A method for reducing the oxalate content in the production of anhydrous citric acid

Abstract

This application discloses a method for reducing the oxalate content in the production of anhydrous citric acid, belonging to the field of organic acid refining technology. The method involves adjusting the pH of the citric acid fermentation broth to 2.5-3.0, adding a MnO2-γ-Al2O3 catalyst, and catalytically oxidizing and degrading oxalic acid into CO2 and H2O under oxygen-containing gas conditions. After the oxalate content reaches the target level, the pH of the reaction solution is raised to 3.5-4.0 to passivate the catalyst and terminate the reaction. After filtration and catalyst recovery, the filtrate is treated to obtain the finished anhydrous citric acid product. This method can be implemented in existing production lines, the catalyst can be recycled more than 15 times, the oxalate removal rate is ≥90%, and the oxalate content of the finished anhydrous citric acid product can be reduced to below 20 ppm. It is suitable for the production of food-grade, pharmaceutical-grade, and electronic-grade anhydrous citric acid.

Description

Technical Field

[0001] This application belongs to the field of organic acid refining technology, and in particular relates to a method for reducing the oxalate content in the production of anhydrous citric acid. Background Technology

[0002] Citric acid (C6H8O7) is currently the world's most produced fermented organic acid, widely used in food, beverage, pharmaceutical, daily chemical, and electronics industries. Industrially, citric acid is mainly produced through the deep fermentation method using Aspergillus niger. During Aspergillus niger fermentation, oxalic acid, as a byproduct of the tricarboxylic acid cycle, is inevitably generated, typically accounting for 2-8% of the total organic acids in the fermentation broth. Existing industrial citric acid production processes generally include solid-liquid separation of the fermentation broth, calcium salt neutralization / acid hydrolysis, ion exchange decolorization, evaporation and concentration, crystallization, drying, and final product. Oxalate removal is crucial across these processes. Current industrial technologies for controlling oxalate content mainly fall into the following categories: 1. Fermentation broth calcium salt precipitation method: Calcium carbonate slurry is added to the fermentation broth. Utilizing the pH difference between calcium oxalate and calcium citrate, calcium oxalate is preferentially precipitated within a specific pH range. This method is the most traditional industrial approach. Its drawback is that both oxalic acid and citric acid are organic acids, resulting in limited pH selectivity for calcium salt precipitation. Some calcium citrate inevitably co-precipitates, leading to a decrease in product yield. Furthermore, the determination of the precipitation endpoint depends on online pH control, which has a lag response, and the oxalic acid removal effect fluctuates greatly in actual production. 2. Two-stage neutralization method: Two pH control stages are set in the neutralization process. First, oxalate is preferentially precipitated at a low pH value, and then the pH is increased for normal neutralization of calcium citrate. This method improves the oxalic acid removal rate, but it is still difficult to meet the standards of high-end products and increases the complexity of the process operation. 3. Chromatographic separation / ion exchange method: Citric acid and oxalic acid are separated from fermentation broth using preparative chromatography techniques or special ion exchange resins. This method has a good separation effect, but the equipment investment is huge, the resin regeneration consumes a lot of acid and alkali, the operating cost is high, and the industrial promotion is limited. 4. Fermentation strain modification method: In recent years, by knocking out key genes for oxalic acid synthesis (such as the OAHS gene) in Aspergillus niger using CRISPR-Cas9 technology to construct low-by-product mutant strains, the accumulation of oxalic acid can be reduced to below 0.5%. However, issues such as fermentation stability of genetically engineered strains, strain degradation, and regulatory approval make it difficult to promote this route on a large scale in industrial production in the short term. In summary, existing technologies suffer from several common drawbacks: First, physical separation methods (precipitation, chromatography, ion exchange) rely on differences in properties between citric acid and oxalic acid in a certain dimension (such as calcium salt solubility and resin affinity). However, since their molecular structures and chemical properties are quite similar, selective separation is inherently difficult, resulting in low separation factors. Second, existing methods are all transfer-based processes; oxalic acid is separated from one location to another, still requiring subsequent waste treatment and environmental disposal. Existing technologies rarely offer chemical processes for the complete degradation and elimination of oxalic acid in the post-treatment stage of the fermentation broth. Therefore, there is an urgent need to develop a new process that can be implemented in the citric acid production process, is simple to operate, has high selectivity, and can directly chemically degrade oxalate without affecting the citric acid product.

[0003] Oxalate oxidase is a type of enzyme in nature that specifically degrades oxalate. It can efficiently catalyze the oxidation of oxalate to CO2 and H2O2, and has almost no catalytic activity towards citric acid. The catalytic mechanism of oxalate oxidase is based on the reaction of Mn... 2+ With the participation of cofactors, molecular oxygen is used as an electron acceptor to oxidize and degrade oxalic acid; however, natural oxalate oxidase has poor stability, low activity, and extremely high cost in the acidic environment of citric acid production, and cannot be directly used in industrial production. Summary of the Invention

[0004] The purpose of this application is to provide a method for reducing the oxalate content in the production of anhydrous citric acid by constructing an artificial catalyst that mimics the active site structure of oxalate oxidase. This catalyst contains a microenvironment of Mn active sites similar to that of natural oxalate oxidase, which can selectively catalyze the oxidation of oxalate under acidic conditions while remaining inert to citric acid, thereby solving the technical problem of high oxalate content in existing citric acid products.

[0005] To achieve the above objectives, the technical solution adopted in this application is: to provide a method for reducing the oxalate content during the production of anhydrous citric acid, specifically including the following steps: (I) Fermentation broth pretreatment: Remove impurities from the citric acid fermentation broth, adjust the pH, and heat to obtain pretreated fermentation broth; (II) Catalytic oxidation reaction: The pretreated fermentation broth and MnO2-γ-Al2O3 catalyst were added to the reaction vessel, and oxygen-containing gas was introduced to carry out the catalytic oxidation reaction. After the reaction was completed, the pH was adjusted, the gas was stopped after stirring, and crude citric acid was obtained. The crude citric acid was filtered, the catalyst was recovered, and the filtrate was post-treated to obtain anhydrous citric acid product with significantly reduced oxalate content.

[0006] In one embodiment, In step (1), the pH is adjusted to 2.5-3.0, and the heating temperature is 25-40 ℃.

[0007] In one embodiment, In the MnO2-γ-Al2O3 catalyst, the loading of MnO2 is 5-8% of the mass of the γ-Al2O3 support.

[0008] In one embodiment, In the MnO2-γ-Al2O3 catalyst, the Mn source is Mn(NO3)2·4H2O or Mn(NO3)2·6H2O, and the particle size of the γ-Al2O3 support is 0.5-2 mm.

[0009] In one embodiment, The MnO2-γ-Al2O3 catalyst was prepared by calcination at a temperature of 380-420 ℃, a heating rate of 2 ℃ / min, and a calcination time of 3-5 h.

[0010] In one embodiment, In step (ii), the loading amount of the MnO2-γ-Al2O3 catalyst is 20-80 g / L of the pretreated fermentation broth.

[0011] In one embodiment, The oxygen content of the oxygen-containing gas in step (ii) is 21-30%, and the ventilation rate is 0.2-0.8 vvm.

[0012] In one embodiment, The catalytic oxidation reaction in step (ii) is carried out at a temperature of 25-40 °C for 1-4 h.

[0013] In one embodiment, The solvent used to adjust the pH in step (ii) is NaOH solution or NH3·H2O; adjust the pH to 3.4-4.0; stop aeration after stirring for 20-30 min.

[0014] In one embodiment, The post-processing in step (ii) specifically includes decolorization, concentration, crystallization, and drying; the crystallization temperature is 35-55℃, and the drying temperature is 50-60℃.

[0015] This application provides a method for reducing oxalate content during the production of anhydrous citric acid. 1. This technical solution ingeniously utilizes the selective adsorption and activation phenomenon of oxalic acid on the surface of manganese oxides. Oxalic acid molecules are symmetrical linear dicarboxylic acids with small molecular size, allowing them to enter specific micropores on the catalyst surface; while citric acid molecules are larger and have difficulty entering the same active micropores. This size sieving effect endows the catalyst with substrate selectivity for oxalic acid, breaking through the existing process of oxalate removal, which is a transfer-type physical separation process, and shifting to a degradation-type chemical conversion. Through MnO2 catalytic oxidation, the C-C bonds of oxalic acid are completely broken and mineralized into CO2 and H2O. This is carbon skeleton degradation, which is irreversible once it occurs. Therefore, as long as sufficient active oxygen species and reaction time are provided, the residual concentration of oxalic acid can theoretically approach zero, fundamentally eliminating oxalate impurities and producing no secondary pollutants or waste streams containing oxalic acid. The catalyst is a supported solid catalyst that can be recycled and reused, does not introduce foreign impurity ions, and has controllable operating costs. 2. A pH-mediated activation-passivation dual-mode process control strategy uses precise pH control as the core process control method. Under low pH conditions, the C-C bonds of oxalic acid are effectively oxidized and broken, and oxalic acid is completely mineralized into CO2 and H2O. After the oxalic acid is fully degraded, the active sites on the catalyst surface are reversibly passivated by slightly increasing the pH, stopping the oxidation reaction and avoiding any side reactions caused by prolonged exposure of citric acid to the oxidizing environment in subsequent concentration and crystallization processes. This pH-mediated activation-passivation dual-mode control strategy ensures the controllability of the oxidation process and the safety of citric acid. 3. This solution operates at the forefront of the production process, rather than in the final reprocessing of the finished product. It is an online control method for the production process, exhibiting high selectivity and low citric acid loss. The selectivity stems from a dual synergistic mechanism of size sieving and pH control. Firstly, the mesoporous structure of the catalyst support γ-Al2O3 forms a size sieving barrier—oxalic acid molecules have a much smaller kinetic diameter than citric acid molecules, making it easier for them to enter the catalyst micropores and contact the active sites. Citric acid, due to its larger molecular volume and steric hindrance, has difficulty effectively contacting the active centers. Secondly, the operating range of pH 2.5-3.0 falls within the range where oxalic acid oxidation activity is highest and citric acid oxidation activity is lowest. This is a chemical-level selectivity, and the synergistic effect of both ensures high selectivity. 4. The entire process does not introduce heavy metal ions or other new impurities. MnO2 has extremely low solubility in water and hardly dissolves Mn under neutral and acidic conditions. 2+ Ions; γ-Al₂O₃ support is chemically inert and also does not dissolve Al. 3+ pH adjustment uses the acid hydrolysis solution from the citric acid production process or analytical grade dilute sulfuric acid, without introducing any new types of chemicals. Passivation uses dilute alkaline solution to introduce trace amounts of sodium. + or NH4 +It is removed in subsequent ion exchange steps, and the oxidation products of oxalic acid are only CO2 and H2O, with no secondary pollution, making it environmentally friendly. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a graph showing the oxalic acid content detection in Example 1. Detailed Implementation

[0018] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, this application will be further described in detail. It should be understood that the specific embodiments described herein are only for explaining this application and are not intended to limit this application.

[0019] Example 1 A method for reducing oxalate content during the production of anhydrous citric acid specifically includes the following steps: (I) Pretreatment of fermentation broth: Citric acid fermentation broth produced by our company was selected to remove mycelium, protein, pigment and other macromolecular impurities. After testing, the citric acid mass fraction was 15.2% and the oxalate content (calculated as oxalic acid) was 520 ppm (relative to the solid content of citric acid). The pH was adjusted to 2.8±0.05 and heated to 33±1 ℃ to obtain the pretreated fermentation broth. Specifically, according to the existing process route, the citric acid fermentation broth is filtered through a plate and frame filter or centrifuged to remove Aspergillus niger mycelium and other solid suspended matter. It then undergoes calcium salt neutralization-acid hydrolysis-decolorization treatment, or ion exchange decolorization treatment, to obtain a pretreated solution with a citric acid mass fraction of 8-20%. This process removes macromolecular impurities such as mycelium, protein, and pigments from the fermentation broth, providing a relatively clean reaction environment for the subsequent oxalic acid catalytic oxidation step. This prevents macromolecular impurities from clogging catalyst micropores or covering active sites, thus reducing catalytic efficiency. The pH is adjusted using analytical grade dilute sulfuric acid or the acid hydrolysis solution from the citric acid production process (acidic effluent from the sulfuric acid hydrolysis step), without introducing any new types of chemicals. (II) Catalytic oxidation reaction S1. Catalyst preparation: Take 500 g of γ-Al2O3 support (particle size 1-2 mm, specific surface area 220 m²) 2 / g, average pore size 8 nm (purchased from supplier), dried at 120 ℃ for 2 h, and prepared 180 mL of aqueous solution containing Mn(NO3)2·4H2O (analytical grade), with a Mn element concentration of 40 g / L; this solution was uniformly impregnated onto the γ-Al2O3 support by equal volume impregnation method, aged at room temperature for 3 h, and dried at 110 ℃ for 4 h; calcined in a muffle furnace at 2 ℃ / min to 400 ℃ under air atmosphere for 4 h, cooled, washed 3 times with deionized water, and dried at 80 ℃ to obtain a MnO2-γ-Al2O3 catalyst with a MnO2 loading of 6.5% (based on Mn); Specifically, the research on MnO2-γ-Al2O3 catalysts is relatively mature, making their preparation and use convenient. Using γ-Al2O3 as a support, its abundant mesoporous structure provides a high specific surface area, which is beneficial for the uniform dispersion of the MnO2 active component and the diffusion and mass transfer of the substrate oxalic acid molecules. The Mn-O bonding structure on the surface of the active component MnO2 provides a similar Mn-O bond structure to that in natural oxalate oxidases. 2+ The coordination microenvironment of the cofactor constitutes the functional simulation basis for catalytic oxalic acid oxidation and degradation; at the same time, since the support γ-Al2O3 and the active component MnO2 are both insoluble solids, the catalyst can be separated from the reaction solution by simple filtration and recycled for reuse without introducing any foreign metal ions or impurities. S2. Catalytic oxidation reaction: Take 200 L of pretreated fermentation broth and add it to the catalytic reaction tank. Add 10 kg of MnO2-γ-Al2O3 catalyst (the loading amount of MnO2-γ-Al2O3 catalyst is 50 g / L of pretreated fermentation broth). Stir at 100 rpm and introduce oxygenated air (oxygen content of 28%, aeration rate of 0.5 vvm) through the aeration head at the bottom of the tank. React for 2 h. Take samples at 30, 60, 90, and 120 min to detect the oxalate content. At 120 min, the oxalate content drops to 9 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 98.3%. Specifically, dissolved oxygen is activated at the active sites on the MnO2 surface, forming reactive oxygen species, and atomic oxygen and superoxide species O2 are adsorbed on the surface. - The active oxygen species further undergo an oxidative cleavage reaction with the C-C bonds of oxalic acid, and the oxalic acid is completely mineralized, i.e., H2C2O4 + [O]active → 2CO2↑ + H2O. The CO2 generated in the reaction is discharged with the tail gas and does not pollute the product. S3, pH passivation to terminate the reaction: After the reaction is completed, slowly add 350 mL of 5% NaOH solution while stirring to adjust the pH to 3.7±0.1. Continue stirring for 25 min, then turn off the aeration to obtain crude citric acid. Specifically, as pH increases, the degree of protonation on the MnO2 surface decreases, and the high-valence active manganese (Mn) becomes more active.4+ It is reduced to Mn with low oxidizing activity. 3+ -O, the catalytic activity decreases significantly, this stage is the termination of the oxalic acid oxidation reaction, to ensure the chemical stability of citric acid in the subsequent concentration and crystallization process; S4. Post-processing: The crude citric acid is filtered through a 200-mesh stainless steel wire mesh to separate the catalyst. The catalyst is washed three times with deionized water and dried at 80°C for future use. The filtrate enters the conventional ion exchange decolorization and evaporation concentration process: evaporation and concentration are carried out at 62°C and a vacuum of -88 kPa until the citric acid mass fraction reaches 76%. The concentrate is then placed in a crystallizer and evaporated at 45°C for crystallization. After centrifugation, the crystals are dried with hot air at 55°C until the moisture content reaches 0.25%, yielding anhydrous citric acid. Figure 1 As shown, the oxalate content in the purified anhydrous citric acid product was determined by the external standard method, and the result was 8.23 ​​ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 98.4% and a total citric acid recovery rate of 97.5%.

[0020] Example 2 The difference between this embodiment and Example 1 is that the citric acid fermentation broth contained 12.8% citric acid by mass and 890 ppm oxalate by mass. The pH was adjusted to 2.7±0.05, and the broth was heated to 35±1 °C to obtain a pretreated fermentation broth. The loading amount of MnO2-γ-Al2O3 catalyst was 12 kg, the aeration rate was 0.6 vvm, and the reaction was carried out for 2.5 h. The oxalate content was measured to be reduced to 18 ppm, with a removal rate of 98.0%. 5% NaOH solution was added dropwise to adjust the pH to 3.6±0.1, and stirring was continued for 20 min before aeration was turned off. The catalyst recovery and post-treatment were the same as in Example 1 to obtain anhydrous citric acid product. The purified anhydrous citric acid product had an oxalate content of 16 ppm (calculated as oxalic acid, relative to the solid content of citric acid), a removal rate of 98.2%, and a total citric acid recovery rate of 97.0%.

[0021] Example 3 This embodiment differs from Example 1 in that the citric acid fermentation broth contained 12.8% citric acid by mass and 890 ppm oxalate by mass. The pH was adjusted to 2.9±0.05, and the broth was heated to 32±1 °C to obtain a pretreated fermentation broth. The MnO2-γ-Al2O3 catalyst contained 50 g / L Mn and 8.0% MnO2 loading (based on Mn). The catalyst was calcined at 420 °C for 5 h. The MnO2-γ-Al2O3 catalyst loading was 8 kg, the aeration rate was 0.4 vvm, and the reaction time was 3 h. The oxalate content was reduced to 25 ppm, with a removal rate of 97.2%. The pH was adjusted to 3.8±0.1 by adding 3% NH3·H2O solution, and the aeration was stopped after stirring for 30 min. Catalyst recovery and post-treatment were the same as in Example 1, yielding anhydrous citric acid. The purified anhydrous citric acid had an oxalate content of 22%. The removal rate was 97.5% (ppm as oxalic acid, relative to the solid content of citric acid), and the total recovery rate of citric acid was 97.3%.

[0022] Example 4 The difference between this embodiment and Example 1 is that the MnO2 loading was 5.0% (calculated as Mn), the calcination was performed at 380 ℃ for 3 h, and the other operations were the same. The oxalate content was reduced to 18 ppm, with a removal rate of 96.5%. The post-treatment yielded anhydrous citric acid product. The oxalate content of the refined anhydrous citric acid product was 11 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 97.9% and a total citric acid recovery rate of 97.5%.

[0023] Example 5 The difference between this embodiment and Example 1 is that the pH of the citric acid fermentation broth was adjusted to 2.5 and heated to 25°C to obtain a pretreated fermentation broth; the remaining operations were the same, and the oxalate content was measured to be reduced to 35 ppm, with a removal rate of 93.3%; after post-treatment, anhydrous citric acid was obtained, and the oxalate content of the refined anhydrous citric acid was 31 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 94.0% and a total citric acid recovery rate of 97.4%.

[0024] Example 6 The difference between this embodiment and Example 1 is that the pH of the citric acid fermentation broth was adjusted to 3.0 and heated to 40 °C to obtain a pretreated fermentation broth; the remaining operations were the same, and the oxalate content was measured to be reduced to 22 ppm, with a removal rate of 95.8%; after post-treatment, anhydrous citric acid was obtained, and the oxalate content of the refined anhydrous citric acid was 19 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 96.3% and a total citric acid recovery rate of 97.2%.

[0025] Example 7 The difference between this embodiment and Example 1 is that Mn(NO3)2·4H2O was replaced with Mn(NO3)2·6H2O, while the rest of the operations were the same. The oxalate content was reduced to 13 ppm, with a removal rate of 97.5%. After post-treatment, anhydrous citric acid was obtained. The oxalate content of the refined anhydrous citric acid product was 12 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 97.7% and a total citric acid recovery rate of 97.5%.

[0026] Example 8 The difference between this embodiment and Example 1 is that the loading amount of the MnO2-γ-Al2O3 catalyst is 4 kg, while the other operations are the same. The oxalate content was reduced to 58 ppm, with a removal rate of 88.9%. After post-treatment, anhydrous citric acid was obtained. The oxalate content of the refined anhydrous citric acid product was 52 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 90.0% and a total citric acid recovery rate of 97.6%.

[0027] Example 9 The difference between this embodiment and Example 1 is that the loading amount of the MnO2-γ-Al2O3 catalyst is 16 kg, while the other operations are the same. The oxalate content was reduced to 8 ppm, with a removal rate of 98.5%. After post-treatment, anhydrous citric acid was obtained. The oxalate content of the refined anhydrous citric acid product was 7 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 98.7% and a total citric acid recovery rate of 97.2%.

[0028] Example 10 The difference between this embodiment and Example 1 is that air with an oxygen content of 21% and an air flow rate of 0.2 vvm were used. The other operations were the same, and the oxalate content was reduced to 42 ppm, with a removal rate of 91.9%. The post-treatment yielded anhydrous citric acid product. The oxalate content of the refined anhydrous citric acid product was 38 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 92.7% and a total citric acid recovery rate of 97.4%.

[0029] Example 11 The difference between this embodiment and Embodiment 1 is that oxygenated air with an oxygen content of 30% and an air flow rate of 0.8 vvm were used. The other operations were the same, and the oxalate content was reduced to 6 ppm, with a removal rate of 98.8%. The post-treatment yielded anhydrous citric acid product, and the oxalate content of the refined anhydrous citric acid product was 5 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 99.0% and a total citric acid recovery rate of 97.3%.

[0030] Example 12 The difference between this embodiment and Example 1 is that the temperature of the catalytic oxidation reaction was 25 °C and the time was 4 h, while the other operations were the same. The oxalate content was reduced to 16 ppm, with a removal rate of 96.9%. The post-treatment yielded anhydrous citric acid product. The oxalate content of the refined anhydrous citric acid product was 14 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 97.3% and a total citric acid recovery rate of 97.4%.

[0031] Example 13 The difference between this embodiment and Example 1 is that the temperature of the catalytic oxidation reaction was 40 ℃ and the time was 1 h, while the other operations were the same. The oxalate content was reduced to 25 ppm, with a removal rate of 95.2%. The post-treatment yielded anhydrous citric acid product. The oxalate content of the refined anhydrous citric acid product was 22 ppm (calculated as oxalic acid, relative to the solid content of citric acid), with a removal rate of 95.8% and a total citric acid recovery rate of 97.3%.

[0032] Example 14 The recovered catalyst was reused to investigate the effect of the number of cycles on the oxalate removal rate. The catalyst recovered in Example 1 was washed and dried, and then reused under the conditions of Example 1. The reaction conditions were exactly the same for each repeated experiment. The experimental results are shown in Table 1. Table 1. Removal rate results of catalyst recycling

[0033] The oxalate removal rate remained at 94.8% after 15 cycles of catalyst recycling, and slightly decreased to 92.7% after 18 cycles. The catalytic activity retention rate (based on removal rate, relative to the first use) was ≥85% after 15 cycles, indicating good catalytic performance.

[0034] Comparative Example 1 The same fermentation broth as in Example 1 (oxalate content 520 ppm) was used. Without pH adjustment (original fermentation broth pH=4.5), the catalyst was directly added and the mixture was aerated for 2 hours. The residual oxalate content was measured at 362 ppm, with a removal rate of 30.4%. Under higher pH conditions, oxalic acid mainly exists as C2O4. 2- In its current form, the activity of the active sites on the MnO2 surface is significantly reduced, and the catalytic oxidation efficiency drops sharply. At the same time, as the pH increases, the MnO2 surface tends to passivate, and its ability to activate oxalic acid C-C bonds is drastically weakened.

[0035] Comparative Example 2 Take the same fermentation broth as in Example 1 (oxalate content 520 ppm), skip step (II) S3pH passivation to terminate the reaction, and directly proceed to the subsequent evaporation, concentration and crystallization processes; the final product was found to contain trace byproducts: decarboxylation products of citric acid (itaconic acid, citraconic acid, etc.), with a content of 0.3-0.5%, and the purity of citric acid decreased; during the high-concentration evaporation and concentration process, the acidity of the concentrate increased (the pH of the concentrated citric acid solution can be as low as 1-2), and the remaining small amount of active oxygen species and catalyst micro-fragments continued to have oxidative activity under low pH and high temperature conditions, and some tertiary alcohols in citric acid molecules underwent β-elimination dehydration to generate itaconic acid, which confirmed the necessity of the oxidation reaction termination step.

[0036] Comparative Example 3 The catalyst was prepared using the same method as in Example 1, except that the γ-Al₂O₃ support was replaced with a silica support of equal particle size (specific surface area 150 m²). 2 / g, with an average pore size of approximately 30 nm). Catalytic oxidation was carried out under the same conditions for 2 h; the residual oxalate content was detected to be 145 ppm, with a removal rate of 72.1%. Furthermore, trace amounts of citric acid oxidation degradation products, such as 3-ketoglutaric acid, were found in the subsequent ion exchange decolorization solution. The non-mesoporous support had excessively large pore sizes, resulting in a near-complete loss of size sieving effect. Citric acid molecules could also freely enter the pores and contact the MnO2 active sites, leading to a significant decrease in selectivity.

[0037] Comparative Example 4 Fe(NO3)3 was used to replace Mn as the source to prepare Fe-γ-Al2O3 catalyst, which replaced MnO2-γ-Al2O3 catalyst, with the rest of the operation remaining the same. The residual oxalate content was found to be 198 ppm, with a removal rate of 61.9%. The residual iron ions reacted with citric acid to form a yellow iron citrate complex, resulting in abnormal color of the finished product and seriously affecting the product grade. The oxalate oxidation activity of iron is significantly lower than that of manganese, but the price of iron source is significantly lower than that of manganese source. It is possible that by extending the reaction time, increasing the catalyst loading, or increasing the reaction temperature, compatibility can be achieved.

[0038] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method of reducing the oxalate content in the production of anhydrous citric acid, characterized in that, Specifically, the following steps are included: (I) Fermentation broth pretreatment: Remove impurities from the citric acid fermentation broth, adjust the pH, and heat to obtain pretreated fermentation broth; (II) Catalytic oxidation reaction: The pretreated fermentation broth and MnO2-γ-Al2O3 catalyst were added to the reaction vessel, and oxygen-containing gas was introduced to carry out the catalytic oxidation reaction. After the reaction was completed, the pH was adjusted, the gas was stopped after stirring, and crude citric acid was obtained. The crude citric acid was filtered, the catalyst was recovered, and the filtrate was post-treated to obtain anhydrous citric acid product.

2. A method of reducing the oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, In step (1), the pH is adjusted to 2.5-3.0, and the heating temperature is 25-40 ℃.

3. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, In the MnO2-γ-Al2O3 catalyst, the loading of MnO2 is 5-8% of the mass of the γ-Al2O3 support.

4. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, In the MnO2-γ-Al2O3 catalyst, the Mn source is Mn(NO3)2·4H2O or Mn(NO3)2·6H2O, and the particle size of the γ-Al2O3 support is 0.5-2 mm.

5. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, The MnO2-γ-Al2O3 catalyst was prepared by calcination at a temperature of 380-420 ℃, a heating rate of 2 ℃ / min, and a calcination time of 3-5 h.

6. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, In step (ii), the loading amount of the MnO2-γ-Al2O3 catalyst is 20-80 g / L of the pretreated fermentation broth.

7. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, The oxygen content of the oxygen-containing gas in step (ii) is 21-30%, and the ventilation rate is 0.2-0.8 vvm.

8. The method for reducing oxalate content in the production of anhydrous citric acid according to claim 1, characterized in that, The catalytic oxidation reaction in step (ii) is carried out at a temperature of 25-40 °C for 1-4 h.

9. A method for reducing oxalate content during the production of anhydrous citric acid according to claim 1, characterized in that, The solvent used to adjust the pH in step (ii) is NaOH solution or NH3·H2O; adjust the pH to 3.4-4.0; stop aeration after stirring for 20-30 min.

10. A method for reducing oxalate content during the production of anhydrous citric acid according to claim 1, characterized in that, The post-processing in step (ii) specifically includes decolorization, concentration, crystallization and drying; the crystallization temperature is 35-55 ℃ and the drying temperature is 50-60 ℃.

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