A method for producing fully water-soluble agricultural polyphosphate ammonium

Abstract

The present application relates to a method for producing fully water-soluble agricultural polyphosphate, comprising mixing an additive with wet-process phosphoric acid purification by-product raffinate having a P2O5 content of 20-25%, to obtain a mixture, the additive comprising a chelating agent and / or a dispersing agent; concentrating the mixture at a low temperature of not more than 75 DEG C; mixing the concentrated raffinate with urea at a low temperature of not more than 55 DEG C, to form a urea phosphate homogeneous melt; aging the urea phosphate homogeneous melt at a constant temperature of 35-50 DEG C; degassing and pre-condensing the aged urea phosphate homogeneous melt at a temperature of not more than 78 DEG C under vacuum, to obtain a pasty urea phosphate homogeneous melt having a solid content of 62-68%; heating the pasty urea phosphate homogeneous melt under vacuum, the heating temperature being a gradient that first increases and then decreases; and cooling, to obtain a polyphosphate semi-finished product. Advantages: reducing the risk of fouling, protecting production equipment, improving product quality, and meeting the requirements of high water solubility, low biuret, and rich functional ingredients.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to a method for producing fully water-soluble agricultural ammonium polyphosphate. Background Technology

[0002] Agricultural ammonium polyphosphate (APP) is an excellent fully water-soluble fertilizer with advantages such as chelating micronutrients, slow-release phosphorus nutrients, and good compatibility with pesticides. Traditional production processes use thermal phosphoric acid or purified wet-process phosphoric acid as raw materials, resulting in high costs. The residual acid, a byproduct of wet-process phosphoric acid purification, is inexpensive but contains a large amount of metallic impurities (Fe). 3+ Al 3+ Ca 2+ Mg 2+ (etc.) and anionic impurities (F - SO4 2- These impurities can cause the following problems during the polymerization process: Equipment scaling and clogging: Metal ions react with phosphate ions, fluoride ions, etc., at high temperatures and through concentration and polymerization to form insoluble phosphate and fluoroaluminate scale, which clogs heat exchangers and pipes.

[0003] The product has poor water solubility: Unlocked metal impurities form insoluble phosphate crystals in the polymerization stage, resulting in a water solubility of only 70-85%, which cannot meet the requirements of drip irrigation and liquid fertilizer.

[0004] The product becomes sticky and difficult to crush: Metal impurities catalyze the side reaction of urea, generating sticky substances such as biuret, which causes the product to become sticky and jam the crusher.

[0005] Slurry foaming and overflow: Impurities trigger uncontrollable gas generation reactions, causing foaming and overflow of the material layer inside the dryer, forcing a reduction in output. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for producing fully water-soluble agricultural ammonium polyphosphate, which effectively overcomes the defects of the prior art.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A method for producing fully water-soluble agricultural ammonium polyphosphate is provided, comprising the following steps: The additive in the formula is mixed with the residual acid from wet phosphoric acid purification byproduct with a P2O5 content of 20%-25% to obtain a mixture. The additive includes a chelating agent and / or a dispersant. The chelating agent is used to chelate metal ions in the residual acid and form chelated trace elements. The dispersant is used to disperse the initial crystal nuclei of metal ions in the residual acid. The mixture was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate. The homogeneous melt of urea phosphate was aged under constant temperature conditions of 35-50℃. The aging homogeneous melt of urea phosphate was degassed and pre-condensed under vacuum conditions at a temperature not exceeding 78°C to obtain a paste-like homogeneous melt of urea phosphate with a solid content of 62%-68%. The paste-like homogeneous melt of urea phosphate was heated under vacuum conditions, with the heating temperature exhibiting a gradient change of first increasing and then decreasing. After cooling, ammonium polyphosphate semi-finished product is obtained; The semi-finished ammonium polyphosphate is crushed and sieved to obtain the finished agricultural water-soluble ammonium polyphosphate.

[0008] Based on the above technical solution, the present invention can be further improved as follows.

[0009] Furthermore, the additive includes a chelating agent, which comprises a citric acid solution with a mass concentration of 30-50%, wherein the amount of the citric acid solution added is 0.1-0.5% of the total mass of impurities in the residual acid.

[0010] Furthermore, it also includes a reinforcing agent, which includes an EDTA complexing agent or a disodium EDTA salt; The mixture is concentrated at a low temperature not exceeding 75°C until the P2O5 content is 38-40%. The steps of mixing concentrated residual acid with urea to generate a homogeneous melt of urea phosphate under low temperature conditions not exceeding 55°C include: The mixture was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. The fortifying agent is mixed with the concentrated raffinate, wherein the amount of fortifying agent added is 0.005%-0.015% of the mass of the raffinate; Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate containing chelated trace elements.

[0011] Furthermore, it also includes a reinforcing agent, which includes an EDTA complexing agent or a disodium EDTA salt; The step of mixing the formulated amount of chelating agent with the residual raffinate from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution includes: The chelating agent and the reinforcing agent in the formulation amount are mixed with the residual acid from wet phosphoric acid purification with a P2O5 content of 20%-25% to obtain a mixed solution; the amount of the reinforcing agent added is 0.5-1.0 times the molar number of metal impurities in the residual acid.

[0012] Furthermore, the additive includes a dispersant, which includes polyaspartic acid; the concentration of the polyaspartic acid is 35%, and the amount added is 0.5‰-1.5‰ of the mass of the residual raffinate; The step of mixing the additive in the prescribed amount with the residual raffinate from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution includes: The dispersant in the formulation is mixed with the residual acid from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution.

[0013] Furthermore, the additives include chelating agents and dispersants. The chelating agent comprises a citric acid solution with a mass concentration of 30-50%, and the amount of the citric acid solution added is 0.1-0.5% of the total mass of impurities in the residual acid. The dispersant comprises polyaspartic acid; the concentration of the polyaspartic acid is 35%, and the amount of the polyaspartic acid added is 0.3‰-0.8‰ of the mass of the residual acid. The steps of mixing the additive in the prescribed amount with the residual raffinate from wet phosphoric acid purification with a P2O5 content of 20%-25% to obtain a mixture; concentrating the mixture at a low temperature not exceeding 75°C to a P2O5 content of 38-40%; and mixing the concentrated residual raffinate with urea at a low temperature not exceeding 55°C to generate a homogeneous melt of urea phosphate include: The chelating agent of the formula amount is mixed with the residual raffinate from the wet phosphoric acid purification process with a P2O5 content of 20%-25% to obtain a mixed solution; Mix the prescribed amount of dispersant with the mixture; The mixture with added dispersant was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate.

[0014] Furthermore, the step of heating the paste-like homogeneous urea phosphate melt under vacuum conditions, with the heating temperature exhibiting a gradient change from initial increase to decrease, includes: The paste-like homogeneous melt of urea phosphate is heated under vacuum conditions, with the heating temperature sequentially in a first temperature zone, a second temperature zone, and a third temperature zone. The temperature in the first temperature zone is 155-165℃, and the heating time is 12-18 minutes. The temperature in the second temperature zone is 175-190℃, and the heating time is 22-30 minutes. The temperature in the third temperature zone is 165-175℃, and the heating time is 8-12 minutes.

[0015] Furthermore, the paste-like homogeneous urea phosphate melt is heated in a vacuum belt dryer. The vacuum belt dryer has three continuously distributed heating zones—a first heating zone, a second heating zone, and a third heating zone—along its material conveying direction. The heating temperature of the first heating zone is 155-165°C, the heating temperature of the second heating zone is 175-190°C, and the heating temperature of the third heating zone is 165-175°C.

[0016] Furthermore, the cooling step to obtain the ammonium polyphosphate semi-finished product includes: The homogeneous melt of urea phosphate was cooled to ≤45℃ to obtain ammonium polyphosphate semi-finished product.

[0017] Furthermore, the molar ratio of the concentrated residual acid to the urea is 1:0.65-0.72.

[0018] The beneficial effects of this invention are: By combining the chemical chelation of citric acid with the physical encapsulation of urea phosphate, a "dual locking" mechanism for metal impurities in raffinate is formed. Citric acid is added before concentration to chelate metal ions at the source, preventing scaling during concentration and polymerization. Urea phosphate is generated during the neutralization reaction, encapsulating all impurities through a hydrogen bond network. The synergistic effect of both ensures that impurities remain homogeneously stable in all subsequent processes, ultimately dispersing uniformly in the product as chelated trace element nutrients, thus improving product quality. Citric acid chelates with metallic impurities in the residual acid before concentration, effectively preventing scaling on the inner wall of the concentration equipment and protecting the equipment. Citric acid chelates exhibit thermal stability up to 250℃ and do not decompose at 175-190℃ in the polymerization stage. The chelated metal ions cannot act as heterogeneous nucleation sites or chain terminators, significantly reducing the amount of gas produced by side reactions and fundamentally inhibiting foaming of the material layer within the dryer. Citric acid transforms metallic impurities such as iron, aluminum, calcium, and magnesium in the residual acid into stable chelated trace elements. These chelates dissolve into the solution along with the product, becoming effective nutrients that can be absorbed by crops, thus realizing the transformation from "harmful impurities" to "beneficial nutrients." The concentration, neutralization with urea, maturation, and prepolymerization processes avoid the urea's violent hydrolysis temperature (>85℃) and the biuret accelerated formation temperature (>120℃), effectively reducing the biuret content in the product, inhibiting side reactions from a thermodynamic perspective, and controlling the biuret content in subsequent products. This effectively solves industry problems such as product stickiness and softness and crushing equipment failure caused by high biuret content. The heating polymerization temperature exhibits a gradient change, first increasing and then decreasing. The process layout of prepolymerization and dehydration, high-temperature main polymerization, and constant-temperature curing forms an N-shaped temperature curve that first heats up and then cools down, precisely controlling the polycondensation reaction rate and preventing biuret proliferation and material coking caused by local overheating, while ensuring that the degree of polymerization of the finished product is uniform. Using a synergistic approach of citric acid with DTA complexing agents or EDTA disodium salt can further reduce the heavy metal content in products and improve product standards. Replacing citric acid with polyaspartic acid can achieve the cost reduction requirement in production; The synergistic approach of using polyaspartic acid and citric acid can meet the requirements of high water solubility, low biuret content, and rich functional ingredients in the product, thereby further improving product quality. Attached Figure Description

[0019] Figure 1 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 1 ; Figure 2 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 2 ; Figure 3 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 3 ; Figure 4 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 4 ; Figure 5 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 5 ; Figure 6 The process flow of the method for producing fully water-soluble agricultural ammonium polyphosphate according to the present invention Figure 6 . Detailed Implementation

[0020] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0021] Example 1 like Figure 1As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment involves using a chelating agent to chelate metal impurities in the residual acid from wet phosphoric acid purification byproducts, forming residual acid containing chelated trace elements. This is then concentrated at low temperature. Subsequently, the residual acid is mixed with urea under low-temperature conditions to generate a homogeneous melt of urea phosphate. This melt is then subjected to constant-temperature curing, degassing, and pre-condensation at low temperature to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated to polymerize and cooled to obtain a semi-finished ammonium polyphosphate product. Finally, it is pulverized and sieved to obtain the finished product. The details are as follows: Step S1: Dissolve citric acid in warm water at 40-50℃ to prepare a citric acid solution with a mass concentration of 30-50%. Then mix the citric acid solution with residual raffinate from wet-process phosphoric acid purification, which has a P2O5 content of 20%-25%, to obtain a mixed solution. The mass of citric acid solution added is equal to the mass of impurities in the residual raffinate (including metallic impurities (Fe2O3)). 3+ Al 3+ Ca 2+ Mg 2+ (etc.) and anionic impurities (F - SO4 2- Add 0.1%-0.5% of the total mass of citric acid and residual raffinate to ensure that the citric acid and the metal impurities in the residual raffinate are fully mixed, so that the citric acid and the metal impurities in the residual raffinate are chelated to form chelated trace (metal) elements. In some embodiments, the citric acid solution is mixed with the residual raffinate in a static mixer to ensure thorough mixing.

[0022] Step S2: The raffinate mixture containing citric acid obtained in step S1 is sent to a concentration system for concentration. The concentration temperature in the concentration system is 55-75℃ and the vacuum degree is -0.085 to -0.095MPa. The raffinate mixture is concentrated to a P2O5 content of 38-40%. In some embodiments, the concentration system employs a double-effect countercurrent MVR concentration system, such as a double-effect MVR evaporator, in which the mixture is forced to circulate at a flow rate ≥1.8 m / s.

[0023] Step S3: Neutralize at low temperature to generate urea phosphate The concentrated raffinate obtained in step S2 is mixed with industrial urea at a molar ratio of 1:0.65-0.72. The mixing temperature is controlled below 55°C. During mixing, the two are stirred under normal pressure for no less than 3 hours to generate a homogeneous melt of urea phosphate containing citric acid chelate (chelated trace (metal) elements) and all the original impurities of raffinate. In some embodiments, residual acid and industrial urea are mixed and stirred in an enamel-lined reactor, which is a prior art device and will not be described in detail here.

[0024] Step S4: Constant temperature curing The homogeneous melt of urea phosphate obtained in step S3 is aged at a low temperature of 35-50℃ for no more than 9 hours. In some embodiments, the homogeneous melt of urea phosphate is cured at a constant temperature in an intermediate storage tank of a set volume. The intermediate storage tank is a container with a constant temperature heating unit and an internal stirrer. It is a conventional device with a relatively simple structure, which will not be described in detail here.

[0025] Step S5: Vacuum prepolymerization The aging homogeneous melt of urea phosphate obtained in step S4 is degassed and pre-condensed under a vacuum of -0.078 to -0.090 MPa and a temperature of 68-78°C to obtain a paste-like homogeneous melt of urea phosphate with a solid content of 62%-68%. In some embodiments, the vacuum prepolymerization step is performed in a vacuum separator of an appropriate model, in which the homogeneous melt of urea phosphate is forced to circulate at a high flow rate (flow velocity ≥ 2.2 m / s), and the residence time of the homogeneous melt of urea phosphate in the vacuum separator does not exceed 1.5 hours.

[0026] Step S6: Vacuum gradient polymerization The paste-like homogeneous melt of urea phosphate obtained in step S5 is fed into a vacuum polymerization device. Under vacuum conditions, the paste-like homogeneous melt of urea phosphate is heated and polymerized, and the heating temperature shows a gradient change of first increasing and then decreasing. In some embodiments, the above-mentioned vacuum gradient polymerization step is carried out in a vacuum belt drying and polymerization integrated machine. Specifically, the vacuum belt drying and polymerization integrated machine can be a three-layer Teflon mesh belt vacuum belt polymerization machine of an appropriate model. The vacuum belt drying and polymerization integrated machine has an absolute pressure of 5-12 kPa and has three continuous and independent first heating zone, second heating zone and third heating zone from the feeding end to the discharging end. The heating temperature of the first heating zone is 155-165℃, the heating temperature of the second heating zone is 175-190℃, and the heating temperature of the third heating zone is 165-175℃. The first heating zone, the second heating zone and the third heating zone respectively form a first temperature zone, a second temperature zone and a third temperature zone. The material passes through the three temperature zones at a uniform speed by the material conveyor belt of the vacuum belt drying and polymerization integrated machine. The time spent in the first temperature zone is 12-18 minutes, the time spent in the second temperature zone is 22-30 minutes, and the time spent in the third temperature zone is 8-12 minutes. More specifically, the first temperature zone is designated as the prepolymerization and dehydration zone, the second temperature zone as the main polymerization zone, and the third temperature zone as the constant temperature maturation zone.

[0027] Step S7: Cooling The material, after being heated in three temperature zones, is cooled until its temperature is ≤45℃ to obtain a sheet-like ammonium polyphosphate semi-finished product. In some embodiments, the discharge end of the vacuum belt drying and polymerization machine is equipped with a water cooling system (using water cooling pipes coiled around the discharge end to cool the discharge end, which is existing technology and will not be described in detail here). When the material passes through the discharge end, the water cooling system is used to cool the heated and polymerized product passing through the discharge end.

[0028] Step S8, Post-processing The semi-finished ammonium polyphosphate is crushed and sieved to obtain the finished agricultural water-soluble ammonium polyphosphate. Specifically, the ammonium polyphosphate semi-finished product is pulverized at a low temperature not exceeding 45℃ and then sieved through an 80-120 mesh sieve to obtain the finished agricultural water-soluble ammonium polyphosphate. The pulverization and sieving are performed using conventional pulverizers and sieves, both of which are existing equipment in the industry and will not be described in detail here.

[0029] The specifications of the obtained finished agricultural water-soluble ammonium polyphosphate are as follows: Total nutrients (N+P2O5) ≥66%, P2O5 content ≥52%, N content ≥13%, moisture ≤2.8%, water solubility ≥92%, biuret content <0.5%, degree of polymerization 12-30, product appearance is white or off-white powder granules, good brittleness.

[0030] The following example illustrates the preparation of agricultural water-soluble ammonium polyphosphate by combining sodium citrate and residual raffinate: A phosphate chemical company's residual acid (P2O5 content 23.5%) contains 1.2% Fe2O3, 2.1% Al2O3, 3.5% CaO, and 1.8% MgO. The ammonium phosphate powder (11-44 monoammonium phosphate) contains 26% insoluble matter. The process is as follows: Example 1: Citric acid was added at 0.3% of the total mass of metal elements in the residual raffinate, prepared into a 40% solution, and injected before the concentration pump. The residual raffinate containing citric acid was fed into a double-effect MVR concentration system, concentrated to a P2O5 content of 38.2% under the conditions of a circulation flow rate of 2.0 m / s, a temperature of 70℃, and a vacuum degree of -0.090 MPa. The concentrated residual raffinate was mixed with urea at a molar ratio of 1:0.68 and stirred at 52℃ for 3.5 hours to obtain a clear homogeneous melt of urea phosphate. The homogeneous melt of urea phosphate was matured at 42℃ for 6 hours. Vacuum prepolymerization was carried out at 72℃ and -0.085 MPa, with a high flow rate circulation of 2.5 m / s and a residence time of 1.2 hours, resulting in a solid content of 65% in the discharge. The temperatures of the three zones of the vacuum belt press were 160℃, 185℃, and 170℃, with residence times of 15 min, 25 min, and 10 min, respectively, and the discharge temperature was 38℃.

[0031] Results: The product had a P2O5 content of 53.5%, a nitrogen content of 13.8%, a moisture content of 2.3%, a water solubility of 93.5%, and a biuret content of 0.28%. The product exhibited good brittleness, and the pulverizer operated continuously without tripping. Compared to the control example without citric acid (using the same residual acid and formulation as in Example 1, but without adding citric acid), the water solubility increased by 8 percentage points, and the foaming frequency of the vacuum belt drying polymerization machine decreased by more than 90%.

[0032] Example 2: The same residual acid as in Example 1 was used, but no ammonium powder was added after concentration.

[0033] Citric acid was added at 0.3% of the total mass of metal elements in the residual raffinate, prepared into a 40% solution, and injected before the concentration pump. The residual raffinate containing citric acid was fed into a double-effect MVR concentration system, concentrated to a P2O5 content of 38.2% under the conditions of a circulation flow rate of 2.0 m / s, a temperature of 70℃, and a vacuum degree of -0.090 MPa. The concentrated residual raffinate was mixed with urea at a molar ratio of 1:0.70 and stirred at 50℃ for 3 hours to obtain a clear homogeneous melt of urea phosphate. The homogeneous melt of urea phosphate was matured at 42℃ for 6 hours. Vacuum prepolymerization was carried out at 72℃ and -0.085 MPa, with a high flow rate circulation of 2.5 m / s and a residence time of 1.2 hours, resulting in a solid content of 65% in the discharge. The temperatures of the three zones of the vacuum belt press were 160℃, 185℃, and 170℃, with residence times of 15 min, 25 min, and 10 min, respectively, and the discharge temperature was 38℃.

[0034] Results: The product contained 54.2% P2O5, 14.5% N, 2.1% moisture, 94.8% water solubility, and 0.22% biuret. The product's bulk density remained stable at 0.85-0.90 g / cm³. 3 The automatic packaging system operated continuously and normally. Compared to the control example without citric acid (using the same residual acid and formulation as in Example 2, but without adding citric acid), significant scaling appeared inside the concentration system after 15 days of operation, and the heat transfer efficiency decreased by 25%. The product's water solubility was 84.2%, and frequent foaming and overflow occurred in the vacuum belt dryer, requiring a reduction in output to 60% of the design capacity for stable operation. The product's bulk density was 0.65-0.85 g / cm³. 3 The fluctuations were significant, leading to frequent shutdowns and adjustments of the subsequent automated packaging system for finished products.

[0035] Example 2 like Figure 2As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment involves using a chelating agent to chelate metal impurities in the residual acid from wet phosphoric acid purification byproducts, forming residual acid containing chelated trace elements. This is then concentrated at low temperature. Next, a strengthening agent is added to the concentrated residual acid, followed by a low-temperature reaction with urea to generate a homogeneous melt of urea phosphate. This melt is then subjected to low-temperature constant-temperature curing, degassing, and pre-condensation to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated and polymerized, cooled, and the resulting ammonium polyphosphate semi-finished product is obtained. Finally, it is pulverized and sieved to obtain the finished product. The difference between this method and Example 1 lies in the slightly different application scenarios. When the content of heavy metals such as arsenic, cadmium, and lead in the residual acid is abnormally high (e.g., As > 5 mg / kg, Cd > 3 mg / kg, Pb > 20 mg / kg), and citric acid alone cannot stably control the heavy metal content of the product within international export standards (e.g., EU 2019 / 1009, As ≤ 10 mg / kg, Cd ≤ 10 mg / kg, Pb ≤ 50 mg / kg), a compound formulation of citric acid and a fortifying agent is adopted, as follows: Step 101: Dissolve citric acid in warm water at 40-50℃ to prepare a citric acid solution with a mass concentration of 30-50%. Then mix the citric acid solution with residual raffinate from the wet-process phosphoric acid purification process, which has a P2O5 content of 20%-25%. The mass of citric acid solution added is equal to the mass of impurities in the residual raffinate (including metallic impurities (Fe2O3)). 3+ Al 3+ Ca 2+ Mg 2+ (etc.) and anionic impurities (F - SO4 2- Add 0.3%-0.5% of the total mass of citric acid and residual raffinate to ensure thorough mixing, so that the citric acid and residual raffinate contain no metallic impurities (mainly Fe). 3+ Al 3+ Ca 2+ Mg 2+ ) chelation, forming chelated trace (metallic) elements; In some embodiments, the citric acid solution is mixed with the residual raffinate in a static mixer to ensure thorough mixing.

[0036] Step 102: Mix the citric acid-containing raffinate mixture obtained in Step 101 with an EDTA complexing agent or EDTA disodium salt; Specifically, the amount of EDTA complexing agent or EDTA disodium salt added is 0.005%-0.015% of the mass of the raffinate. When adding, the EDTA complexing agent or EDTA disodium salt is pre-dissolved in warm water at 40-50℃ and prepared into a 5% solution. The solution is stirred for 10-15 minutes to allow the EDTA complexing agent or EDTA disodium salt to fully chelate the heavy metal ions remaining in the raffinate. In some embodiments, the residual acid mixture is mixed with an EDTA complexing agent or EDTA disodium salt in a low-temperature neutralization reactor, such as an enamel-lined reactor.

[0037] Next, the procedures S2-S8 in Example 1 are carried out, with other conditions the same as in Example 1.

[0038] The specifications of the obtained finished agricultural water-soluble ammonium polyphosphate are as follows: The product's heavy metal content meets EU 2019 / 1009 export standards, with water solubility ≥93% and biuret content <0.5%. The trace addition of EDTA complexing agent or EDTA disodium salt (<0.02%) will not significantly increase costs, and its impact on product biodegradability is within regulatory acceptable limits.

[0039] It is particularly important to emphasize that in this second example, the EDTA complexing agent or disodium EDTA salt must be added after citric acid, specifically for chelating trace heavy metals that citric acid fails to completely lock in. The amount of EDTA complexing agent or disodium EDTA salt added must not exceed 0.2‰ of the mass of the residual raffinate. Excessive amounts will result in the presence of persistent EDTA-metal complexes in the product, failing to meet the environmental regulations of some international markets.

[0040] Example 3 like Figure 3 As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment involves chelating metal impurities in the residual acid from wet phosphoric acid purification byproducts with a chelating agent and a reinforcing agent to form residual acid containing chelated trace elements. This is then concentrated at low temperature. Next, a reinforcing agent is added to the concentrated residual acid, followed by a low-temperature reaction with urea to generate a homogeneous melt of urea phosphate. This melt is then subjected to low-temperature isothermal ripening, degassing, and pre-condensation to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated, polymerized, and cooled to obtain a semi-finished ammonium polyphosphate product. Finally, it is pulverized and sieved to obtain the finished product. The difference from Example 2 lies in the slightly different application scenarios. In this embodiment, the reinforcing agent (EDTA complexing agent or disodium EDTA salt) is used as an emergency treatment agent for extreme impurity conditions. When the source of residual acid is temporarily changed, the impurity content is abnormally high (such as total metal oxide content >8%, or containing abnormally high concentrations of heavy metals such as chromium and nickel), and citric acid alone cannot guarantee the water solubility and compliance of the product, EDTA is used as an emergency reinforcing agent, as detailed below: Before production, the total content of all metal impurities in the raffinate is tested, and the total number of metal moles is calculated. The following steps are then performed: Step S1001: Citric acid is pre-dissolved in warm water at 40-50℃ to prepare a citric acid solution with a mass concentration of 30-50%. Simultaneously, an EDTA complexing agent or EDTA disodium salt solution is prepared. The EDTA complexing agent or EDTA disodium salt is first pre-dissolved in warm water at 40-50℃ to prepare a 5-10% solution. The amount of EDTA complexing agent or EDTA disodium salt added is 0.5-1.0 times the molar number of metal impurities in the raffinate. Then, the citric acid solution and the EDTA complexing agent or EDTA disodium salt solution are simultaneously mixed with raffinate from wet-process phosphoric acid purification, which has a P2O5 content of 20%-25%. This allows the citric acid and the EDTA complexing agent or EDTA disodium salt to chelate with the metal impurities in the raffinate, with the citric acid primarily reacting with the metal impurities (mainly Fe). 3+ Al 3+ Ca 2+ Mg 2+ Chelation: EDTA complexing agent or disodium EDTA salt fully chelates residual heavy metal ions in the raffinate that have not been chelated with citric acid; Next, the procedures S2-S8 in Example 1 are carried out, with other conditions the same as in Example 1.

[0041] The specifications of the obtained finished agricultural water-soluble ammonium polyphosphate are as follows: The product has a water solubility of ≥90%, meets the HG / T 2770-2020 Type IV standard, and its heavy metal content meets domestic fertilizer standards.

[0042] It is important to emphasize that the method described in Example 3 is only applicable for temporary, single-batch emergency treatment and should not be used as a routine production process. After use, the equipment must be thoroughly cleaned to prevent EDTA residue from affecting subsequent batches.

[0043] Example 4 like Figure 4As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment involves mixing a dispersant with residual raffinate, a byproduct of wet phosphoric acid purification, to disperse the initial crystal nuclei of metal impurities (metal ions) in the residual raffinate, replacing the citric acid in Example 1. This high-molecular dispersion effectively prevents the sedimentation of suspended solids in water and the deposition and scaling of acidic substances. Then, low-temperature concentration is performed, followed by a low-temperature reaction between the residual raffinate and urea to generate a homogeneous melt of urea phosphate. This melt is then subjected to low-temperature constant-temperature curing, degassing, and pre-condensation to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated, polymerized, and cooled to obtain a semi-finished ammonium polyphosphate product. Finally, it is pulverized and sieved to obtain the finished product. The difference between this method and Example 1 lies in the application scenario: when the market price of citric acid fluctuates significantly, or when enterprises have a need to reduce costs, fertilizer-grade polyaspartic acid (PASP) is used to partially or completely replace citric acid as the main dispersant. The specific process is as follows: Step S1000: Fertilizer-grade polyaspartic acid (PASP) liquid (with an effective polyaspartic acid content of 35%) is mixed with residual acid from wet phosphoric acid purification with a P2O5 content of 20%-25% at a concentration of 0.5‰-1.5‰ of the residual acid mass to obtain a mixed solution. In some embodiments, the polyaspartic acid solution is mixed with the residual raffinate in a static mixer to ensure thorough mixing.

[0044] Next, the procedures S2-S8 in Example 1 are carried out, with other conditions the same as in Example 1.

[0045] The specifications of the obtained finished agricultural water-soluble ammonium polyphosphate are as follows: The product has a water solubility of ≥92%, a biuret content of <0.5%, and a whiteness of ≥90. Polyaspartic acid molecules are stable at high polymerization temperatures (decomposition temperature >300℃) and do not produce carbonized black spots.

[0046] It is particularly important to emphasize that polyaspartic acid (PASP) solution is in liquid form and requires a dedicated metering pump and storage tank. Its effective content may fluctuate depending on the supplier and batch; samples must be taken before each batch is used to test the effective content and adjust the dosage accordingly. Polyaspartic acid exhibits good stability in acidic slurries and does not react with urea phosphate systems to form flocculation.

[0047] Example 5 like Figure 5 As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment employs a synergistic approach of dispersant and chelating agent, which are mixed together with residual acid from the wet phosphoric acid purification process. The chelating agent is responsible for chemically chelating Fe. 3+ Al 3+ The dispersant is responsible for dispersing Ca. 2+ Mg 2+The initial crystal nuclei are formed, followed by low-temperature concentration. Then, under low-temperature conditions, the residual acid and urea are mixed and reacted to generate a homogeneous melt of urea phosphate. This is then subjected to low-temperature isothermal ripening, degassing, and pre-condensation to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated and polymerized, cooled, to obtain a semi-finished product of ammonium polyphosphate. Finally, it is pulverized and sieved to obtain the finished product. The difference between this scheme and Example 4 lies in the application scenario: if further cost reduction is required, or if the enterprise has a need for cost reduction, the dispersant and chelating agent can be compounded and used in a specific ratio. The specific process is as follows: Step S10001: Dissolve citric acid in warm water at 40-50℃ to prepare a citric acid solution with a mass concentration of 30-50%. At the same time, prepare a composite solution by mixing fertilizer-grade polyaspartic acid (PASP) liquid (with an effective polyaspartic acid content of 35%) with the citric acid solution at a mass ratio of 1:1 to 3:1. Mix the composite solution with the residual acid from the wet-process phosphoric acid purification byproduct with a P2O5 content of 20%-25% to obtain a mixed solution. In some embodiments, the mixing of the three liquids is carried out in a static mixer.

[0048] Next, the procedures S2-S8 in Example 1 are carried out, with other conditions the same as in Example 1.

[0049] Example 6 like Figure 6 As shown, the method for producing fully water-soluble agricultural ammonium polyphosphate in this embodiment involves mixing a dispersant and a chelating agent in synergistic action with residual urea, a byproduct of wet-process phosphoric acid purification. The chelating agent and dispersant work together to form a triple stable system of "small molecule chelation + high molecular weight dispersion + urea phosphate complexation." Then, low-temperature concentration is performed. Following this, the residual urea is mixed with urea under low-temperature conditions to generate a homogeneous melt of urea phosphate. This melt is then subjected to low-temperature isothermal ripening, degassing, and pre-condensation to generate a paste-like homogeneous melt of urea phosphate. The paste is then heated, polymerized, and cooled to obtain a semi-finished ammonium polyphosphate product. Finally, it is pulverized and sieved to obtain the finished product. The difference between this method and Example 4 lies in the application scenario: producing APP products for high-end liquid fertilizers and export-grade water-soluble fertilizers, which simultaneously need to meet the requirements of high water solubility (≥94%), low biuret (<0.3%), and rich in functional components. The specific process is as follows: Step S100001: Dissolve citric acid in warm water at 40-50℃ to prepare a citric acid solution with a mass concentration of 30-50%. Then mix the citric acid solution with residual raffinate from the wet-process phosphoric acid purification process, which has a P2O5 content of 20%-25%. The mass of citric acid solution added is equal to the mass of impurities in the residual raffinate (including metallic impurities (Fe2O3)). 3+ Al 3+ Ca 2+ Mg2+ (etc.) and anionic impurities (F - SO4 2- Add 0.2%-0.3% of the total mass of citric acid and residual raffinate to ensure that the citric acid and the metal impurities in the residual raffinate are fully mixed, so that the citric acid and the metal impurities in the residual raffinate are chelated to form chelated trace (metal) elements; In some embodiments, the citric acid solution is mixed with the residual raffinate in a static mixer to ensure thorough mixing.

[0050] Step S100002: Mix the citric acid-containing raffinate mixture obtained in step S100001 with fertilizer-grade polyaspartic acid (PASP) liquid. The amount of polyaspartic acid (PASP) liquid added is 0.3‰-0.8‰ of the mass of raffinate (PASP) by weight. Before adding, the polyaspartic acid (PASP) is diluted with a small amount of water and then slowly injected while stirring for 10 minutes. In some embodiments, the residual acid mixture is mixed with fertilizer-grade polyaspartic acid (PASP) liquid in a static mixer.

[0051] Next, the procedures S2-S8 in Example 1 are carried out, with other conditions the same as in Example 1.

[0052] The specifications of the obtained finished agricultural water-soluble ammonium polyphosphate are as follows: The product has a water solubility of ≥94%, a biuret content of <0.3%, and a whiteness of ≥92. The synergistic effect of polyaspartic acid (PASP) in fertilizer application results in better crop absorption and yield increase in agricultural applications.

[0053] It is particularly important to emphasize that in this embodiment six, the amount of polyaspartic acid (PASP) added is relatively low, mainly to play its role in dispersing and enhancing, rather than completely replacing the chelating function of citric acid. The ratio of the two compounds should be flexibly adjusted according to the actual impurity content of the residual acid and the quality of the target product.

[0054] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0055] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0056] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0057] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first and second features are in direct contact, or that they are in indirect contact through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0058] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0059] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for producing fully water-soluble agricultural ammonium polyphosphate, characterized in that, Includes the following steps: The additive in the formula is mixed with the residual acid from wet phosphoric acid purification byproduct with a P2O5 content of 20%-25% to obtain a mixture. The additive includes a chelating agent and / or a dispersant. The chelating agent is used to chelate metal ions in the residual acid and form chelated trace elements. The dispersant is used to disperse the initial crystal nuclei of metal ions in the residual acid. The mixture was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate. The homogeneous melt of urea phosphate was aged under constant temperature conditions of 35-50℃. The aging homogeneous melt of urea phosphate was degassed and pre-condensed under vacuum conditions at a temperature not exceeding 78°C to obtain a paste-like homogeneous melt of urea phosphate with a solid content of 62%-68%. The paste-like homogeneous melt of urea phosphate was heated under vacuum conditions, with the heating temperature exhibiting a gradient change of first increasing and then decreasing. After cooling, ammonium polyphosphate semi-finished product is obtained; The semi-finished ammonium polyphosphate is crushed and sieved to obtain the finished agricultural water-soluble ammonium polyphosphate.

2. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The additive includes a chelating agent, which comprises a citric acid solution with a mass concentration of 30-50%, wherein the amount of the citric acid solution added is 0.1-0.5% of the total mass of impurities in the residual acid.

3. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 2, characterized in that, It also includes a fortifying agent, which includes an EDTA complexing agent or a disodium EDTA salt; The mixture is concentrated at a low temperature not exceeding 75°C until the P2O5 content is 38-40%. The steps of mixing concentrated residual acid with urea to generate a homogeneous melt of urea phosphate under low temperature conditions not exceeding 55°C include: The mixture was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. The fortifying agent is mixed with the concentrated raffinate, wherein the amount of fortifying agent added is 0.005%-0.015% of the mass of the raffinate; Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate containing chelated trace elements.

4. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, It also includes a fortifying agent, which includes an EDTA complexing agent or a disodium EDTA salt; The step of mixing the formulated amount of chelating agent with the residual raffinate from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution includes: The chelating agent and the reinforcing agent in the formulation amount are mixed with the residual acid from wet phosphoric acid purification with a P2O5 content of 20%-25% to obtain a mixed solution; the amount of the reinforcing agent added is 0.5-1.0 times the molar number of metal impurities in the residual acid.

5. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The additive includes a dispersant, which includes polyaspartic acid; the concentration of the polyaspartic acid is 35%, and the amount added is 0.5‰-1.5‰ of the mass of the residual raffinate. The step of mixing the additive in the prescribed amount with the residual raffinate from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution includes: The dispersant in the formulation is mixed with the residual acid from the wet phosphoric acid purification process, which has a P2O5 content of 20%-25%, to obtain a mixed solution.

6. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The additives include chelating agents and dispersants. The chelating agent includes a citric acid solution with a mass concentration of 30-50%, and the amount of the citric acid solution added is 0.1-0.5% of the total mass of impurities in the residual acid. The dispersant includes polyaspartic acid; the concentration of the polyaspartic acid is 35%, and the amount of the polyaspartic acid added is 0.3‰-0.8‰ of the mass of the residual acid. The additive in the formulation is mixed with residual raffinate from wet phosphoric acid purification with a P2O5 content of 20%-25% to obtain a mixture; the mixture is then concentrated at a low temperature not exceeding 75°C to a P2O5 content of 38-40%. The steps for mixing concentrated residual acid with urea to generate a homogeneous melt of urea phosphate under low-temperature conditions not exceeding 55°C include: The chelating agent of the formula amount is mixed with the residual raffinate from the wet phosphoric acid purification process with a P2O5 content of 20%-25% to obtain a mixed solution; Mix the prescribed amount of dispersant with the mixture; The mixture with added dispersant was concentrated at a low temperature not exceeding 75°C until the P2O5 content was 38-40%. Under low temperature conditions not exceeding 55°C, the concentrated residual acid is mixed with urea to generate a homogeneous melt of urea phosphate.

7. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The step of heating the paste-like homogeneous urea phosphate melt under vacuum conditions, with the heating temperature exhibiting a gradient change from first increasing to decreasing, includes: The paste-like homogeneous melt of urea phosphate is heated under vacuum conditions, with the heating temperature sequentially in a first temperature zone, a second temperature zone, and a third temperature zone. The temperature in the first temperature zone is 155-165℃, and the heating time is 12-18 minutes. The temperature in the second temperature zone is 175-190℃, and the heating time is 22-30 minutes. The temperature in the third temperature zone is 165-175℃, and the heating time is 8-12 minutes.

8. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 7, characterized in that, The paste-like homogeneous urea phosphate melt is heated in a vacuum belt dryer. The vacuum belt dryer has three continuously distributed heating zones—a first heating zone, a second heating zone, and a third heating zone—along its material conveying direction. The heating temperature of the first heating zone is 155-165℃, the heating temperature of the second heating zone is 175-190℃, and the heating temperature of the third heating zone is 165-175℃.

9. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The cooling process to obtain the ammonium polyphosphate semi-finished product includes: The homogeneous melt of urea phosphate was cooled to ≤45℃ to obtain ammonium polyphosphate semi-finished product.

10. The method for producing fully water-soluble agricultural ammonium polyphosphate according to claim 1, characterized in that, The molar ratio of the concentrated residual acid to the urea is 1:0.65-0.72.

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