A method for separating and recovering oxalic acid from wastewater containing oxalic acid

Abstract

This invention provides a method for separating and recovering oxalic acid from oxalic acid-containing wastewater, relating to the field of oxalic acid recovery and utilization technology. The method includes the following preparation steps: rare earth precipitation wastewater is mixed with a composite extractant for extraction to obtain a loaded organic phase and hydrochloric acid raffinate; porous silica balls are mixed and stirred with a urea aqueous solution, impregnated under reduced pressure, filtered, and dried to obtain composite particles; deionized water and the loaded organic phase are added to a reaction vessel, the composite particles are placed in a sieve basket at the interface between the deionized water and the loaded organic phase, nitrogen gas is introduced, and the mixture is pressurized and heated. The sieve basket is removed from the liquid phase, the reaction continues, heating is stopped, and the mixture is cooled and allowed to stand, separating to obtain an ammonium oxalate-rich aqueous phase; calcium chloride solution is added to the ammonium oxalate-rich aqueous phase, filtered to obtain a wet calcium oxalate material, sulfuric acid solution is added, filtered to obtain an oxalic acid filtrate, evaporated under reduced pressure, cooled to crystallize, filtered, and dried to obtain oxalic acid crystals. This invention can improve the recovery rate of oxalic acid and obtain high-purity oxalic acid crystals.

Description

Technical Field

[0001] This invention relates to the field of oxalic acid recovery and utilization technology, specifically to a method for separating and recovering oxalic acid from oxalic acid-containing wastewater. Background Technology

[0002] In the rare earth hydrometallurgical process, rare earth raw materials are typically leached, extracted, and back-extracted to obtain a rare earth chloride solution. Oxalic acid is then added to convert rare earth ions into insoluble rare earth oxalates. After aging, filtration, washing, and calcination, rare earth oxides are obtained. During oxalic acid precipitation and subsequent washing, wastewater containing unreacted oxalic acid and hydrochloric acid is generated. This wastewater has a high acidity; direct discharge without treatment not only wastes oxalic acid resources but also increases the burden on wastewater treatment. If it is directly returned to the production line, the residual oxalic acid may form precipitates with rare earth ions, affecting rare earth leaching, extraction, and product quality. Therefore, it is necessary to effectively separate oxalic acid and hydrochloric acid from the wastewater and recycle the oxalic acid to reduce production costs and wastewater treatment pressure.

[0003] Existing technologies have included studies on the separation and recovery of oxalic acid from rare earth precipitation wastewater using solvent extraction. For example, CN104610043A discloses a method for recovering oxalic acid from rare earth industrial wastewater. This method uses an extractant composed of TBP and sulfonated kerosene to perform multi-stage extraction of oxalic acid from the wastewater, obtaining an oxalic acid-loaded organic phase. This is followed by multi-stage back-extraction with water, and the resulting oxalic acid aqueous solution is recycled for further back-extraction enrichment, ultimately yielding a high-concentration oxalic acid solution. This approach can achieve the separation of oxalic acid from hydrochloric acid and the recovery of oxalic acid. However, its back-extraction process mainly relies on multi-stage contact and cyclic enrichment between the loaded organic phase and water, and does not address the directional mass transfer and transformation of oxalic acid at the oil-water interface. Furthermore, during the back-extraction process, there is still room for improvement in how to concentrate the back-extraction-promoting substances at the two-phase interface and promptly stop the oxalic acid from entering the aqueous phase after it has been largely back-extracted, thus balancing the oxalic acid recovery rate and the purity of the subsequent oxalic acid product.

[0004] In summary, there is a need to provide a method for separating and recovering oxalic acid from oxalic acid-containing wastewater in order to solve the problems existing in the prior art. Summary of the Invention

[0005] In view of this, the present invention provides a method for separating and recovering oxalic acid from oxalic acid-containing wastewater, so as to improve the recovery rate and purity of oxalic acid.

[0006] To achieve the above objective, a method for separating and recovering oxalic acid from oxalic acid-containing wastewater includes the following preparation steps:

[0007] S1. The rare earth precipitation wastewater is mixed with a composite extractant and extracted to obtain a loaded organic phase and hydrochloric acid raffinate. S2. Mix and stir porous silica balls with urea aqueous solution, impregnate under reduced pressure, filter and dry under vacuum to obtain composite particles; S3. Add deionized water and the supported organic phase to the reaction vessel in sequence. Pour the composite particles into the sieve basket and place it in the interface area between the deionized water and the supported organic phase. Let it stand, introduce nitrogen gas, pressurize and heat to carry out the reaction. Remove the sieve basket from the liquid phase, continue heating the reaction, stop heating, cool and let it stand, and separate the back-extracted organic phase and the ammonium oxalate-rich aqueous phase. S4. Add calcium chloride solution to the ammonium oxalate-rich aqueous phase to react, filter to obtain calcium oxalate wet material, add sulfuric acid solution to react, filter to obtain oxalic acid filtrate, evaporate under reduced pressure, cool to crystallize, filter and dry to obtain oxalic acid crystals. The density of the composite particles is greater than the density of the supported organic phase and less than the density of deionized water. The composite extractant comprises bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 260# solvent oil.

[0008] It should be understood that the supported organic phase in this invention is an organic phase supported on oxalic acid, which is obtained by mixing and extracting rare earth precipitation wastewater with a composite extractant; the densities of the composite particles, the supported organic phase, and the deionized water are all tested under normal temperature conditions.

[0009] This invention achieves the separation and recovery of oxalic acid from rare earth precipitation wastewater by constructing a synergistic treatment method of "liquid-liquid extraction-interfacial back-extraction-conversion crystallization". In the initial extraction stage, bis-2-(ethylhexyl) phosphonate is used as the extractant. With the synergistic effect of diluent 260# solvent oil, the selective extraction ability of the loaded organic phase relative to oxalic acid is improved, thereby achieving the initial separation of oxalic acid and hydrochloric acid in rare earth precipitation wastewater and reducing the interference of hydrochloric acid on the subsequent oxalic acid recovery process.

[0010] In the oxalic acid back-extraction stage, porous silica composite particles loaded with urea are introduced as an interface control carrier. The composite particles are loaded into a sieve basket, which is then placed at the interface between the loaded organic phase and deionized water. During the room-temperature stratification stage, because the density of the composite particles is between that of the loaded organic phase and the deionized water, the composite particles naturally tend to move towards the interface between the two phases, which is beneficial for determining their initial action position. During the subsequent heating and pressurization process, the sieve basket can limit the position of the composite particles, reducing the displacement of the composite particles caused by temperature rise and changes in the density of deionized water, so that the composite particles continue to act on the interface between the loaded organic phase and the deionized water. The urea loaded on the composite particles is continuously released near the interface and hydrolyzes under high temperature and high pressure to generate ammonium ions. The deionized water and ammonium ions in the interface region come into full contact with the oxalic acid in the loaded organic phase, converting the oxalic acid into ammonium oxalate. This conversion process helps to weaken the interaction between oxalic acid and the composite extractant, reducing the stability of oxalic acid in the loaded organic phase. The generated ammonium oxalate is difficult to exist stably in the loaded organic phase and continuously transfers to the deionized water, causing the oxalic acid inside the loaded organic phase to continuously migrate to the interface region, thereby promoting the back-extraction of oxalic acid from the loaded organic phase to the deionized water and improving the back-extraction efficiency of oxalic acid. Furthermore, this invention removes the composite particles entirely from the liquid phase using a sieve basket, cutting off the continuous release source of urea. This facilitates control over the amount of urea added to deionized water, preventing excessive urea release from affecting subsequent oxalic acid purification. Simultaneously, the urea that has already entered the deionized water continues to undergo hydrolysis and participates in oxalic acid back-extraction and the formation of ammonium oxalate, reducing urea residue in the subsequent ammonium oxalate-rich aqueous phase and thus improving the purity of oxalic acid recovery.

[0011] Subsequently, calcium chloride was used to convert ammonium oxalate in the ammonium oxalate-rich aqueous phase into insoluble calcium oxalate, thereby achieving solid-phase enrichment of oxalate ions. Then, calcium oxalate was converted into oxalic acid by sulfuric acid, while calcium sulfate precipitate was generated. High-purity oxalic acid crystals were obtained by solid-liquid separation and low-temperature crystallization.

[0012] Optionally, the porous silica spheres are prepared by adding polyoxyethylene polyoxypropylene ether and hexadecyltrimethylammonium bromide to an aqueous ethanol solution, stirring until clear, adding tetraethyl orthosilicate, stirring for 15-20 min, adding hydrochloric acid, and continuing stirring; dropping the resulting mixture into liquid paraffin, continuing the reaction for 10-12 h, filtering, washing 3-4 times with acetone and deionized water respectively, drying to constant weight at 55-60 °C, calcining at 650-660 °C for 3-4 h, and sieving.

[0013] In this invention, porous silica spheres are used as the urea carrier, which exhibits good structural stability. They are less prone to breakage or structural collapse during vacuum impregnation, stirring, and high-temperature pressure back-extraction, maintaining a relatively stable carrier morphology, which is beneficial for urea loading and release. Simultaneously, porous silica has relatively stable chemical properties, making it less likely to cause side reactions under the reaction conditions of this invention, and less likely to introduce impurities such as metal ions into the system, thus reducing the burden on subsequent oxalic acid purification.

[0014] Optionally, the porous silica spheres have a particle size of 1.5~2 mm and a density of 0.8~0.83 g / cm³. 3 .

[0015] In this invention, the particle size of the porous silica spheres is controlled to be 1.5~2mm, which facilitates the removal of the composite particles as a whole in subsequent processes and reduces the amount of composite particles remaining.

[0016] Optionally, in step S1, the extraction method is multi-stage countercurrent extraction, with 15 to 20 countercurrent stages, an extraction temperature of 20 to 30°C, and an extraction mixing time of 10 to 20 minutes; the weight ratio of 2-ethylhexylphosphonic acid bis-2-(ethylhexyl) ester to 260# solvent oil is 3:1, and the weight ratio of rare earth precipitation wastewater to composite extractant is 1:1.

[0017] In this invention, the weight ratio of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid to 260# solvent oil is controlled at 3:1, which can balance extraction capacity and organic phase flowability, and reduce the impact of excessively high organic phase viscosity on mass transfer and stratification. The weight ratio of rare earth precipitation wastewater to composite extractant is controlled at 1:1, which helps to ensure sufficient contact between oxalic acid and composite extractant, thereby improving the extraction efficiency and separation effect of oxalic acid.

[0018] Optionally, in step S2, porous silica spheres are mixed with urea aqueous solution, stirred at a speed of 100~200 rpm, depressurized to -0.085~-0.06 MPa and maintained for 20~30 min, restored to normal pressure, and the depressurization and restoration to normal pressure operation is repeated 2~3 times. After filtration, the composite particles are vacuum dried at 55~60℃ to constant weight to obtain composite particles.

[0019] Optionally, under normal temperature conditions, the density of the composite particles is 0.94~0.96 g / cm³. 3 The density of the supported organic phase is 0.9~0.91 g / cm³. 3 .

[0020] In this invention, the density of the composite particles is controlled to be 0.94~0.96 g / cm³, which is greater than the density of the density-loaded organic phase and less than the density of deionized water. This is beneficial for the composite particles to naturally tend towards the interface region between the two phases, which facilitates their initial positioning in the stainless steel sieve basket and reduces the obvious floating or sinking of particles during the sieve basket setting process.

[0021] Optionally, in step S3, deionized water and the supported organic phase are poured into the reaction vessel in sequence, the composite particles are poured into a stainless steel sieve basket and placed in the interface area between the deionized water and the supported organic phase, and allowed to stand for 2-3 minutes. The reaction vessel is then sealed, nitrogen gas is introduced for pre-pressurization, and the mixture is heated to 140-145°C. The reaction pressure is controlled at 0.6-0.65 MPa, and the reaction is carried out at constant temperature and pressure for 3-5 hours. The stainless steel sieve basket is removed from the liquid phase, and the reaction is continued to be heated for 2-3 hours. Heating is then stopped, and the mixture is cooled and allowed to stand for 60-90 minutes. The pressure is then released to atmospheric pressure, and the back-extracted organic phase and the ammonium oxalate-rich aqueous phase are separated.

[0022] Optionally, in step S1, the initial concentration of oxalic acid in the supported organic phase is 17~18 g / L; in step S3, when the concentration of oxalic acid in the supported organic phase decreases to 0.7~0.8 g / L, the stainless steel sieve basket is removed from the liquid phase.

[0023] In this invention, when the concentration of oxalic acid in the supported organic phase decreases from 17~18 g / L to 0.7~0.8 g / L, the composite particles are removed to stop the continued release of urea, thereby reducing the amount of unreacted urea entering the ammonium oxalate-rich aqueous phase and preventing it from being entrained during subsequent precipitation, acidification and crystallization processes, thus balancing the oxalic acid recovery rate and the purity of oxalic acid crystals.

[0024] Optionally, in step S4, the ammonium oxalate-rich aqueous phase is heated to 40-60°C, calcium chloride solution is added, the mixture is stirred for 50-60 minutes, and then filtered to obtain wet calcium oxalate material; sulfuric acid solution is added to the wet calcium oxalate material, heated to 70-90°C, stirred for 30-50 minutes, and then filtered to remove calcium sulfate precipitate to obtain oxalic acid filtrate; the oxalic acid filtrate is heated to 55-60°C, evaporated under reduced pressure to -0.08 to -0.07 MPa for 1-2 hours, then cooled to 0-5°C for 2-3 hours to crystallize, filtered, and vacuum dried to obtain oxalic acid crystals.

[0025] Optionally, the composite particles comprise the following raw materials in parts by weight: 10-10.5 parts of porous silica spheres and 20-25 parts of 35wt% urea aqueous solution; the porous silica spheres comprise the following raw materials in parts by weight: 12-13 parts of polyoxyethylene polyoxypropylene ether, 3-4 parts of hexadecyltrimethylammonium bromide, 38-40 parts of 60wt% ethanol solution, 40-42 parts of tetraethyl orthosilicate, 7-9 parts of 37wt% hydrochloric acid, and 760-780 parts of liquid paraffin.

[0026] The above-described technical solution of the present invention has at least the following beneficial effects: This invention achieves the separation and recovery of oxalic acid from rare earth precipitation wastewater by constructing a synergistic treatment method of "liquid-liquid extraction-interfacial back-extraction-conversion crystallization". In the initial extraction stage, bis-2-(ethylhexyl) phosphonate is used as the extractant. With the synergistic effect of diluent 260# solvent oil, the selective extraction ability of the loaded organic phase relative to oxalic acid is improved, thereby achieving the initial separation of oxalic acid and hydrochloric acid in rare earth precipitation wastewater and reducing the interference of hydrochloric acid on the subsequent oxalic acid recovery process.

[0027] During the oxalic acid back-extraction stage, at room temperature, the composite particles naturally tend towards the interface between the loaded organic phase and deionized water. A stainless steel sieve basket confines the composite particles, ensuring their continued action on the interface during heating and pressurization. The urea loaded on the composite particles is released and hydrolyzed near the interface, promoting the formation of water-soluble ammonium oxalate from oxalic acid and its transfer to deionized water, thereby improving the back-extraction efficiency of oxalic acid. Furthermore, removing the stainless steel sieve basket from the liquid phase in the later stages of back-extraction stops the continued release of urea from the composite particles, reducing the burden on subsequent purification and improving the purity of the oxalic acid crystals. Subsequently, calcium chloride was used to convert ammonium oxalate in the ammonium oxalate-rich aqueous phase into insoluble calcium oxalate, thereby achieving solid-phase enrichment of oxalate ions. Calcium oxalate was then converted into oxalic acid by sulfuric acid, while calcium sulfate precipitate was generated. Oxalic acid crystals were obtained through solid-liquid separation and low-temperature crystallization. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The described embodiments are some embodiments of the present invention, and all other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0029] Example 1 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 20 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 30℃, and the extraction mixing time was 20 min. The density of the loaded organic phase was 0.91 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 18 g / L.

[0030] S2. Add 13 parts of polyoxyethylene polyoxypropylene ether (F127) and 4 parts of hexadecyltrimethylammonium bromide (CTAB) to 40 parts of 60wt% ethanol solution, stir until clear, add 42 parts of tetraethyl orthosilicate (TEOS), stir for 20 min, then add 9 parts of 37wt% hydrochloric acid, and continue stirring for 8 h; slowly drop the resulting mixture into 780 parts of liquid paraffin at 65℃, continue the reaction for 12 h, filter, wash 4 times with acetone and deionized water respectively, dry to constant weight at 60℃, calcine at 660℃ for 4 h, and sieve to obtain porous silica spheres; mix 10.5 parts of porous silica spheres with 25 parts of 35wt% urea aqueous solution, stir continuously at 200 rpm, reduce pressure to -0.06 MPa and maintain for 30 min, restore to normal pressure, repeat the "reduced pressure-normal pressure" operation 3 times, filter, and vacuum dry at 60℃ to constant weight to obtain composite particles. The porous silica spheres have a particle size of 1.5~2mm and a density of 0.83g / cm³. 3 The density of the composite particles is 0.96 g / cm³. 3 .

[0031] S3. Add 640 parts of deionized water to the reaction vessel, then add 160 parts of the loaded organic phase. Let it stand for 20 minutes to separate the phases. Load 12.1 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the loaded organic phase and the deionized water, and let it stand for 3 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 145℃, adjust the nitrogen flow rate to control the pressure at 0.65MPa, and react at constant temperature and pressure for 5 hours. When the oxalic acid concentration in the loaded organic phase decreases to 0.8g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 3 hours, stop heating, cool and stand for 90 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0032] S4. Heat the ammonium oxalate-rich aqueous phase to 60°C, add 13 parts of 30wt% calcium chloride solution, stir for 60 min, age for 90 min, and filter to obtain wet calcium oxalate material; add 8.7 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 90°C, stir for 50 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 60°C, evaporate under reduced pressure to -0.07 MPa for 2 h, then cool to 5°C to crystallize for 3 h, filter, and vacuum dry at 50°C for 10 h to obtain oxalic acid crystals.

[0033] Example 2 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 18 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 27℃, and the extraction mixing time was 15 min. The density of the loaded organic phase was 0.9 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 17.5 g / L.

[0034] S2. Add 12.5 parts of polyoxyethylene polyoxypropylene ether (F127) and 3.5 parts of hexadecyltrimethylammonium bromide (CTAB) to 39 parts of 60wt% ethanol solution, stir until clear, add 41 parts of tetraethyl orthosilicate (TEOS), stir for 15 min, then add 8 parts of 37wt% hydrochloric acid, and continue stirring for 7 h; slowly dropwise add the resulting mixture to 770 parts of liquid paraffin at 63℃, continue the reaction for 11 h, filter, and rinse with acetone. Washed three times with deionized water, dried to constant weight at 58℃, calcined at 655℃ for 3 hours, and sieved to obtain porous silica spheres. 10.2 parts of the porous silica spheres were mixed with 23 parts of a 35wt% urea aqueous solution, stirred continuously at 100 rpm, and the pressure was reduced to -0.07 MPa and maintained for 25 minutes. The pressure was then restored to normal, and this "reduced pressure-normal pressure" operation was repeated twice. After filtration, the mixture was vacuum dried to constant weight at 57℃ to obtain composite particles. The porous silica spheres had a particle size of 1.5~2 mm and a density of 0.82 g / cm³. 3 The density of the composite particles is 0.95 g / cm³. 3 .

[0035] S3. Add 635 parts of deionized water to the reaction vessel, then add 157 parts of the loaded organic phase. Let it stand for 15 minutes to separate the phases. Pack 12 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the loaded organic phase and the deionized water, and let it stand for 2 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 143℃, adjust the nitrogen flow rate to control the pressure at 0.6MPa, and react at constant temperature and pressure for 4 hours. When the oxalic acid concentration in the loaded organic phase decreases to 0.7g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 2.5 hours, stop heating, cool and stand for 75 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0036] S4. Heat the ammonium oxalate-rich aqueous phase to 50°C, add 12.5 parts of 30wt% calcium chloride solution, stir for 50 min, age for 80 min, and filter to obtain wet calcium oxalate material; add 8 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 80°C, stir for 40 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 57°C, evaporate under reduced pressure to -0.075 MPa for 1.5 h, then cool to 2°C to crystallize for 2.5 h, filter, and vacuum dry at 45°C for 8.5 h to obtain oxalic acid crystals.

[0037] Example 3 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 15 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 25℃, and the extraction mixing time was 10 min. The density of the loaded organic phase was 0.9 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 17 g / L.

[0038] S2. Add 12 parts of polyoxyethylene polyoxypropylene ether (F127) and 3 parts of hexadecyltrimethylammonium bromide (CTAB) to 38 parts of 60wt% ethanol solution, stir until clear, add 40 parts of tetraethyl orthosilicate (TEOS), stir for 15 min, then add 7 parts of 37wt% hydrochloric acid, and continue stirring for 6 h; slowly drop the resulting mixture into 760 parts of liquid paraffin at 60℃, continue the reaction for 10 h, filter, wash three times with acetone and deionized water respectively, dry to constant weight at 55℃, calcine at 650℃ for 3 h, and sieve to obtain porous silica spheres; mix 10 parts of porous silica spheres with 20 parts of 35wt% urea aqueous solution, stir continuously at 100 rpm, reduce pressure to -0.085 MPa and maintain for 20 min, restore to normal pressure, repeat the "reduced pressure-normal pressure" operation twice, filter, and vacuum dry to constant weight at 55℃ to obtain composite particles. The porous silica spheres have a particle size of 1.5~2mm and a density of 0.8g / cm³. 3 The density of the composite particles is 0.94 g / cm³. 3 .

[0039] S3. Add 600 parts of deionized water to the reaction vessel, then add 150 parts of the loaded organic phase. Let it stand for 10 minutes to separate the phases. Load 11.8 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the loaded organic phase and the deionized water, and let it stand for 2 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 140℃, adjust the nitrogen flow rate to control the pressure at 0.6MPa, and react at constant temperature and pressure for 3 hours. When the oxalic acid concentration in the loaded organic phase decreases to 0.8g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 2 hours, stop heating, cool and stand for 60 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0040] S4. Heat the ammonium oxalate-rich aqueous phase to 40°C, add 11.2 parts of 30wt% calcium chloride solution, stir for 50 min, age for 70 min, and filter to obtain wet calcium oxalate material; add 7.4 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 70°C, stir for 30 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 55°C, evaporate under reduced pressure to -0.08 MPa for 1 h, then cool to 0°C to crystallize for 2 h, filter, and vacuum dry at 40°C for 6 h to obtain oxalic acid crystals.

[0041] Example 4 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 15 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 26℃, and the extraction mixing time was 15 min. The density of the loaded organic phase was 0.91 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 17.7 g / L.

[0042] S2. Add 12.4 parts of polyoxyethylene polyoxypropylene ether (F127) and 3.7 parts of hexadecyltrimethylammonium bromide (CTAB) to 38.6 parts of 60wt% ethanol solution, stir until clear, add 41.6 parts of tetraethyl orthosilicate (TEOS), stir for 15 min, then add 8.3 parts of 37wt% hydrochloric acid, and continue stirring for 7 h; slowly dropwise add the resulting mixture to 765 parts of liquid paraffin at 65℃, continue the reaction for 11 h, filter, and then... The silica spheres were washed four times with acetone and deionized water, dried to constant weight at 58°C, calcined at 660°C for 3 hours, and sieved to obtain porous silica spheres. 10.3 parts of the porous silica spheres were mixed with 22 parts of a 35wt% urea aqueous solution, stirred continuously at 200 rpm, and the pressure was reduced to -0.085 MPa and maintained for 20 minutes. The pressure was then restored to normal, and this "reduced pressure-normal pressure" operation was repeated three times. After filtration, the spheres were vacuum dried to constant weight at 57°C to obtain composite particles. The porous silica spheres had a particle size of 1.5–2 mm and a density of 0.82 g / cm³. 3 The density of the composite particles is 0.95 g / cm³. 3 .

[0043] S3. Add 635 parts of deionized water to the reaction vessel, then add 157 parts of the loaded organic phase. Let it stand for 15 minutes to separate the phases. Load 11.9 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the loaded organic phase and the deionized water, and let it stand for 3 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 143℃, adjust the nitrogen flow rate to control the pressure at 0.6MPa, and react at constant temperature and pressure for 4.5 hours. When the oxalic acid concentration in the loaded organic phase decreases to 0.75g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 2.5 hours, stop heating, cool and stand for 70 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0044] S4. Heat the ammonium oxalate-rich aqueous phase to 55°C, add 11.5 parts of 30wt% calcium chloride solution, stir for 55 min, age for 80 min, and filter to obtain wet calcium oxalate material; add 7.9 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 80°C, stir for 40 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 58°C, evaporate under reduced pressure to -0.07 MPa for 1.5 h, then cool to 3°C to crystallize for 3 h, filter, and vacuum dry at 40°C for 9 h to obtain oxalic acid crystals.

[0045] Example 5 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 17 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 28℃, and the extraction mixing time was 18 min. The density of the loaded organic phase was 0.91 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 17.7 g / L.

[0046] S2. Add 12.8 parts of polyoxyethylene polyoxypropylene ether (F127) and 3.8 parts of hexadecyltrimethylammonium bromide (CTAB) to 39.5 parts of 60wt% ethanol solution, stir until clear, add 41.5 parts of tetraethyl orthosilicate (TEOS), stir for 17 min, then add 8.5 parts of 37wt% hydrochloric acid, and continue stirring for 7.5 h; slowly dropwise add the resulting mixture to 775 parts of liquid paraffin at 64℃, continue the reaction for 11.5 h, and filter. The silica spheres were washed four times with acetone and deionized water, dried to constant weight at 55°C, calcined at 658°C for 3.5 h, and sieved to obtain porous silica spheres. 10.4 parts of the porous silica spheres were mixed with 24 parts of a 35 wt% urea aqueous solution, stirred continuously at 150 rpm, and the pressure was reduced to -0.065 MPa and maintained for 26 min. The pressure was then restored to normal, and this "reduced pressure-normal pressure" operation was repeated twice. After filtration, the mixture was vacuum dried to constant weight at 55°C to obtain composite particles. The porous silica spheres had a particle size of 1.5–2 mm and a density of 0.83 g / cm³. 3 The density of the composite particles is 0.96 g / cm³. 3 .

[0047] S3. Add 630 parts of deionized water to the reaction vessel, then add 158 parts of the supported organic phase. Let it stand for 17 minutes to separate the phases. Load 11.9 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the supported organic phase and the deionized water, and let it stand for 2.5 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 142℃, adjust the nitrogen flow rate to control the pressure at 0.63MPa, and react at constant temperature and pressure for 4.5 hours. When the oxalic acid concentration in the supported organic phase decreases to 0.7g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 2.5 hours, stop heating, cool and stand for 70 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0048] S4. Heat the ammonium oxalate-rich aqueous phase to 55°C, add 11.7 parts of 30wt% calcium chloride solution, stir for 55 min, age for 75 min, and filter to obtain wet calcium oxalate material; add 7.7 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 80°C, stir for 35 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 56°C, evaporate under reduced pressure to -0.08 MPa for 2 h, then cool to 3°C to crystallize for 3 h, filter, and vacuum dry at 40°C for 10 h to obtain oxalic acid crystals.

[0049] Example 6 S1. Rare earth precipitation wastewater was mixed with a composite extractant and subjected to 20 stages of countercurrent extraction to separate the loaded organic phase and hydrochloric acid raffinate. The composite extractant consisted of 75 parts of bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 25 parts of 260# solvent oil. The weight ratio of rare earth precipitation wastewater to the composite extractant was 1:1. The extraction temperature was 27℃, and the extraction mixing time was 15 min. The density of the loaded organic phase was 0.9 g / cm³. 3 The initial concentration of oxalic acid in the supported organic phase was 17 g / L.

[0050] S2. Add 12.6 parts of polyoxyethylene polyoxypropylene ether (F127) and 3.3 parts of hexadecyltrimethylammonium bromide (CTAB) to 38.5 parts of 60wt% ethanol solution, stir until clear, add 41.5 parts of tetraethyl orthosilicate (TEOS), stir for 15 min, then add 8.5 parts of 37wt% hydrochloric acid, and continue stirring for 7.5 h; slowly dropwise add the resulting mixture to 770 parts of liquid paraffin at 61℃, continue the reaction for 11.5 h, and filter. The silica spheres were washed three times with acetone and three times with deionized water, dried to constant weight at 57°C, calcined at 655°C for 3.5 h, and sieved to obtain porous silica spheres. 10.4 parts of the porous silica spheres were mixed with 22 parts of a 35 wt% urea aqueous solution, stirred continuously at 150 rpm, and the pressure was reduced to -0.085 MPa and maintained for 30 min. The pressure was then restored to normal, and this "reduced pressure-normal pressure" operation was repeated three times. After filtration, the mixture was vacuum dried to constant weight at 57°C to obtain composite particles. The porous silica spheres had a particle size of 1.5–2 mm and a density of 0.81 g / cm³. 3 The density of the composite particles is 0.94 g / cm³. 3 .

[0051] S3. Add 625 parts of deionized water to the reaction vessel, then add 153 parts of the supported organic phase. Let it stand for 20 minutes to separate the phases. Load 11.9 parts of composite particles into a stainless steel sieve basket (the pore size of the stainless steel sieve basket is 0.15~1mm, and the filling volume of the composite particles is 60% of the effective volume of the stainless steel sieve basket). Place the stainless steel sieve basket in the interface area between the supported organic phase and the deionized water, and let it stand for 3 minutes. Seal the reaction vessel, introduce nitrogen for pre-pressurization, heat to 143℃, adjust the nitrogen flow rate to control the pressure at 0.6MPa, and react at constant temperature and pressure for 5 hours. When the oxalic acid concentration in the supported organic phase decreases to 0.8g / L, remove the stainless steel sieve basket from the liquid phase, continue heating for 2 hours, stop heating, cool and stand for 70 minutes, depressurize to atmospheric pressure, and separate and collect the back-extracted organic phase and the ammonium oxalate-rich aqueous phase.

[0052] S4. Heat the ammonium oxalate-rich aqueous phase to 45°C, add 11.5 parts of 30wt% calcium chloride solution, stir for 55 min, age for 85 min, and filter to obtain wet calcium oxalate material; add 7.9 parts of 40wt% sulfuric acid solution to the wet calcium oxalate material, heat to 85°C, stir for 30 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat the oxalic acid filtrate to 58°C, evaporate under reduced pressure to -0.08 MPa for 2 h, then cool to 3°C to crystallize for 2.5 h, filter, and vacuum dry at 45°C for 9 h to obtain oxalic acid crystals.

[0053] The present invention also includes comparative examples and related experiments.

[0054] Comparative Example 1 The only difference from Example 1 is that in step S3, the composite particles were not loaded into a stainless steel sieve basket, but were directly added into the reaction vessel. The other components and preparation steps were completely consistent, and the product was used to recover oxalic acid crystals.

[0055] Comparative Example 2 The only difference from Example 1 is that in step S3, after adding the composite particles, no heating and pressurization treatment was performed; the treatment was carried out only under normal temperature and pressure conditions. The other components and preparation steps were completely consistent, and the product was used to recover oxalic acid crystals.

[0056] Comparative Example 3 The only difference from Example 1 is that in step S3, the stainless steel sieve basket was not removed from the liquid phase (i.e., the composite particles were not removed). The other components and preparation steps are completely the same, and the product is used to recover oxalic acid crystals.

[0057] Performance testing: Referring to the national standard GB / T 1626-2008 "Oxalic Acid for Industrial Use", the mass fractions of oxalic acid, sulfate, residue on ignition, chloride, and calcium in the oxalic acid crystals prepared in Examples 1-6 and Comparative Examples 1-3 were tested. The recovery rate of oxalic acid crystals was calculated based on the volume of the supported organic phase, the initial concentration of oxalic acid therein, and the mass of the final obtained oxalic acid crystals. The test and calculation results are shown in Table 1.

[0058] Table 1

[0059] As shown in Table 1, the mass fraction of oxalic acid in the oxalic acid crystals obtained in Examples 1-6 ranges from 99.16% to 99.73%, while the mass fractions of sulfate, ignition residue, chloride, and calcium are all low. The recovery rate of oxalic acid crystals ranges from 93.2% to 95.8%, indicating that the present invention can improve the recovery rate of oxalic acid crystals while reducing the residue of sulfate, chloride, calcium, and other inorganic impurities in oxalic acid crystals, thereby improving the recovery purity of oxalic acid crystals.

[0060] Compared to Example 1, Comparative Example 1 did not use a stainless steel sieve basket for containment. The composite particles entered the deionized aqueous phase after heating, increasing the release of urea and thus slightly improving the oxalic acid recovery rate. However, the increased amount of back-extraction aids entering the ammonium oxalate-rich aqueous phase increased the burden on subsequent purification, leading to a decrease in the purity of the oxalic acid crystals. Compared to Example 1, Comparative Example 2 did not undergo heating and pressurization under sealed conditions. Urea was difficult to hydrolyze in deionized water, resulting in a significant decrease in the recovery rate of the obtained oxalic acid crystals. This indicates that sealed heating and pressurization is beneficial for promoting the back-extraction of oxalic acid and improving the recovery rate of oxalic acid crystals. Compared to Example 1, Comparative Example 3 did not remove the stainless steel sieve basket from the liquid phase in the later stage of back-extraction. The composite particles continuously released urea into the aqueous phase, increasing the burden on subsequent purification. The oxalic acid mass fraction of the obtained oxalic acid crystals decreased significantly, indicating that timely removal of the sieve basket from the liquid phase is beneficial for improving the purity of the oxalic acid crystals.

[0061] The above are preferred embodiments of the present invention. Those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for separating and recovering oxalic acid from an oxalic acid-containing wastewater, characterized by, The preparation steps include the following: S1. The rare earth precipitation wastewater is mixed with a composite extractant and extracted to obtain a loaded organic phase and hydrochloric acid raffinate. S2. Mix and stir porous silica balls with urea aqueous solution, impregnate under reduced pressure, filter and dry under vacuum to obtain composite particles; S3. Add deionized water and the supported organic phase to the reaction vessel in sequence. Pour the composite particles into the sieve basket and place it in the interface area between the deionized water and the supported organic phase. Let it stand, introduce nitrogen gas, pressurize and heat to carry out the reaction. Remove the sieve basket from the liquid phase, continue heating the reaction, stop heating, cool and let it stand, and separate the back-extracted organic phase and the ammonium oxalate-rich aqueous phase. S4. Add calcium chloride solution to the ammonium oxalate-rich aqueous phase to react, filter to obtain calcium oxalate wet material, add sulfuric acid solution to react, filter to obtain oxalic acid filtrate, evaporate under reduced pressure, cool to crystallize, filter and dry to obtain oxalic acid crystals. The density of the composite particles is greater than the density of the supported organic phase and less than the density of deionized water. The composite extractant comprises bis-2-(ethylhexyl) 2-ethylhexylphosphonic acid and 260# solvent oil.

2. The method of claim 1, wherein the method is characterized by, The porous silica spheres are prepared by adding polyoxyethylene polyoxypropylene ether and hexadecyltrimethylammonium bromide to an aqueous ethanol solution, stirring until clear, adding tetraethyl orthosilicate, stirring for 15-20 min, adding hydrochloric acid, and continuing stirring; adding the resulting mixture dropwise into liquid paraffin, continuing the reaction for 10-12 h, filtering, washing 3-4 times with acetone and deionized water respectively, drying to constant weight at 55-60℃, calcining at 650-660℃ for 3-4 h, and sieving.

3. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 2, characterized in that, The porous silica sphere has a particle size of 1.5-2 mm and a density of 0.8-0.83 g / cm 3 .

4. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 1, characterized in that, In step S1, the extraction method is multi-stage countercurrent extraction, with 15 to 20 countercurrent stages, an extraction temperature of 20 to 30°C, and an extraction mixing time of 10 to 20 minutes. The weight ratio of bis-2-(ethylhexyl) phosphonate 2-ethylhexyl ester to 260# solvent oil is 3:1, and the weight ratio of rare earth precipitation wastewater to composite extractant is 1:

1.

5. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 1, characterized in that, In step S2, porous silica spheres are mixed with urea aqueous solution and stirred at a speed of 100~200 rpm. The pressure is reduced to -0.085~-0.06 MPa and maintained for 20~30 min. The pressure is then restored to normal. The pressure reduction and restoration operation is repeated 2~3 times. After filtration, the composite particles are vacuum dried at 55~60℃ to constant weight to obtain composite particles.

6. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 1, characterized in that, The density of the composite particles is 0.94-0.96 g / cm 3 under normal temperature conditions, and the density of the loaded organic phase is 0.9-0.91 g / cm 3 .

7. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 1, characterized in that, In step S3, deionized water and the supported organic phase are poured into the reaction vessel in sequence. The composite particles are poured into a stainless steel sieve basket and placed in the interface area between the deionized water and the supported organic phase. After standing for 2-3 minutes, the reaction vessel is sealed, nitrogen gas is introduced for pre-pressurization, and the temperature is heated to 140-145°C. The reaction pressure is controlled at 0.6-0.65 MPa, and the reaction is carried out at constant temperature and pressure for 3-5 hours. The stainless steel sieve basket is removed from the liquid phase, and the reaction is continued to be heated for 2-3 hours. Heating is stopped, and the mixture is cooled and allowed to stand for 60-90 minutes. The pressure is released to atmospheric pressure, and the back-extracted organic phase and the ammonium oxalate-rich aqueous phase are separated.

8. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 7, characterized in that, In step S1, the initial concentration of oxalic acid in the supported organic phase is 17~18 g / L; in step S3, when the concentration of oxalic acid in the supported organic phase decreases to 0.7~0.8 g / L, the stainless steel sieve basket is removed from the liquid phase.

9. The method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 1, characterized in that, In step S4, the ammonium oxalate-rich aqueous phase is heated to 40-60°C, calcium chloride solution is added, the mixture is stirred and reacted for 50-60 minutes, and then filtered to obtain wet calcium oxalate material. Add sulfuric acid solution to wet calcium oxalate, heat to 70-90℃, stir for 30-50 min, filter to remove calcium sulfate precipitate, and obtain oxalic acid filtrate; heat oxalic acid filtrate to 55-60℃, reduce pressure to -0.08-0.07 MPa and evaporate for 1-2 h, then cool to 0-5℃ to crystallize for 2-3 h, filter and vacuum dry to obtain oxalic acid crystals.

10. A method for separating and recovering oxalic acid from oxalic acid-containing wastewater according to claim 2, characterized in that, The composite particles comprise the following raw materials in parts by weight: 10-10.5 parts of porous silica spheres and 20-25 parts of 35wt% urea aqueous solution; the porous silica spheres comprise the following raw materials in parts by weight: 12-13 parts of polyoxyethylene polyoxypropylene ether, 3-4 parts of hexadecyltrimethylammonium bromide, 38-40 parts of 60wt% ethanol solution, 40-42 parts of tetraethyl orthosilicate, 7-9 parts of 37wt% hydrochloric acid, and 760-780 parts of liquid paraffin.

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