decreasing the pressure can lower the partial pressure of fluorides such as SiF4, promoting their volatilization and removal.

[0045] Reaction conditions: The amount of active silica added is 10% of the mass of the raw acid; the reaction time is 2 hours; the pressure of the stripping distillation system is <1 kPa; and the controlled temperature is 110℃. By optimizing the conditions, this technology can reduce the fluoride content in phosphoric acid from 2-3% to <0.01%, meeting the defluorination requirements of wet-process phosphoric acid.

[0046] S4. Phosphoric acid is recovered through double salt precipitation and alkali dissolution. Utilizing the basicity of melamine and its ability to form water-insoluble melamine phosphate double salt crystals with phosphate ions, melamine phosphate is generated and separated from the impurity-containing mother liquor by filtration. Reaction process: C3H6N6+H3PO4→C3H6N6·H3PO4↓ Common impurity metal ions in wet-process phosphoric acid include Ca. 2+ Mg 2+ Al³ + Fe³ + These substances are unstable or have high solubility under the reaction conditions, so most of them remain in the mother liquor, thus achieving the initial purification of phosphoric acid.

[0047] Reaction conditions: reaction temperature 20-50℃, pH 1.5-2.5, melamine addition (molar ratio) 1:1.1, stirring reaction time 60min, aging time 30min, followed by filtration separation.

[0048] Dissociation steps: The filtered melamine phosphate precipitate was washed three times with deionized water. After washing with deionized water, the metal ion content in the melamine phosphate was <1 mg / kg salt. Alkaline dissolution with ammonia water was used to destroy the complex salt structure, allowing melamine to enter the aqueous phase. The temperature was controlled at 50℃ to release phosphate ions, forming a solution containing ammonium dihydrogen phosphate. This achieved a wet-process phosphoric acid recovery rate >95%.

[0049] S5. Using bipolar membrane electrodialysis to convert ammonium dihydrogen phosphate into phosphoric acid and ammonia. Its principle diagram is as follows Figure 2 In bipolar membrane electrodialysis, the core lies in the bipolar membrane, which works synergistically with conventional ion exchange membranes (anion exchange membranes and cation exchange membranes) under the drive of an electric field. Under reverse bias, water molecules in the bipolar membrane interface layer undergo catalytic dissociation to generate H₂. + and OH - Reaction principle: In the salt chamber: Ammonium dihydrogen phosphate solution (NH4H2PO4→NH4) is introduced. + +H2PO4 - NH4 under the action of a DC electric field + The cations cross the cation membrane and enter the alkaline chamber on the right. H₂PO₄ - Anions pass through the anion membrane and enter the acid chamber on the left.

[0050] In the acid chamber: H2PO4 migrated from the salt chamber. - H migrated from the bipolar film + They combine to form phosphoric acid. Reaction: H₂PO₄ - +H + →H3PO4.

[0051] In the alkali chamber: NH4 migrated from the salt chamber. + With OH groups migrating from the bipolar membrane - They combine to form ammonia. Reaction: NH4 + +OH - →NH3·H2O.

[0052] Operating parameters: Ammonium dihydrogen phosphate concentration ~200g / L, current density 600A / m 2 The temperature was 35℃. The obtained phosphoric acid concentration was >10%, and the ammonia concentration was ~2%. The ammonia solution was obtained by stripping and reabsorption to obtain a 20% ammonia solution, which was returned to the alkaline dissolution process for double salt precipitation.

[0053] S6. Phosphoric acid solution produced by the bipolar membrane system is treated by evaporation and concentration. The 10% phosphoric acid obtained from the bipolar membrane system is first concentrated to over 20% via conventional electrodialysis, and then evaporated to obtain purified phosphoric acid with a concentration of 85%. Compared with conventional solvent extraction, this method can increase the phosphoric acid recovery rate to over 90%, significantly improving economic benefits.

[0054] Example 2 The only difference between this embodiment and Example 1 is that in the pretreatment desulfurization of step S1, the calcium-containing compound is Ca(OH)2 and the barium salt is BaCO3, with a molar ratio of 3:1.

[0055] Example 3 The only difference between this embodiment and Embodiment 1 is that in the pretreatment for arsenic removal in step S2, phosphorus pentasulfide is used as the sulfide, and the reaction temperature is controlled at 75-85℃.

[0056] Example 4 The only difference between this embodiment and Embodiment 1 is that in the deep removal of fluoride ions in step S3, the amount of active SiO2 added is the same as that added to wet-process phosphoric acid. -The molar ratio of ions is 2:1, and the stripping distillation temperature is controlled at 90-110℃.

[0057] Example 5 The only difference between this embodiment and Example 1 is that in the double salt precipitation and alkali dissolution recovery of phosphoric acid in step S4, the N-containing organic precipitant is urea, with a molar ratio of urea to phosphate of 1.5:1 and a reaction time of 2 hours.

[0058] Example 6 The only difference between this embodiment and Embodiment 1 is that in the dissociation step of step S4, the alkaline solution used is NaOH solution, and the alkaline dissolution reaction temperature is controlled at 60℃.

[0059] Example 7 The only difference between this embodiment and Embodiment 1 is that in step S4, the centrifugal filtration and washing speed is 6000 rpm, and the number of times the double salt precipitate is washed with deionized water is 4.

[0060] Example 8 The only difference between this embodiment and Example 1 is that in the double salt precipitation in step S4, the N-organic precipitant is a mixture of 8-hydroxyquinoline and dimethylglyoxime (molar ratio 1:1), and the aging time is 45 min.

[0061] Example 9 The only difference between this embodiment and Embodiment 1 is that in the alkaline dissolution treatment of step S4, the alkaline solution used is Na2CO3 solution with an alkaline concentration of 18%.

[0062] Comparative Example 1 This comparative example provides a method for the purification of wet-process phosphoric acid. 24% wet-process phosphoric acid is pretreated with calcium oxide for desulfurization. Operating conditions: temperature 30-70℃, reaction time 4h, Ca... 2+ The molar ratio of Ca:S to S was 1.1-2. The results showed that the desulfurization rate was the highest when the reaction time was 4 hours. At 70℃, when the Ca:S excess ratio was 1.1:1 to 2:1, the desulfurization rate of wet-process phosphoric acid increased from 31% to 57.4%, an increase of 87.5%.

[0063] Comparative Example 2 This comparative example provides a method for removing arsenic using wet phosphoric acid with phosphoric acid, such as phosphorus pentasulfide (P2S5). The reaction principle is basically the same as that for arsenic removal with sodium sulfide. Specific reaction equation: 2P2S5 + 16H2O → 4H3PO4 + 10H2S↑ H3AsO4 + H2S → H3AsO3 + H2O + S H3AsO3 + 3H2S → 6H2O + As2S3↓ The arsenic removal reaction temperature in the phosphorus pentasulfide method is controlled at 70℃. P2S5 is pumped in as a suspension at approximately four times the theoretical amount, achieving an arsenic removal rate of over 99% with an As concentration of less than 0.5 mg / kg. However, this method also has some drawbacks. Phosphorus pentasulfide is highly toxic and produces toxic H2S gas; improper operation can cause environmental pollution and harm to operator health. Compared to adding Na2S, the advantage of the phosphorus pentasulfide method is that it does not introduce Na. + Ionic impurities.

[0064] Comparative Example 3 This comparative example provides a method for removing fluoride from wet-process phosphoric acid using chemical precipitation. This is the most commonly used and mature method in industry. The core principle is to remove fluoride ions by adding chemical reagents, causing them to form insoluble fluoride precipitates. The sodium salt method involves adding soluble NaCl salt to phosphoric acid, which reacts with fluorosilicic acid (H₂SiF₆) in the phosphoric acid to form sodium fluorosilicate (Na₂SiF₆) precipitate. Chemical reaction: H2SiF6+2Na + →Na₂SiF₆↓+2H₂ + The advantages of this method are its mature technology, simple operation, and relatively low cost. The disadvantages are its limited defluorination rate, typically reducing the fluoride content to only 1-2%, which is insufficient for high-purity phosphoric acid. Furthermore, the solubility of Na₂SiF₆ in phosphoric acid affects the depth of defluorination, and the precipitate may encapsulate and trap phosphoric acid, resulting in phosphorus loss.

[0065] Comparative Example 4 This comparative example provides a precipitation separation method for wet-process phosphoric acid using a "urea precipitation method." The reaction principle involves generating urea phosphate adduct crystals, followed by evaporation and concentration, and then cooling to extract and purify the phosphoric acid from the complex wet-process acid system, thus achieving separation from impurities. Urea and phosphoric acid react in a 1:1 molar ratio to generate urea phosphate crystals. The reaction process is as follows: H3PO4 + CO(NH2)2 → CO(NH2)2 H3PO4↓ Experimental results showed that at a temperature of 30℃ and a ratio of 48% phosphoric acid to urea of ​​1:1.2, the urea phosphate formed by the reaction of urea and phosphoric acid dissolved in water. Evaporation and concentration were required to crystallize the urea phosphate, achieving a phosphoric acid recovery rate of over 90% in crude phosphoric acid. However, the precipitate obtained using this method contained a significant amount of metal ion impurities, preventing effective separation of the urea phosphate crystals from the impurities in the crude phosphoric acid. Furthermore, washing the precipitate with water resulted in significant back-dissolution of urea phosphate, drastically reducing the phosphoric acid recovery rate. These results indicate that using urea as a precipitant for the separation and purification of crude phosphoric acid has significant limitations, and its feasibility requires further verification. The relevant experimental conditions are shown in the table below.

[0066] Table 1. Different reaction conditions for urea precipitation with crude phosphoric acid

[0067] Experimental Example 1 Experimental objective: To optimize the key parameters of "stepwise desulfurization using calcium-containing compounds + barium salts", determine the optimal molar ratio of CaO to S, and verify the deep desulfurization effect; Experimental materials: 46% wet-process crude phosphoric acid, CaO, BaCl2 2H2O; Experimental steps: This pretreatment process first involves calcium oxide desulfurization testing, followed by barium chloride deep desulfurization. The feed ratios were determined based on Ca:S (molar ratio) ratios of 1.1, 1.3, and 1.4. Experimental conditions included a 40℃ water bath temperature, a 4-hour reaction time, and stirring at 410 rpm.

[0068] The experimental results are shown in Tables 2 and 3: Table 2. Component analysis of the supernatant after reaction with different CaO addition ratios (top)

[0069] Table 3. Component analysis of the supernatant after reaction with different CaO addition ratios (Part 2)

[0070] During the experiment, it was found that when calcium oxide was added to wet-process phosphoric acid and the mixture was stirred, the solution became turbid. After standing for a long time, obvious stratification occurred. The supernatant was taken for analysis (the results are shown in the table above). It was found that when the molar ratio Ca:S = 1.3, the sulfur content in crude phosphoric acid could be reduced to 2 g / L, with a sulfur removal rate of 89.64%. When the molar ratio Ca:S = 1.4, the results were better than Ca:S = 1.3, which could reduce the sulfur content to 1.1 g / L, with a sulfur removal rate of 94.5%. However, residual Ca in the supernatant... 2+ An increase in ion concentration means that adding excessive CaO will lead to an increase in the concentration of metal impurity ions in crude phosphoric acid, and some phosphoric acid reacts with CaO. 2+ Ions form calcium sulfate precipitate, resulting in losses. Further deep desulfurization using barium chloride was conducted, and the desulfurization rate was found to be >98.5%.

[0071] Experimental Example 2 Experimental objective: To determine the optimal operating conditions for the formation of a double salt precipitate from melamine and crude phosphoric acid, to verify the purity of the product, and to clarify the separation effect of the precipitate on impurities and the phosphoric acid recovery rate.

[0072] Experimental materials: 46% wet-process crude phosphoric acid Experimental steps: (1) Preparation of double salt precipitation: Take 1L of the above 46% crude phosphoric acid, dilute it 11 times, heat it to 95℃, add melamine precipitant step by step according to 1.2 times the stoichiometric amount of phosphate, stir continuously for 1.5h, then age at a constant temperature for 30min, and cool to room temperature. At the same time, a control group is set up: phosphoric acid is added directly to melamine slurry, and other conditions are the same.

[0073] (2) Solid-liquid separation: After the reaction, the liquid is centrifuged and filtered to collect the double salt precipitate and the precipitate mother liquor. The mother liquor is retained for testing.

[0074] (3) Precipitation washing: Wash the double salt precipitate with deionized water 1-5 times, each time using twice the mass of the precipitate, and collect the washing solution each time.

[0075] (4) Detection and analysis: XRD analysis was performed on the two groups of precipitates to confirm the product composition; the content of metal impurities such as Ca, Fe, Al, and Mg in the precipitates and the content of components such as Ca, Na, Fe, S, P, and F in the mother liquor and each washing liquid were detected by elemental analyzer, and the phosphoric acid recovery rate was calculated.

[0076] Experimental results: (1) Verification of operating conditions: Under the conditions of 95℃, stepwise addition of melamine, and aging for 30 minutes, the reaction system is stable, with no excessive water evaporation, and a large amount of uniform double salt precipitate can be obtained. In large-scale production, the use of a closed reaction tank can further reduce water loss and the dilution factor of crude phosphoric acid.

[0077] (2) Product purity verification: XRD analysis revealed that the precipitate formed when the crude phosphoric acid solution was first heated and then melamine precipitant was added in steps included phosphate-melamine double salt, basic aluminosilicate, and excess melamine, etc. (e.g.) Figure 5 (Left test results); If phosphoric acid is added to melamine slurry, the resulting product is a complex aluminum phosphate compound (such as...) Figure 5 The test results on the right show that different methods of adding the precipitant affect the type and properties of the product. In order to promote the separation of phosphoric acid from other impurities in the crude phosphoric acid stock solution, in actual operation, it is advisable to first heat the crude phosphoric acid solution and then add melamine precipitant step by step to generate a double salt precipitate, which is beneficial to the separation and purification of crude phosphoric acid.

[0078] (3) Composition analysis of the mother liquor and filter residue washing liquid Table 4: Analysis results (g / L) of mother liquor and filter residue washing liquid.

[0079] Table 5: Analysis results (g / L) of mother liquor and filter residue washing liquid.

[0080] Compositional analysis of the precipitate filtrate and different washing batches of the precipitate revealed that under the current reaction conditions, the phosphoric acid in the crude phosphoric acid did not completely precipitate with the precipitant melamine. A small amount of phosphoric acid remained in the filtrate, possibly due to adsorption by the precipitate sludge or back dissolution from the phosphate double salt precipitate during the washing process. Furthermore, the low content of impurity ions adsorbed by the precipitate sludge indicates that the double salt precipitation method has a good impurity removal effect on phosphoric acid. Based on the total phosphorus content in the double salt precipitation filtrate and multiple washing solutions, the recovery rate of crude phosphoric acid using melamine to generate double salt precipitate under the current conditions can be estimated to be approximately 99%.

[0081] To more clearly demonstrate the technical advantages of the method of the present invention, the core operating conditions and experimental results of the above embodiments and comparative examples are summarized in the following table: Table 6. Results of different embodiments and comparative examples

[0082] The results analysis shows that the method uses stepwise precipitation of calcium compounds and barium salts to remove sulfate ions from wet-process phosphoric acid; sulfides are used to remove arsenic from wet-process phosphoric acid; active SiO2 is added to the system and combined with stripping distillation to remove fluoride ions; a composite filtration method is used to separate and remove the precipitate; an N-containing organic precipitant is added to the filtrate to form a double salt precipitate with phosphate ions; the double salt precipitate is separated by filtration, washed with deionized water, treated with alkali, and filtered to obtain a phosphate-containing solution; the phosphate-containing solution is treated with bipolar membrane electrodialysis to generate an alkaline solution and a phosphoric acid solution; and 85% purified phosphoric acid is obtained by evaporation and concentration, and the phosphoric acid purity meets the product quality requirements. This invention's process has the core advantages of thorough impurity removal, high phosphoric acid recovery rate, low waste discharge, and low production cost, effectively avoiding the problems of resource waste, serious pollution, and high cost associated with traditional refining methods.

[0083] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for wet-process phosphoric acid purification, characterized in that, Includes the following steps: S1: Sulfate ions in wet-process phosphoric acid are removed by a stepwise compound precipitation reaction involving calcium-containing compounds and barium salts. S2: Arsenic is removed from wet-process phosphoric acid using sulfides; S3: Add active SiO2 to the system and remove fluoride ions by stripping distillation, and then remove the corresponding precipitates of sulfate, arsenic and fluorine by composite filtration. S4: Add an N-containing organic precipitant to the filtered liquid to form a double salt precipitate with phosphate ions. After filtration, separate the double salt precipitate and wash it with deionized water. Then, treat the washed double salt precipitate with alkaline solution and filter it to obtain a phosphate-containing solution. S5: The phosphate-containing solution is treated with bipolar membrane electrodialysis to generate an alkaline solution and a phosphoric acid solution; S6: The phosphoric acid solution is evaporated and concentrated to obtain refined phosphoric acid.

2. The method for wet-process phosphoric acid purification according to claim 1, characterized in that: The total amount of calcium-containing compounds and barium salts added in step S1 is in a molar ratio of 1:1 to 3:1 to the S content in wet-process phosphoric acid. The molar ratio of the added calcium-containing compound to the barium salt is 1:1–10:

1.

3. The method for wet-process phosphoric acid purification according to claim 1, characterized in that: The calcium-containing compound in step S1 is one or a combination of several of the following: CaO, Ca(OH)2, CaCO3, Ca(HCO3)2, Ca3(PO3)2, CaHPO4, and Ca(H2PO4)2. The barium salt is one or a combination of BaCl2, BaCO3, and Ba(NO3)2.

4. The method for wet-process phosphoric acid purification according to claim 1, characterized in that: The sulfide in step S2 is one or more of sodium sulfide, sodium persulfide, and phosphorus pentasulfide; The amount of sulfide added is 1-2 times the stoichiometric amount.

5. A method for wet-process phosphoric acid purification according to claim 1, characterized in that: In step S3, the amount of active SiO2 added is equal to the amount of F in the wet-process phosphoric acid. - The molar ratio of ions is 1:1 – 5:

1.

6. The method for wet-process phosphoric acid purification according to claim 1, characterized in that: In step S3, the composite filtration includes any two or more combinations of multi-media filtration, activated carbon filtration, precision filtration, and membrane filtration.

7. A method for wet-process phosphoric acid purification according to claim 1, characterized in that: The N-organic precipitant in step S4 is any one or a combination of urea, melamine, 8-hydroxyquinoline, dimethylglyoxime, quinomolybdate and their derivatives; The molar ratio of the N-containing organic precipitant added in step S4 to the phosphate ions in wet-process phosphoric acid is 1:1 to 3:

1.

8. A method for wet-process phosphoric acid purification according to claim 1, characterized in that: In step S4, the alkaline solution used for the alkaline dissolution of the double salt precipitation is any one or a combination of ammonia, NaOH, KOH, Na2CO3, and NaHCO3.

9. A method for wet-process phosphoric acid purification according to claim 1, characterized in that: The N-containing organic precipitant precipitated during the alkali dissolution process in step S4 is separated, recovered, and then returned to the double salt precipitation step for recycling.

10. A method for wet-process phosphoric acid purification according to claim 1, characterized in that: The alkaline solution generated by the bipolar membrane electrodialysis system in step S5 is returned to the alkaline dissolution process of the complex salt precipitation in step S4 for recycling.

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