Method for producing urea formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid raffinate

Abstract

The invention relates to the technical field of fertilizer production and industrial solid waste resource utilization, and particularly discloses a method for producing a urea formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid raffinate. According to the method, phosphorus-potassium mixed acid slurry, polyglutamic acid and urea formaldehyde are used as main raw materials, and the preparation method comprises the steps of raw material reaction and mixed acid preparation, multi-stage neutralization and functional agent mixing, slow-release nitrogen source mixing, returned material pretreatment and granulation, screening and wrapping, process waste heat recovery and the like. The product disclosed by the invention can be applied as a special compound fertilizer with nutrient slow release and biological stimulation functions, and has the advantages of balanced nutrients, high utilization rate and capability of improving soil. Besides, according to the preparation method, high-valued resource utilization of the phosphorus-potassium mixed acid raffinate is achieved, and through ammoniation pretreatment of returned materials and systematic recovery of reaction waste heat, the preparation method has the advantages that the raw material cost is reduced, the product performance is improved, and the production energy consumption is reduced.

Description

Technical Field

[0001] This application relates to the field of fertilizer production and industrial solid waste resource utilization technology, and more specifically, it relates to a method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue. Background Technology

[0002] With the advancement of policies promoting the resource utilization of industrial solid waste and the upgrading of agricultural demand for high-efficiency functional fertilizers, the phosphorus and potassium mixed acid extraction residue, as a byproduct of phosphorus chemical production, has become a high-quality, low-cost raw material for the production of compound fertilizers due to its rich phosphorus and potassium nutrients. Furthermore, the slow-release nitrogen properties of urea-formaldehyde combined with the growth-promoting and stress-resistance functions of polyglutamic acid have become a core technological pathway to improve fertilizer nutrient utilization and enhance field application effects. However, the phosphorus and potassium mixed acid extraction residue itself has a complex composition, containing various impurities and exhibiting high natural acidity with a wide fluctuation range, posing a severe challenge to the stability control of subsequent processes.

[0003] In existing technologies, the neutralization reaction in the production of this type of compound fertilizer is often completed in a single step, which makes it difficult to accurately offset the acidity fluctuations of the raffinate. This results in a persistently unstable pH value in the neutralization system, often leading to localized over-acidity or severe acid-base fluctuations. Furthermore, the subsequent spray granulation process involves a high-temperature environment. Polyglutamic acid, as a bioactive substance, is highly susceptible to structural degradation and destruction of active groups under the combined effects of unstable acid-base conditions and high temperatures, significantly weakening its core functions of promoting growth and resisting stress. This problem not only wastes functional components but also prevents the final compound fertilizer product from fully achieving the dual expectations of slow release and synergistic effects, impacting the deep utilization of phosphorus and potassium mixed acid raffinate resources and failing to fully meet the actual needs of agricultural production for highly active functional fertilizers. Summary of the Invention

[0004] To address the problem of activity loss due to acid-base fluctuations and high temperatures in the production of polyglutamic acid in the prior art, this application provides a method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the residual liquid of phosphorus-potassium mixed acid extraction.

[0005] A method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue includes the following steps: S1. Raw material reaction and preparation of mixed acid: After reacting sulfuric acid with potassium chloride, potassium hydrogen slurry is separated and mixed with phosphoric acid to obtain potassium phosphoric acid mixed acid slurry; S2. Neutralization and mixing with functional agents: The phosphorus-potassium mixed acid slurry is subjected to a multi-stage neutralization reaction, and polyglutamic acid solution is added during the neutralization process to obtain a composite slurry; S3. Slow-release nitrogen source mixing: Urea-formaldehyde is added to the composite slurry and mixed to form a spray slurry; S4. Pretreatment and granulation of returned material: Ammonia pretreatment is performed on a portion of the screened fine powder returned material to obtain pretreated returned material; the spray slurry and the pretreated returned material are sprayed together for granulation and drying to form wet fertilizer granules. S5. Screening and coating: After drying the wet fertilizer granules, they are screened, and qualified granules are coated with a spray coating to obtain compound fertilizer products. S6. Process waste heat recovery: The chemical reaction heat and / or absorption heat generated during the manufacturing process are recovered and utilized.

[0006] By adopting the above technical solution, this production method focuses on the resource utilization of phosphorus and potassium mixed acid extraction residue, constructing a multi-step synergistic production system. Sulfuric acid and potassium chloride undergo a double decomposition reaction to generate potassium hydrogen slurry, and hydrogen chloride gas is separated and recovered. This slurry is then mixed with phosphoric acid to form a nutrient-balanced phosphorus and potassium mixed acid slurry through precise phosphorus and potassium element ratio, laying the nutrient foundation for compound fertilizer. Addressing the complex composition and large acidity fluctuations of the phosphorus and potassium mixed acid extraction residue, a multi-stage neutralization reaction mode is adopted. First, the system acidity is initially controlled. Then, after precisely adjusting the pH to a suitable range, a γ-type polyglutamic acid solution is added. The stable acid-base environment reduces the damage to its active groups, enhancing its binding capacity with phosphorus and potassium nutrients. Subsequently, urea-formaldehyde is added and stirred for dispersion. Its slow-release properties synergize with the chelating function of polyglutamic acid, delaying nutrient release. A portion of the screened fine powder is pretreated with ammoniation to form a slightly alkaline surface layer, which is mixed with the spray slurry to promote agglomeration and simultaneously buffer the system acidity to protect functional components. After granulation, the wet granules are dried and screened, and then coated with a composite coating liquid to form a protective film. The heat of chemical reaction and heat of absorption during the production process are recovered and used to preheat the combustion air or pre-concentrate the slurry, realizing energy recycling and constructing a production process that takes into account resource utilization, functional stability and energy efficiency.

[0007] Preferably, in step S1, the reaction is a metathesis reaction carried out in a conversion reaction tank at a temperature of 80-120°C; after the reaction, the mixture is subjected to gas-liquid separation, the gas is treated by a hydrochloric acid absorption system to obtain by-product hydrochloric acid, and the liquid is a potassium hydrogen slurry; the effective potassium content in the potassium hydrogen slurry, calculated as potassium oxide, is 15-25%.

[0008] By adopting the above technical solution, the conversion reactor provides a closed and stable environment for the metathesis reaction of sulfuric acid and potassium chloride. The two react in the range of 80-120℃, ensuring that potassium chloride is fully converted into usable potassium salt, while avoiding impurities generated by high-temperature side reactions. The reaction mixture is subjected to gas-liquid separation, and hydrogen chloride gas is sprayed into the hydrochloric acid absorption system for recycling as a by-product hydrochloric acid, realizing the resource utilization of volatile components. The separated potassium hydrogen slurry is used to stabilize the effective potassium content (calculated as potassium oxide) at 15-25% by adjusting the feed ratio and reaction residence time, providing a stable potassium source for subsequent mixing and blending with phosphoric acid.

[0009] Preferably, in step S2, the multi-stage neutralization reaction is carried out in a neutralization reaction system, including a first neutralization zone and a second neutralization zone; the pH value of the material is first adjusted to 3.5 to 4.5 in the first neutralization zone, and then the pH value of the material is adjusted to 5.0 to 6.5 in the second neutralization zone; and a polyglutamic acid solution is added in the second neutralization zone.

[0010] By adopting the above technical solution, the neutralization reaction system employs a zoned design to achieve precise pH step-by-step control. In the first neutralization zone, gaseous ammonia is introduced to initially adjust the slurry pH to 3.5 to 4.5, rapidly neutralizing most of the free acid, reducing fluctuations in subsequent adjustments, and preventing acid-base imbalances caused by impurities. After the material enters the second neutralization zone, gaseous ammonia continues to be introduced to precisely control the pH to 5.0 to 6.5, providing a mild and stable environment for polyglutamic acid and reducing the damage to its molecular structure caused by drastic acid-base changes. After the pH stabilizes in the second neutralization zone, a polyglutamic acid solution is added. The stable system formed by the step-by-step neutralization ensures the integrity of its active groups and guarantees sufficient binding with phosphorus and potassium nutrients.

[0011] Preferably, in step S2, the material temperature in the second neutralization zone is maintained at 60-85°C; the amount of polyglutamic acid solution added is such that the mass percentage of polyglutamic acid in the final product is 0.2-0.5%; the polyglutamic acid is γ-type polyglutamic acid with a molecular weight of 100-300 kDa.

[0012] By adopting the above technical solution, the temperature in the second neutralization zone is stabilized at 60-85℃ using a temperature control device. This avoids the degradation of polyglutamic acid due to high temperatures, improves its mixing uniformity with the slurry, and promotes its interaction with phosphorus and potassium nutrients. The amount of polyglutamic acid solution added is strictly controlled to maintain its mass percentage in the final product at 0.2-0.5%, ensuring sufficient chelation with phosphorus and potassium nutrients while avoiding waste of functional components and increased costs. γ-type polyglutamic acid is selected, utilizing its carboxyl-rich molecular chain structure to enhance the chelation stability with phosphorus and potassium ions. The molecular weight is limited to 100-300 kDa to balance its dispersibility and structural stability in the slurry, ensuring its continued core role in nutrient chelation.

[0013] Preferably, in step S3, the amount of urea-formaldehyde added, calculated as nitrogen element, accounts for 30-60% of the total nitrogen mass of the spray slurry; the mixing is carried out under the conditions of a stirring speed of 30-60 r / min and a mixing time of 10-30 min.

[0014] By adopting the above technical solution, urea-formaldehyde, as a slow-release nitrogen source, is added at 30-60% of the total nitrogen mass of the spray slurry, based on elemental nitrogen. This forms a synergistic system of rapid and long-lasting phosphorus and potassium nutrients chelated with polyglutamic acid, while avoiding nutrient release problems caused by excessively high or low urea-formaldehyde proportions. The mixing process uses a stirring speed of 30-60 rpm to ensure thorough contact and dispersion of urea-formaldehyde with the composite slurry, preventing localized aggregation that could affect subsequent granulation and nutrient release uniformity. It also prevents high-speed stirring from damaging the polyglutamic acid structure and chelation system. A mixing time of 10-30 minutes is set to ensure complete integration of urea-formaldehyde into the slurry, forming a stable spray matrix and providing structural support for the balanced and slow-release of nutrients from the particles.

[0015] Preferably, in step S4, the amount of fine powder returned during partial screening accounts for 20% to 50% of the total returned material mass; the ammoniation pretreatment is carried out by contact treatment using ammonia-containing tail gas or diluted gaseous ammonia from the neutralization reaction system, with a contact time of 60-90 seconds; and the inlet temperature of the hot air used for granulation is 180-250°C.

[0016] By adopting the above technical solution, the amount of fine powder returned from the ammoniation pretreatment is controlled to 20% to 50% of the total returned material mass. This provides sufficient core materials with agglomeration activity for spray granulation, ensuring rapid adhesion and agglomeration of slurry droplets, while avoiding slurry viscosity problems caused by abnormal proportions of pretreatment returned material. Ammoniation pretreatment prioritizes the use of ammonia-containing tail gas from the neutralization reaction; if insufficient, diluted ammonia gas is used as a supplement. A contact time of 60 to 90 seconds ensures that the returned material surface fully adsorbs ammonia components to form a stable, slightly alkaline surface layer, preventing excessive ammoniation from causing subsequent abrupt pH changes. The granulation hot air inlet temperature is set at 180-250℃ to rapidly evaporate slurry moisture, ensuring particle drying and shaping, and improving strength. Simultaneously, temperature gradient control prevents localized high temperatures from damaging the functional component structure, balancing drying efficiency and functional stability.

[0017] Preferably, the pretreated return material formed by the ammoniation pretreatment has a surface pH value between 7.5 and 9.0.

[0018] By adopting the above technical solution, the pH value of the returned material surface after ammoniation pretreatment is precisely controlled between 7.5 and 9.0, forming a mild acid-base compatible system with the subsequent acidic spray slurry. By adjusting the ammonia source flow rate and contact time, a stable alkaline adsorption layer is formed on the returned material surface, avoiding sudden local pH changes. This pH range promotes the wetting, adhesion, and coagulation of slurry droplets on the returned material surface, while simultaneously creating a mild acid-base buffer environment inside the particles, protecting the functional stability of urea-formaldehyde and polyglutamic acid, and ensuring the synergistic effect of particle forming quality and nutrient function.

[0019] Preferably, in step S5, the conditions for spray coating are: particle temperature inside the coating cylinder 45-65℃, relative humidity 25-45%; the coating liquid contains polyglutamic acid, an anti-caking agent and a hydrophilic natural colloid, wherein the hydrophilic natural colloid is selected from at least one of xanthan gum, guar gum and sodium alginate.

[0020] By adopting the above technical solution, the environmental parameters and coating solution components inside the coating cylinder are precisely controlled during the spray coating process. The particle temperature is controlled at 45-65℃ to promote rapid film formation of the coating solution while preventing high temperatures from damaging the internal functional components. The relative humidity is controlled at 25-45% to ensure adequate solvent evaporation and prevent coating layer cracking and particle adhesion. The coating solution uses a multi-component formulation: polyglutamic acid enhances nutrient chelation, forming a synergistic effect with the inner functional components; an anti-caking agent reduces particle hygroscopicity and adhesion; and a hydrophilic natural colloid, selected from at least one of xanthan gum, guar gum, and sodium alginate, utilizes its film-forming and dispersing properties to form a dense protective film, improving the particle's dispersion and solubility in water. This multi-component synergistic construction creates a coating layer with protective, slow-release, and solubilizing functions.

[0021] Preferably, in step S6, the specific method of recycling is as follows: the recovered heat is used through a heat exchange device to preheat the combustion air required for granulation in step S4, or to pre-concentrate the phosphorus-potassium mixed acid slurry in step S1.

[0022] By adopting the above technical solution, the heat of neutralization reaction and the heat absorbed by hydrochloric acid generated during the production process are recovered and efficiently utilized through a heat exchange device, employing two suitable pathways. First, the recovered heat is used to preheat the combustion air for granulation, increasing the initial air temperature, enhancing fuel combustion efficiency, reducing consumption, and stabilizing the hot air temperature to prevent energy supply fluctuations from affecting the granulation process. Second, the recovered heat is used for the pre-concentration of the phosphorus-potassium mixed acid slurry, gently removing some free moisture through heating, reducing the subsequent moisture evaporation load during granulation, and maintaining a stable nutrient concentration in the slurry to ensure uniformity of subsequent reactions and mixing. Both methods reduce energy consumption through energy recycling, achieving efficient energy allocation.

[0023] Preferably, the neutralization reaction system is a first neutralization tank and a second neutralization tank arranged in series, or a single-unit multi-stage neutralization tank with internal partitions and partitioned stirring devices; the return material pretreatment device is a fluidized bed pretreatment chamber or a rotary ammoniation cylinder; the production scale of the method is 90,000-110,000 tons of compound fertilizer per year.

[0024] By adopting the above technical solutions, the neutralization reaction system employs two structures adapted to different production scenarios: a first and second neutralization tank connected in series, each independently equipped with temperature control, ammonia gas introduction, and stirring devices, allowing for precise control of parameters in each area, suitable for large-scale continuous production; and a single-unit multi-stage neutralization tank with internal partitions and zoned stirring devices, dividing independent neutralization zones through partitions to avoid material mixing interference, simplify equipment space, and suitable for small- to medium-scale production. The return material pretreatment device offers two options: a fluidized bed pretreatment chamber uses airflow disturbance to fluidize the return material, improving ammoniation uniformity and efficiency; and a rotary ammoniation cylinder uses cylinder rotation to tumble the return material, adapting to different particle size return material requirements, both ensuring stable pH levels on the return material surface. The production scale is set at 90,000-110,000 tons per year, matching the processing capacity of each key unit. Large-scale production reduces unit energy consumption and costs while avoiding excessively large scale that increases process control difficulty, ensuring stable product quality.

[0025] In summary, this application has the following beneficial effects: 1. The method of this application provides a relatively mild and stable mixing environment for polyglutamic acid by setting up a multi-stage neutralization reaction system including a first neutralization zone and a second neutralization zone with a specific pH range, and by adding a polyglutamic acid solution of a specific type and molecular weight range in the second neutralization zone. This process design helps to reduce the risk of structural degradation or loss of activity in subsequent strongly acidic or high-temperature processing steps, thereby better maintaining its functional properties as a biostimulant and providing a foundation for the final product to exert potential effects such as growth promotion and stress resistance.

[0026] 2. The method of this application realizes the comprehensive utilization of resources from multiple materials in the production process. Specifically, the method recovers hydrochloric acid, a byproduct of the reaction between sulfuric acid and potassium chloride, uses the resulting potassium hydrogen slurry as a carrier for phosphorus and potassium nutrients, and utilizes the ammonia-containing tail gas generated from the neutralization reaction to pretreat the returned material with ammoniation. This design transforms materials that might otherwise be treated as waste or exhaust gas into valuable raw materials or intermediate products, providing effective nutrients for fertilizers while reducing dependence on external raw materials and emissions during the production process, aligning with the concepts of clean production and circular economy.

[0027] 3. The method of this application modifies the surface properties of some returned materials through ammoniation pretreatment, and combines this with the subsequent coating process to optimize the physical properties of compound fertilizer granules. The slightly alkaline layer formed on the surface of the returned materials by ammoniation pretreatment may interact with the acidic spray slurry, affecting the internal structure of the granules. Simultaneously, spraying a coating liquid containing hydrophilic colloids under controlled temperature and humidity conditions can form a functional film on the granule surface. These processes work together to improve the mechanical strength of the finished granules and reduce the tendency for granules to clump together during storage and transportation due to moisture absorption or compression. Attached Figure Description

[0028] Figure 1 This is a flowchart of the method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the residual extract of phosphorus and potassium mixed acid provided in this application; Figure 2 This is a process flow diagram of the spraying workshop for producing urea-formaldehyde polyglutamic acid balanced compound fertilizer from the residual extract of phosphorus and potassium mixed acid provided in this application. Detailed Implementation

[0029] The present application will be further described in detail below with reference to embodiments and comparative examples. Unless otherwise specified, the experimental methods used below are conventional methods. Unless otherwise specified, the materials, reagents, methods and instruments used are all conventional materials, reagents, methods and instruments in the art, which can be obtained by those skilled in the art through commercial channels or prepared according to literature methods.

[0030] Technical concept: In the related technologies for the resource utilization of phosphorus and potassium mixed acid extraction residues to produce urea-formaldehyde polyglutamic acid compound fertilizer, the core problem is that polyglutamic acid is prone to loss of activity during the production process, making it difficult to fully exert its biostimulant function. The reasons for this are twofold: firstly, the phosphorus and potassium mixed acid extraction residues have complex compositions and large acidity fluctuations; existing technologies mostly employ a single-step neutralization mode, which cannot achieve precise and stable pH control, easily leading to drastic local acid-base fluctuations; secondly, subsequent processing steps such as spray granulation involve high-temperature environments. These dual harsh conditions cause the degradation of the polyglutamic acid molecular structure and the destruction of active groups, ultimately affecting the overall synergistic effect of the compound fertilizer.

[0031] This technical solution addresses the aforementioned issues by constructing a stable production system through multi-dimensional synergistic techniques: It employs a multi-stage neutralization reaction system comprising a first and second neutralization zone, precisely controlling the pH value in a stepwise manner. A mild environment suitable for polyglutamic acid is created in the second neutralization zone, where γ-type polyglutamic acid with a specific configuration and molecular weight is added. Ammonia pretreatment of recycled materials forms a slightly alkaline surface layer, buffering the system's acidity when mixed with acidic spray slurry. Temperature and humidity parameters are precisely controlled during granulation and coating processes to prevent high-temperature damage. Simultaneously, a composite coating layer further protects the functional components. These techniques, through synergistic effects of acid-base environment control, temperature control, system buffering, and surface protection, effectively reduce the loss of activity during polyglutamic acid production, ensuring its full functional performance.

[0032] Preparation example: Preparation of γ-type polyglutamic acid solution A strain of *Corynebacterium glutamicum* was inoculated into a seed culture medium containing 20 g / L glucose, 15 g / L glutamic acid, 5 g / L yeast extract, 2 g / L potassium dihydrogen phosphate, and 0.5 g / L magnesium sulfate. Activation was achieved by shaking culture at 30°C and pH 7.0 for 12 hours. The activated seed culture was then inoculated into a fermentation medium at a 10% inoculation rate. The fermentation medium composition was identical to the seed culture medium. Fermentation was carried out at 32°C and pH 6.8-7.2 with aeration and stirring for 48 hours. During fermentation, pH was maintained by adding ammonia dropwise. After fermentation, 2 mol of [acid / formula] was added to the fermentation broth. Adjust the pH to 3.0 with / L hydrochloric acid, let stand for 2 hours, then centrifuge to remove bacterial precipitate. Take the supernatant and concentrate it through an ultrafiltration membrane with a molecular weight cutoff of 100kDa to remove small molecule impurities. Then, add sterile water to the concentrate to dilute and adjust the polyglutamic acid concentration to 50g / L, thus obtaining a γ-type polyglutamic acid solution. Testing shows that the molecular weight of γ-type polyglutamic acid in this solution is 100-300kDa, with a purity ≥95%. It can be directly used in step S2 for mixing with the composite slurry. The addition amount should be precisely controlled according to the required polyglutamic acid mass percentage of 0.2-0.5% in the final product. Example 1 This embodiment provides a method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue, including the following steps: S1. Raw material reaction and mixed acid preparation: Sulfuric acid and potassium chloride are fed into a conversion reaction tank for metathesis reaction to obtain a mixture containing hydrogen chloride gas; the mixture is subjected to gas-liquid separation, the gas part is treated by hydrochloric acid absorption system to obtain by-product hydrochloric acid, and the liquid part is potassium hydrogen slurry; the potassium hydrogen slurry and phosphoric acid are transported to a mixed acid tank for mixing to obtain potassium phosphorus mixed acid slurry.

[0033] The reaction is carried out in a conversion reaction tank at a temperature of 100°C. After the reaction, the mixture is separated into gas and liquid. The gas is treated by a hydrochloric acid absorption system to obtain hydrochloric acid as a byproduct, and the liquid is a potassium hydrogen slurry. The effective potassium content in the potassium hydrogen slurry, calculated as potassium oxide, is 20%.

[0034] S2. Neutralization and mixing of functional agents: The phosphorus-potassium mixed acid slurry is transported to the neutralization reaction system and gaseous ammonia is introduced into it for neutralization reaction; the neutralization reaction system is set to a multi-stage reaction mode, including a first neutralization zone and a second neutralization zone; the pH value of the material is first adjusted to 4.0 in the first neutralization zone, and then the pH value of the material is adjusted to 5.75 in the second neutralization zone. Polyglutamic acid solution is added in the second neutralization zone and mixed to obtain a composite slurry containing polyglutamic acid.

[0035] The material temperature in the second neutralization zone is maintained at 72.5℃; the amount of polyglutamic acid solution added is such that the mass percentage of polyglutamic acid in the final product is 0.35%; the polyglutamic acid is γ-type polyglutamic acid with a molecular weight of 200kDa.

[0036] S3. Slow-release nitrogen source mixing: Add urea-formaldehyde to the composite slurry containing polyglutamic acid and mix to form a spray slurry.

[0037] The amount of urea-formaldehyde added, calculated as nitrogen element, accounts for 45% of the total nitrogen mass of the spray slurry; the mixing is carried out under the conditions of a stirring speed of 45 r / min and a mixing time of 20 min.

[0038] S4. Pretreatment and Granulation of Returned Material: Part of the screened fine powder returned material is subjected to ammoniation pretreatment to obtain pretreated returned material; the spray slurry and the pretreated returned material are sprayed together for granulation and drying to form fertilizer wet granules.

[0039] Of these, the amount of fine powder returned from screening accounts for 35% of the total returned material mass; the ammoniation pretreatment is carried out by contact treatment using ammonia-containing tail gas or diluted gaseous ammonia from the neutralization reaction system, with a contact time of 75s; the inlet temperature of the hot air used for granulation is 215℃.

[0040] Among them, the pretreated return material formed by ammoniation pretreatment has a surface pH value of 8.25.

[0041] S5. Screening and Coating: After drying the wet fertilizer granules, they are screened, and qualified granules are coated with a coating to obtain compound fertilizer products.

[0042] The conditions for spray coating are: particle temperature inside the coating cylinder is 55℃, and relative humidity is 35%; the coating solution contains polyglutamic acid, anti-caking agent and xanthan gum.

[0043] S6. Process waste heat recovery: The chemical reaction heat and absorption heat generated during the manufacturing process are recovered and utilized.

[0044] The specific method of recycling is as follows: the recovered heat is used through a heat exchange device to preheat the combustion air required for granulation in step S4.

[0045] In this embodiment, the neutralization reaction system consists of a first neutralization tank and a second neutralization tank connected in series; the return material pretreatment device is a fluidized bed pretreatment chamber; and the production scale of the method is 100,000 tons of compound fertilizer per year.

[0046] Example 2 This embodiment provides a method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue, including the following steps: S1. Raw material reaction and mixed acid preparation: Sulfuric acid and potassium chloride are fed into a conversion reaction tank for metathesis reaction to obtain a mixture containing hydrogen chloride gas; the mixture is subjected to gas-liquid separation, the gas part is treated by hydrochloric acid absorption system to obtain by-product hydrochloric acid, and the liquid part is potassium hydrogen slurry; the potassium hydrogen slurry and phosphoric acid are transported to a mixed acid tank for mixing to obtain potassium phosphorus mixed acid slurry.

[0047] The reaction is carried out in a conversion reaction tank at a temperature of 80°C. After the reaction, the mixture is separated into gas and liquid. The gas is treated by a hydrochloric acid absorption system to obtain hydrochloric acid as a byproduct, and the liquid is a potassium hydrogen slurry. The effective potassium content in the potassium hydrogen slurry, calculated as potassium oxide, is 15%.

[0048] S2. Neutralization and mixing of functional agents: The phosphorus-potassium mixed acid slurry is transported to the neutralization reaction system and gaseous ammonia is introduced into it for neutralization reaction; the neutralization reaction system is set to a multi-stage reaction mode, including a first neutralization zone and a second neutralization zone; the pH value of the material is first adjusted to 3.5 in the first neutralization zone, and then the pH value of the material is adjusted to 5.0 in the second neutralization zone. Polyglutamic acid solution is added in the second neutralization zone and mixed to obtain a composite slurry containing polyglutamic acid.

[0049] The material temperature in the second neutralization zone is maintained at 60℃; the amount of polyglutamic acid solution added is such that the mass percentage of polyglutamic acid in the final product is 0.2%; the polyglutamic acid is γ-type polyglutamic acid with a molecular weight of 100kDa.

[0050] S3. Slow-release nitrogen source mixing: Add urea-formaldehyde to the composite slurry containing polyglutamic acid and mix to form a spray slurry.

[0051] The amount of urea-formaldehyde added, calculated as nitrogen element, accounts for 30% of the total nitrogen mass of the spray slurry; the mixing is carried out under the conditions of a stirring speed of 30 r / min and a mixing time of 10 min.

[0052] S4. Pretreatment and Granulation of Returned Material: Part of the screened fine powder returned material is subjected to ammoniation pretreatment to obtain pretreated returned material; the spray slurry and the pretreated returned material are sprayed together for granulation and drying to form fertilizer wet granules.

[0053] Of these, the amount of fine powder returned from screening accounts for 20% of the total returned material mass; the ammoniation pretreatment is carried out by contact treatment using ammonia-containing tail gas or diluted gaseous ammonia from the neutralization reaction system, with a contact time of 60s; the inlet temperature of the hot air used for granulation is 180℃.

[0054] Among them, the pretreated return material formed by ammoniation pretreatment has a surface pH value of 7.5.

[0055] S5. Screening and Coating: After drying the wet fertilizer granules, they are screened, and qualified granules are coated with a coating to obtain compound fertilizer products.

[0056] The conditions for spray coating are: particle temperature inside the coating cylinder is 45℃, and relative humidity is 25%; the coating solution contains polyglutamic acid, anti-caking agent and guar gum.

[0057] S6. Process waste heat recovery: The chemical reaction heat generated during the manufacturing process is recovered and utilized.

[0058] The specific method of recycling is as follows: the recovered heat is used to pre-concentrate the phosphorus-potassium mixed acid slurry in step S1 through a heat exchange device.

[0059] In this embodiment, the neutralization reaction system is a single-unit multi-stage neutralization tank with internal partitions and zoned stirring devices; the return material pretreatment device is a rotary ammoniation cylinder; and the production scale of the method is 90,000 tons of compound fertilizer per year.

[0060] Example 3 This embodiment provides a method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue, including the following steps: S1. Raw material reaction and mixed acid preparation: Sulfuric acid and potassium chloride are fed into a conversion reaction tank for metathesis reaction to obtain a mixture containing hydrogen chloride gas; the mixture is subjected to gas-liquid separation, the gas part is treated by hydrochloric acid absorption system to obtain by-product hydrochloric acid, and the liquid part is potassium hydrogen slurry; the potassium hydrogen slurry and phosphoric acid are transported to a mixed acid tank for mixing to obtain potassium phosphorus mixed acid slurry.

[0061] The reaction is carried out in a conversion reaction tank at a temperature of 120°C. After the reaction, the mixture is separated into gas and liquid. The gas is treated by a hydrochloric acid absorption system to obtain hydrochloric acid as a byproduct, and the liquid is a potassium hydrogen slurry. The effective potassium content in the potassium hydrogen slurry, calculated as potassium oxide, is 25%.

[0062] S2. Neutralization and mixing of functional agents: The phosphorus-potassium mixed acid slurry is transported to the neutralization reaction system and gaseous ammonia is introduced into it for neutralization reaction; the neutralization reaction system is set to a multi-stage reaction mode, including a first neutralization zone and a second neutralization zone; the pH value of the material is first adjusted to 4.5 in the first neutralization zone, and then the pH value of the material is adjusted to 6.5 in the second neutralization zone. Polyglutamic acid solution is added in the second neutralization zone and mixed to obtain a composite slurry containing polyglutamic acid.

[0063] The material temperature in the second neutralization zone is maintained at 85℃; the amount of polyglutamic acid solution added is such that the mass percentage of polyglutamic acid in the final product is 0.5%; the polyglutamic acid is γ-type polyglutamic acid with a molecular weight of 300kDa.

[0064] S3. Slow-release nitrogen source mixing: Add urea-formaldehyde to the composite slurry containing polyglutamic acid and mix to form a spray slurry.

[0065] The amount of urea-formaldehyde added, calculated as nitrogen element, accounts for 60% of the total nitrogen mass of the spray slurry; the mixing is carried out under the conditions of a stirring speed of 60 r / min and a mixing time of 30 min.

[0066] S4. Pretreatment and Granulation of Returned Material: Part of the screened fine powder returned material is subjected to ammoniation pretreatment to obtain pretreated returned material; the spray slurry and the pretreated returned material are sprayed together for granulation and drying to form fertilizer wet granules.

[0067] Of these, the amount of fine powder returned from screening accounts for 50% of the total returned material mass; the ammoniation pretreatment is carried out by contact treatment using ammonia-containing tail gas or diluted gaseous ammonia from the neutralization reaction system, with a contact time of 90s; the inlet temperature of the hot air used for granulation is 250℃.

[0068] Among them, the pretreated return material formed by ammoniation pretreatment has a surface pH value of 9.0.

[0069] S5. Screening and Coating: After drying the wet fertilizer granules, they are screened, and qualified granules are coated with a coating to obtain compound fertilizer products.

[0070] The conditions for spray coating are: particle temperature inside the coating cylinder is 65℃, and relative humidity is 45%; the coating solution contains polyglutamic acid, anti-caking agent and sodium alginate.

[0071] S6. Process waste heat recovery: The chemical reaction heat and absorption heat generated during the manufacturing process are recovered and utilized.

[0072] The specific method of recycling is as follows: the recovered heat is used through a heat exchange device to preheat the combustion air required for granulation in step S4.

[0073] In this embodiment, the neutralization reaction system consists of a first neutralization tank and a second neutralization tank connected in series; the return material pretreatment device is a fluidized bed pretreatment chamber; and the production scale of the method is 110,000 tons of compound fertilizer per year.

[0074] Comparative Example 1 The only difference between this comparative example and Example 1 is that the multi-stage neutralization reaction is omitted in step S2. Ammonia gas is introduced into a single neutralization tank to neutralize and adjust the pH of the material to approximately 6.0 in one step. Then, a polyglutamic acid solution is added to this single neutralization tank, and the subsequent steps are exactly the same as in Example 1.

[0075] Comparative Example 2 The only difference between this comparative example and Example 1 is that in step S4, the ammoniation pretreatment of the returned material is omitted. A portion of the sieved fine powder returned material is introduced directly into the granulator along with the sprayed slurry for granulation without any treatment. Subsequent steps are exactly the same as in Example 1.

[0076] Comparative Example 3 The only difference between this comparative example and Example 1 is that, in step S6, the heat from steps S2 and S1 is not specifically recovered and utilized. The heat from the neutralization reaction and the heat absorbed by the hydrochloric acid are directly dissipated through a conventional cooling system and are not used to preheat the combustion air or pre-concentrate the slurry.

[0077] Comparative Example 4 The only difference between this comparative example and Example 1 is that, in step S2, the polyglutamic acid used is α-polyglutamic acid with a molecular weight of 50 kDa. The amount added is the same as in Example 1, so that the mass percentage of polyglutamic acid in the final product is 0.35%.

[0078] Comparative Example 5 The only difference between this comparative example and Example 1 is that in step S1, the conversion reaction between sulfuric acid and potassium chloride is not carried out, and the self-produced potassium hydrogen slurry is not used. Instead, commercially available industrial-grade monoammonium phosphate and potassium sulfate are used directly as phosphorus and potassium sources, and mixed with phosphoric acid and water to prepare a mixed slurry with the same nutrient content as in Example 1. Then, the process proceeds to step S2, and the subsequent steps are exactly the same as in Example 1.

[0079] I. Fertilizer Nutrient Slow-Release Performance Test According to the constant temperature static water release method in GB / T23348-2020 "Slow-Release Fertilizers", 5.0 g of each of the compound fertilizer samples from Examples 1 to 3 and Comparative Examples 1 to 5 were weighed, uniformly wrapped with a breathable membrane, and placed in 500 mL Erlenmeyer flasks. 300 mL of deionized water was added to each Erlenmeyer flask, and then the flasks were placed in a 25°C constant temperature water bath and continuously shaken at 100 r / min. The supernatant from each Erlenmeyer flask was collected on days 1, 3, 7, 14, 28, 60, and 90. The nitrogen content in the supernatant was determined by the Kjeldahl method, the phosphorus content by the molybdenum-antimony colorimetric method, and the potassium content by the flame photometry method. After each sampling, an equal amount of deionized water was immediately added to the Erlenmeyer flask to maintain a constant liquid-solid ratio. The cumulative release rate of nitrogen, phosphorus, and potassium nutrients was calculated based on the measured nutrient content at each time point. Nutrient cumulative release curves for each sample were plotted, and the time required for different samples to reach 80% cumulative nutrient release was compared to evaluate the nutrient slow-release performance of each compound fertilizer sample.

[0080] II. Fertilizer utilization rate and crop growth promotion effect test According to NY / T1115-2020 "Technical Specifications for Field Trials to Identify Fertilizer Effects", a pot experiment was conducted. The test crop was Zhengdan 958 maize, and the test soil was neutral loam with an organic matter content of 1.5%, available nitrogen of 80 mg / kg, available phosphorus of 20 mg / kg, and available potassium of 100 mg / kg. Each compound fertilizer sample was divided into three replicates, with each replicate using a 5L pot containing 5 kg of soil. The compound fertilizers from Examples 1 to 3 and Comparative Examples 1 to 5 were applied precisely according to the dosages of pure nitrogen of 150 mg / kg, pure phosphorus of 60 mg / kg, and pure potassium of 120 mg / kg. A blank control group without any fertilizer was also included. After pre-germination treatment, uniformly growing seedlings were transplanted into individual pots, with two seedlings per pot. All pots were then placed in a light incubator with the following conditions: 16 hours of light per day, day and night temperatures of 25℃ and 18℃ respectively, and a relative humidity of 60%. Regular watering was maintained during cultivation to keep the soil moisture content at 70% of field capacity. After 60 days of cultivation, all plants were harvested, and the plant height, fresh weight, and dry weight of each corn plant were measured. The cumulative uptake of nitrogen, phosphorus, and potassium in the plants was determined using the Kjeldahl method, the molybdenum-antimony colorimetric method, and flame photometry, respectively. The fertilizer nutrient utilization rate (the ratio of nutrient absorbed by the plant to the applied fertilizer nutrient) was calculated for each sample. Based on the combined plant growth indicators and fertilizer nutrient utilization rate, the crop growth-promoting effect and fertilizer utilization efficiency of different compound fertilizer samples were evaluated.

[0081] III. Physical Stability Test of Fertilizer Particles According to GB / T15063-2020 "Compound Fertilizers", the granular compressive strength and anti-caking properties of the compound fertilizer samples from Examples 1 to 3 and Comparative Examples 1 to 5 were tested. Granular compressive strength test: 50 intact granules (1.0-4.0 mm) were randomly selected from each sample. Using a granular strength tester, pressure was applied uniformly along the diameter of each granule, and the maximum pressure value at the moment of granule breakage was recorded. The average pressure value of the 50 granules was taken as the granular compressive strength of the sample. Anti-caking property test: 200 g of each sample was weighed and evenly placed in a cylindrical container with a diameter of 10 cm and a height of 15 cm. A constant pressure of 0.02 MPa was applied to the top of the container, and then the container was placed in a constant temperature and humidity chamber at 30℃ and 80% relative humidity for 30 days. After the period, the container was removed, and the sample was sieved using a 2.0 mm standard sieve. The mass of the sample passing through the sieve was weighed, and the agglomeration rate was calculated, which is the ratio of the mass of the sample that did not pass through the sieve to the total sample mass. By combining granular compressive strength data and agglomeration rate results, the physical stability of each compound fertilizer sample is comprehensively evaluated, reflecting the impact of granulation process and coating treatment on product transportation and storage performance.

[0082] The results of the fertilizer nutrient slow-release performance test are shown in Table 1.

[0083] Table 1:

[0084] Note: 1. The nitrogen release period refers to the time required for the cumulative release rate to reach 80%.

[0085] 2. The release trends of phosphorus and potassium are basically consistent with those of nitrogen, so the release data of nitrogen, which is the most representative, will be presented first.

[0086] The results of the fertilizer utilization rate and crop growth promotion effect test are shown in Table 2.

[0087] Table 2:

[0088] The results of the physical stability test of fertilizer granules are shown in Table 3.

[0089] Table 3:

[0090] As can be seen from Examples 1-3 and Comparative Example 1, and from Tables 1 and 2, the use of a single neutralization step instead of a multi-stage neutralization reaction failed to optimize the environment for polyglutamic acid addition. Polyglutamic acid exhibits insufficient stability in a one-time pH adjustment environment, making its activity more susceptible to adverse effects during subsequent mixing with acidic slurry and granulation. This not only weakens its function as a biostimulant but also indirectly affects the formation of a stable, sustained-release structure of urea-formaldehyde within the composite particles, leading to an accelerated initial nutrient release rate and a decrease in the overall stability of the release curve.

[0091] As can be seen from Examples 1-3 and Comparative Example 2, and in conjunction with Tables 1-3, omitting the ammoniation pretreatment step of the returned material and directly using untreated fine powder returned material for granulation fundamentally affects the overall performance of the product. Due to the lack of a pre-constructed slightly alkaline interface on the surface of the returned material, the acidic components in the spray slurry cannot form the designed pH gradient environment when in contact with the returned material. This directly leads to difficulties in achieving the expected on-demand condensation and orderly encapsulation of urea-formaldehyde within the granules. As a result, the slow-release mechanism of the fertilizer granules is weakened, the nutrient release pattern is closer to that of fast-acting fertilizers, and the internal binding force and final structural density of the granules are also affected, resulting in a decline in the product's compressive strength and anti-caking ability.

[0092] Combining Examples 1-3 and Comparative Example 3 with Tables 1-3, it can be seen that not implementing targeted recovery and utilization of process waste heat does not change the final agronomic and physical properties of the compound fertilizer product. This comparative result clearly demonstrates that process waste heat recovery is an independent technical unit primarily responsible for energy consumption management in the production process. It is functionally decoupled from core process steps involving product composition and structure, such as multi-stage neutralization and return material pretreatment. Whether or not waste heat is recovered does not affect the retention of polyglutamic acid activity, the slow-release structure construction of urea-formaldehyde, or the granule forming and coating quality, confirming its specific energy-saving role in the solution.

[0093] As can be seen from Examples 1-3 and Comparative Example 4, and in conjunction with Tables 1 and 2, replacing γ-type polyglutamic acid with α-type polyglutamic acid and altering its molecular weight resulted in significant differences in the functional effects of the product. Polyglutamic acid with different spatial configurations and molecular weights exhibits variations in its solubility in the slurry system, its molecular chain extension state, and its tolerance to acidic and alkaline environments. These differences in physicochemical properties affect its activity retention rate under specific process conditions, thereby weakening its biostimulatory functions such as promoting nutrient absorption by crops and improving the rhizosphere environment, ultimately resulting in a decrease in fertilizer utilization efficiency and crop growth promotion effects.

[0094] As can be seen from Examples 1-3 and Comparative Example 5, and in conjunction with Tables 1-3, using commercially available pure monoammonium phosphate and potassium sulfate instead of the self-produced phosphorus and potassium mixed acid leaching residue as raw materials, while providing the same nitrogen, phosphorus, and potassium nutrients, alters the material basis of the production system. The chemical form, purity, and associated ion composition of the purchased raw materials differ from those of the self-produced leaching residue. These factors may affect the progress of subsequent neutralization reactions, the rheological properties of the slurry, and the aggregation and granulation behavior of the material during granulation. This change in material source has a cascading effect on the nutrient release behavior, particle mechanical strength, and physical stability of the final fertilizer product during storage.

[0095] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue, characterized in that: Includes the following steps: S1. Raw material reaction and preparation of mixed acid: After reacting sulfuric acid with potassium chloride, potassium hydrogen slurry is separated and mixed with phosphoric acid to obtain potassium phosphoric acid mixed acid slurry; S2. Neutralization and mixing with functional agents: The phosphorus-potassium mixed acid slurry is subjected to a multi-stage neutralization reaction, and polyglutamic acid solution is added during the neutralization process to obtain a composite slurry; S3. Slow-release nitrogen source mixing: Urea-formaldehyde is added to the composite slurry and mixed to form a spray slurry; S4. Pretreatment and granulation of returned material: Ammonia pretreatment is performed on a portion of the screened fine powder returned material to obtain pretreated returned material; the spray slurry and the pretreated returned material are sprayed together for granulation and drying to form wet fertilizer granules. S5. Screening and coating: After drying the wet fertilizer granules, they are screened, and qualified granules are coated with a spray coating to obtain compound fertilizer products. S6. Process waste heat recovery: The chemical reaction heat and / or absorption heat generated during the manufacturing process are recovered and utilized.

2. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S1, the reaction is a metathesis reaction carried out in a conversion reaction tank at a temperature of 80-120°C. After the reaction, the mixture is separated into gas and liquid. The gas is treated by a hydrochloric acid absorption system to obtain hydrochloric acid as a byproduct, and the liquid is a potassium hydrogen slurry. The effective potassium content in the potassium hydrogen slurry, calculated as potassium oxide, is 15-25%.

3. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S2, the multi-stage neutralization reaction is carried out in a neutralization reaction system, including a first neutralization zone and a second neutralization zone; firstly, the pH value of the material is adjusted to 3.5 to 4.5 in the first neutralization zone, and then the pH value of the material is adjusted to 5.0 to 6.5 in the second neutralization zone; and a polyglutamic acid solution is added in the second neutralization zone.

4. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 3, characterized in that: In step S2, the material temperature in the second neutralization zone is maintained at 60-85℃; the amount of polyglutamic acid solution added is such that the mass percentage of polyglutamic acid in the final product is 0.2-0.5%; the polyglutamic acid is γ-type polyglutamic acid with a molecular weight of 100-300kDa.

5. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S3, the amount of urea-formaldehyde added, calculated as nitrogen element, accounts for 30-60% of the total nitrogen mass of the spray slurry; the mixing is carried out under the conditions of a stirring speed of 30-60 r / min and a mixing time of 10-30 min.

6. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S4, the amount of fine powder returned during partial screening accounts for 20% to 50% of the total returned material mass; the ammoniation pretreatment is carried out by contact treatment using ammonia-containing tail gas or diluted gaseous ammonia from the neutralization reaction system, with a contact time of 60-90 seconds; the inlet temperature of the hot air used for granulation is 180-250℃.

7. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 6, characterized in that: The pretreated return material formed by the ammoniation pretreatment has a surface pH value between 7.5 and 9.

0.

8. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S5, the conditions for spray coating are: particle temperature inside the coating cylinder 45-65℃, relative humidity 25-45%; the coating liquid contains polyglutamic acid, anti-caking agent and hydrophilic natural colloid, wherein the hydrophilic natural colloid is selected from at least one of xanthan gum, guar gum and sodium alginate.

9. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: In step S6, the specific method of recycling is as follows: the recovered heat is used through a heat exchange device to preheat the combustion air required for granulation in step S4, or to pre-concentrate the phosphorus-potassium mixed acid slurry in step S1.

10. The method for producing urea-formaldehyde polyglutamic acid compound fertilizer from the phosphorus and potassium mixed acid extraction residue according to claim 1, characterized in that: The neutralization reaction system is a first neutralization tank and a second neutralization tank connected in series, or a single-unit multi-stage neutralization tank with internal partitions and zoned stirring devices; the return material pretreatment device is a fluidized bed pretreatment chamber or a rotary ammoniation cylinder; the production scale of the method is 90,000-110,000 tons of compound fertilizer per year.

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