Preparation process of slow-release composite phosphate fertilizer based on multi-stage activation

Abstract

The invention relates to the technical field of compound fertilizers, in particular to a preparation process of a slow-release compound phosphate fertilizer based on multistage activation. The preparation process comprises the following steps: carrying out powder refining treatment on a phosphorus-based raw material to obtain a pretreated phosphorus material; mixing the pretreated phosphorus material with a weak acid or organic acid activation medium to obtain a dissolution activation material; s3, adding a salifying regulation component and a complexing regulation component into the dissolved activated material to obtain a secondary activated mixture; s4, granulating the secondarily activated mixture to obtain fertilizer core particles; s5, interface activation: performing interface activation treatment on the surfaces of the fertilizer core particles to form an interface activation layer; s6, sequentially constructing an inner quick-release layer, a middle reaction slow-release layer and an outer controlled-release layer on the surfaces of the fertilizer core particles; and S7, drying the coated particles, and screening and finishing the particles to obtain the slow-release composite phosphate fertilizer finished product. The problems that in the prior art, the available phosphorus activation rate is low, the coating binding force is poor, the release curve cannot be adjusted, and the batch consistency is poor are solved.

Description

Technical Field

[0001] This application relates to the field of compound fertilizer technology, and in particular to a process for preparing a slow-release compound phosphate fertilizer based on multi-stage activation. Background Technology

[0002] Slow-release compound phosphate fertilizer is used to gradually release nutrients such as phosphorus to meet the needs of plants at different growth stages. It combines phosphorus sources with other components (such as nitrogen and potassium) to control the release rate, thereby reducing nutrient waste and environmental pollution.

[0003] Existing slow-release compound phosphate fertilizers typically employ single coating or simple compounding methods, which present the following problems:

[0004] Insufficient activation of mineral-derived phosphorus or sparingly soluble phosphorus results in low effective phosphorus content and slow onset of action;

[0005] Poor bonding between the coating and the fertilizer core interface makes the coating prone to detachment and cracking, resulting in "sudden release in the early stage + insufficient release in the later stage";

[0006] It is difficult to simultaneously achieve "early phosphorus supply, continuous phosphorus supply in the rhizosphere, inhibition of phosphorus fixation and improvement of utilization rate";

[0007] Release curves are difficult to replicate stably, batch consistency is poor, and there is a lack of quantifiable process control windows. Summary of the Invention

[0008] This application provides a process for preparing a slow-release compound phosphate fertilizer based on multi-stage activation to solve the above-mentioned problems. The preparation process includes:

[0009] S1. Raw material pretreatment: Provide phosphorus-based raw materials containing sparingly soluble phosphorus sources, and perform powder refinement treatment on the phosphorus-based raw materials to obtain pretreated phosphorus materials;

[0010] S2, Dissolution Activation: The pretreated phosphorus material is mixed with a weak acid or organic acid activation medium to cause local dissolution on the surface of the phosphorus-based raw material and form reactive active sites. After pre-drying, the dissolved activated material is obtained.

[0011] S3, Complexation / Salt Formation Activation: Salt formation regulating components containing Ca and / or Mg and complexation regulating components are added to the leaching activation material to form ion-bridged or complexed intermediates and to construct a salt formation reaction system for subsequent shell formation, thereby obtaining a secondary activation mixture.

[0012] S4. Granulation and nucleation: Granulate the secondary activated mixture to obtain fertilizer core particles;

[0013] S5. Interface activation: The surface of the fertilizer core particles is subjected to interface activation treatment to form an interface activation layer on the surface of the fertilizer core particles for shell anchoring and nucleation.

[0014] S6. Gradient reaction shell one-time molding and coating: An inner fast-release layer, an intermediate reaction slow-release layer and an outer controlled-release layer are sequentially constructed on the surface of the fertilizer core particles;

[0015] S7. Drying and granulation: The coated granules are dried and sieved to obtain the slow-release compound phosphate fertilizer product.

[0016] The multi-stage activation includes dissolution activation, complexation / salt formation activation, and interface activation, and the gradient reaction shell is used to achieve the segmented release of the finished product.

[0017] Through the above technical solution, a seven-step progressive process chain is established, enabling the insoluble phosphorus source to undergo a process of powder refinement to expose the reaction interface, selective dissolution with weak acid to construct surface micropores and active sites, synergistic formation of ion-bridging intermediates by Ca / Mg and complexing components, granulation to provide a structural carrier, construction of chemical anchoring points on the fertilizer core surface by interfacial activation liquid, and sequential in-situ reaction / coating of a three-layer gradient shell to achieve "fast-stable-controlled" release functional zones. Ultimately, the transformation from insoluble phosphorus to slow-release functional particles is completed within a single process line. This multi-stage activation and gradient reaction... The coupling mechanism of the coating enables precise control over the entire process of phosphorus dissolution, complexation, nucleation, reaction, and controlled release in both spatial and temporal dimensions. This solves the problems of low effective phosphorus activation rate, poor coating binding force, unadjustable release curve, and poor batch consistency in existing technologies. Its beneficial effects include significantly improving the initial release rate at 7 days to ≥15%, the mid-term release rate at 30 days to ≥40%, and the total release rate at 90 days to ≥75%. At the same time, it increases the compressive strength of the particles to ≥40N / particle and reduces the shell shedding rate during transportation and application to ≤3%.

[0018] Optionally, in step S1, the insoluble phosphorus source includes one or more of phosphate rock powder, phosphate rock associated phosphorus, phosphate slag phosphorus, and insoluble phosphates.

[0019] The goal of the powder refining process is to control the particle size D50 of the pretreated phosphate material within the range of 20–120 μm.

[0020] Through the above technical solution, by limiting the types of sparingly soluble phosphorus sources and the D50 particle size range, phosphorus-based raw materials possess both resource utilization potential and a sufficient reaction kinetic basis. Phosphate rock powder, associated phosphorus from phosphate rock, and phosphorus slag are all low-grade phosphorus resources, and their introduction reduces raw material costs and improves the utilization rate of industrial solid waste. Controlling the D50 within the range of 20–120 μm avoids agglomeration or dust dispersion during spray activation caused by excessively fine powder, while ensuring sufficient specific surface area (measured specific surface area reaches 1.8–3.2 m² / g) to support uniform acid penetration and surface dissolution reactions, thereby forming a density ≥8 × 10⁻⁶ in step S2. 4The presence of local active sites per mm² provides ample anchoring points for subsequent complexation / salt formation reactions. Its beneficial effect is to increase the formation rate of complex intermediates in step S3 to over 92%, which is 37% higher than the control sample with D50 > 150 μm, and to ensure that the granulation yield in step S4 remains stable at 94%–96%.

[0021] Optionally, in step S2, the weak acid or organic acid activating medium is selected from one or more of citric acid, humic acid, lactic acid, tartaric acid and their salts.

[0022] The mixing process of the pretreated phosphate material with the weak acid or organic acid activation medium is carried out by spraying and mixing. The ratio of the activation medium to the pretreated phosphate material is 0.05 to 0.25 kg / kg, and the pH of the activation system is controlled at 3.0 to 6.0.

[0023] The pre-drying conditions include: a pre-drying temperature of 50–90°C and a pre-drying time of 10–40 min.

[0024] Through the above technical solution, by limiting the type of organic acid, the spray-mixing method, the material-liquid ratio, and the pH window, and by using stepped pre-drying parameters, controllable etching rather than destructive dissolution of the phosphate mineral lattice is achieved. Citric acid and humic acid possess both proton-donating and metal-chelating capabilities, preferentially attacking the Ca-O bonds at the edge of apatite crystals at pH 3.0–6.0, inducing local Ca²⁺ dissolution and exposing PO₄³⁻ active sites. Simultaneously, humate ions can in situ complex and dissolve Ca²⁺, inhibiting redeposition. The spray-mixing method ensures that the acid solution forms a uniform (±5%) liquid film on the surface of 20–120 μm particles, avoiding the internal diffusion lag caused by the impregnation method. The liquid film coverage rate corresponding to 0.05-0.25 kg / kg is 98%-100%, ensuring that all particles are effectively wetted; the pre-drying at 50-90℃ for 10-40 min removes free water while retaining surface bound water (content 0.8%-2.1%), maintaining the Ca / Mg ion migration activity in step S3. Its beneficial effect is that it increases the water-soluble phosphorus content of the soluble activated material obtained in S2 from 0.12% in the untreated sample to 0.85%-1.32%, and the XRD pattern shows that the half-width at half maximum (FWHM) of the hydroxyapatite (002) peak is broadened by 18%-25%, confirming that the degree of lattice distortion is controllable, providing a structurally flexible basis for subsequent complexation / salt formation reactions.

[0025] Optionally, in step S3, the salt formation regulating component includes one or more of magnesium oxide, magnesium hydroxide, dolomite powder, limestone powder, and calcium hydroxide.

[0026] The amount of the salt-forming regulating component added satisfies the following conditions: the Ca / P molar ratio is 0.3 to 2.0 and / or the Mg / P molar ratio is 0.05 to 0.8, wherein P is calculated based on the phosphorus content in the leaching activation material;

[0027] The complexing regulating component includes humate and / or polycarboxylic acid polymer, and the amount of the complexing regulating component added is 0.2% to 3% of the mass of the insoluble phosphorus source.

[0028] Through the above technical solution, by synergistically regulating the Ca / P and Mg / P molar ratios and introducing humate-based complexing agents, the active sites on the surface of the leaching activation material are directionally transformed into ion-bridging intermediates with sustained-release function. Dolomite powder and magnesium oxide provide a bimetallic source of Ca²⁺ and Mg²⁺. Under pH 4.0–5.5 conditions, Mg²⁺ preferentially coordinates with surface PO₄³⁻ to form a six-coordinate octahedral structure, while Ca²⁺ bridges adjacent PO₄ units to form a three-dimensional network. This network exhibits optimal stability (Zeta potential absolute value reaches 32–38 mV) when Ca / P = 0.8–1.2 and Mg / P = 0.2–0.5. Potassium humate acts as a complexing modulator. The controlled component, with its quinone and carboxyl groups, can simultaneously complex Fe³⁺ / Al³⁺ to inhibit soil phosphorus fixation, and its macromolecular skeleton plays a binding and enhancing role in S4 granulation; the addition amount of 0.2% to 3% corresponds to a humate coverage rate of 65% to 95% on the particle surface, ensuring sufficient complexation sites without affecting sphericity. Its beneficial effect is that the phosphorus fixation rate of the secondary activated mixture obtained in S3 in simulated soil solution (pH 6.5, 0.01 mol / L CaCl2) is reduced from 41% in the control sample to 12% to 18%, and the proportion of Ca–Mg composite phosphate in the intermediate layer formed after S6 reaction and solidification reaches 63% to 79%, which is significantly better than the single Ca or Mg phosphate system.

[0029] Optionally, in step S4, the granulation target includes: the granulation moisture content is 6% to 14%, and the particle size of the fertilizer core particles is 2 to 5 mm.

[0030] Through the above technical solution, by precisely controlling the granulation moisture and target particle size, the secondary activated mixture forms dense, regular fertilizer core particles with a surface microporous structure during the rolling nucleation process. A moisture content of 6%–14% corresponds to a material plasticity index (PI) of 12–18. At this level, starch-based binders (such as dextrin) fully swell and form a continuous bonding network, resulting in a particle compaction density of 1.15–1.32 g / cm³. The particle size is 2–5 mm. Within this size range, the particle specific surface area to volume ratio is in the optimal range (0.8–1.2 mm⁻¹), ensuring the optimal spray density of the S5 interface activation liquid. The uniform adhesion during coating (coverage ≥97%) provides a stable geometric base for the S6 three-layer coating; when the moisture content is controlled at 10%±1% and the particle size is concentrated at 3~4mm, the particle roundness reaches 0.89~0.93, and the initial compressive strength reaches 32~38N / particle. Its beneficial effect is to compress the fluctuation range of the weight gain rate of the S5 interface activation layer to ±0.05%, and to provide a uniform heat conduction path for the in-situ reaction of the S6 intermediate reaction slow-release layer, ensuring that the temperature difference between the particle center and the surface is ≤3℃ when cured at 90℃, and the thickness deviation of the reaction layer is controlled within ±0.08mm.

[0031] Optionally, in step S5, the interface activation process includes:

[0032] An interface activation liquid is sprayed onto the surface of the fertilizer core particles and cured, wherein the interface activation liquid contains a silane coupling agent and / or a polycarboxyl polymer, and the weight gain of the interface activation layer is 0.1% to 1.5%.

[0033] The curing temperature for the interface activation treatment is 40–90°C, and the curing time is 5–25 min.

[0034] Through the above technical solution, a bimodal interface layer with both covalent bonding and coordination anchoring functions is constructed by compounding a silane coupling agent with a multi-carboxyl polymer and acting under specific weight gain and thermosetting conditions. γ-aminopropyltriethoxysilane (KH-550) hydrolyzes to generate Si–OH, which condenses with Si–OH or Mg–OH on the fertilizer core surface to form Si–O–Si / Mg covalent bonds. Its amino terminus undergoes an amidation reaction with the carboxyl groups in the outer PBAT coating. Sodium polyacrylate (PAAS), as a multi-carboxyl polymer, has multiple –COO⁻ groups that form an ionic crosslinking network with Ca²⁺ / Mg²⁺ on the fertilizer core surface. A weight gain of 0.1%–1.5% corresponds to an interface layer thickness of 80–1200 nm, ensuring the anchoring density (KH-550 molecular density ≥ 2.1 × 10¹). 9While avoiding excessive thickening that could lead to stress cracking, the curing process at 40–90℃ for 5–25 min resulted in a silane condensation degree of 85%–93% and a PAAS crosslinking degree of 76%–89%. The beneficial effect was that it increased the peel strength between the S6 three-layer shell and the fertilizer core from 1.2 N / mm in the unactivated sample to 4.8–6.3 N / mm, and after 10 freeze-thaw cycles (−20℃ / 2h→25℃ / 2h), it still maintained ≥4.1 N / mm, significantly inhibiting shell detachment during transportation and application.

[0035] Optionally, in step S6, the inner fast-release layer contains a water-soluble phosphorus source, which is selected from one or more of ammonium dihydrogen phosphate, monoammonium phosphate, and potassium dihydrogen phosphate, and the weight gain rate of the inner fast-release layer is 0.5% to 6%.

[0036] Through the above technical solution, by selecting a highly water-soluble phosphorus source and controlling its weight gain rate, the inner layer dissolves rapidly upon contact with soil moisture, establishing an initial phosphorus concentration gradient that drives the subsequent diffusion and release in the middle and outer layers. Ammonium dihydrogen phosphate (MAP), possessing both NH4⁺ and H2PO4⁻, has a higher solubility (52.9 g / 100 g H2O, 25℃) than monoammonium phosphate (MAP 36.8 g / 100 g) and potassium dihydrogen phosphate (MKP 22.6 g / 100 g), and NH4⁺ can locally lower the microenvironment pH, promoting the slow dissolution and release of the slightly soluble phase in the middle layer; weight gain... A release rate of 0.5%–6% corresponds to an inner layer thickness of 15–180 μm. This ensures a 7-day release rate ≥15%, while avoiding excessive thickness that could lead to an initial burst release (burst release rate >30%). When the weight gain rate is 2.0% ± 0.2%, the MAP dissolution rate in deionized water at 25℃ reaches 98.3% in 5 minutes. Its beneficial effect is that the effective phosphorus concentration in the rhizosphere can be detected to be 0.8–1.2 mg / kg within 3 days after sowing, 4–5 days earlier than the control without an inner layer. Furthermore, the cumulative release rate over 7 days remains stable at 18%–22%, with a coefficient of variation ≤4.2%.

[0037] Optionally, in step S6, the intermediate reaction slow-release layer is generated by "spraying a salt-forming activation liquid containing Ca and / or Mg + sprinkling solid powder containing Ca and / or Mg" to form a slightly soluble phosphate multiphase, thereby forming a reaction shell, and is reacted and cured at 45-120℃ for 10-40 min, and the weight gain rate of the intermediate reaction slow-release layer is 1%-10%;

[0038] The slightly soluble phosphate complex phase includes a Ca-Mg complex phosphate phase and / or a Ca phosphate phase and / or a Mg phosphate phase.

[0039] Through the above technical solution, a micro-soluble phosphate multiphase reaction shell with controllable solubility is generated in situ on the interface activation layer by a three-step synergistic process of "liquid phase spraying + solid phase spreading + thermally induced reaction". Spraying an activation liquid containing Mg(NO3)2 and Ca(NO3)2 (solid content 8%–12%) provides ion precursors, while spreading fine dolomite powder (D90 < 45 μm) supplements the solid phase reaction source. Heat treatment at 45–120℃ promotes the decomposition of NO3⁻, generating a localized acidic microenvironment (pH 4.2–5.8), driving a quasi-solid phase reaction between Ca²⁺ / Mg²⁺ and PO4³⁻ from the fertilizer core. When the curing temperature is 90℃ and the time is 25 min, XRD and SEM-EDS confirm the formation of Ca–Mg–P–O. The quaternary composite phase (main diffraction peaks 2θ=25.9°, 31.8°) has a solubility (25℃, pH 6.5) of 0.018–0.023 g / 100 g H2O, which is between that of CaHPO4 (0.002 g / 100 g) and MgNH4PO4 (0.032 g / 100 g). The weight gain rate of 1%–10% corresponds to a reaction shell thickness of 30–300 μm. Its beneficial effect is that it increases the cumulative release rate over 30 days from 32% in the single-layer coated sample to 48%–55%, and reduces the release rate fluctuation range (dQ / dt standard deviation) to 0.15–0.21% / d, which is significantly better than the physical coating control (0.38% / d), confirming that the reaction shell has both sustained-release stability and phosphorus inhibition function.

[0040] Optionally, in step S6, the outer controlled-release layer comprises a biodegradable polymer coating material and / or a resin wax coating material and / or an inorganic-organic hybrid sol coating material, and the weight gain rate of the outer controlled-release layer is 1% to 12%.

[0041] The curing temperature of the outer controlled-release layer is 50–110°C, and the curing time is 10–60 min.

[0042] Through the above technical solution, by selecting three types of materials—biodegradable polymer (PBAT / starch blend), resin wax (microcrystalline wax C32–C50), or silica sol / acrylate hybrid sol—and matching their curing windows, an outer controlled-release barrier with gradient barrier properties is constructed. The PBAT / starch system (mass ratio 7:3) forms a microphase separation structure with a crystallinity of 38%–42% after curing at 80℃ for 30 min, with a water vapor transmission rate (WVTR) of 120–150 g·mm / (m²·d·kPa). The microcrystalline wax, after melt spraying at 65℃ and air-cooled curing at 55℃, forms a dense layered stacked structure with a WVTR of 85–110 g·mm / (m²·d·kPa). The silica sol / acrylate hybrid layer (SiO2 content 25wt%)... After curing at 95℃ for 20 min, a Si–O–C covalent network is generated with a WVTR of 60–80 g·mm / (m²·d·kPa). The wear resistance reaches 500 cycles of Taber abrasion with a weight loss of <0.8 mg. The weight gain rate of 1%–12% corresponds to a coating thickness of 20–480 μm. Under the premise of ensuring a release rate of ≥75% in 90 days, it avoids insufficient release in the later stage due to excessive thickness. Its beneficial effect is that it increases the cumulative release rate in 90 days to 78%–85%, and the average daily release rate in the final stage of release (60–90 days) is stable at 0.08%–0.12% / day, which is 2.3 times higher than the sample without outer layer. After 90 days of field soil burial test (pH 6.2, CEC 18 cmol(+) / kg), the particle integrity rate still reaches 91%–94%.

[0043] Optionally, in step S6, the process of one-time molding and coating of the gradient reaction shell is as follows: it is completed continuously in the same coating device in the order of "inner fast-release layer - intermediate reaction slow-release layer - outer controlled-release layer", and the interlayer mixing is controlled by spraying, powdering and curing rhythm.

[0044] Through the above technical solution, by integrating three independent feeding and temperature control modules into a single rotary drum coating machine and setting precise cycle timing, a spatiotemporal separation construction of the three-layer functional shell is achieved. The inner layer uses high-pressure atomization spraying (pressure 0.3MPa, spray distance 200mm), followed by 60℃ hot air pre-curing for 30s (surface dry only). The middle layer uses a composite mode of "spraying activation liquid + synchronous powder spreading", and immediately enters a 75℃ constant temperature zone for reaction curing for 25min after spraying. The outer layer uses low-pressure atomization spraying (pressure 0.15MPa) and is then transferred to a 9-layer rotary drum coating machine. Curing at 5℃ for 20 minutes; isolation air curtains (wind speed 1.2m / s) are set between each layer to prevent splashing and mixing; this integrated process controls the single batch coating cycle to ≤120 minutes and the CV value of the three-layer thickness to ≤5.8%. Its beneficial effect is to ensure the spatial independence and temporal controllability of the "fast-stable-controlled" release function, so that the RSD of the 7 / 30 / 90-day release rate of 10 batches of samples produced by the same equipment is ≤3.1%, ≤2.7%, and ≤2.4% respectively, meeting the stringent requirements of industrial continuous production for the consistency of release curves. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a flowchart illustrating a slow-release compound phosphate fertilizer preparation process based on multi-stage activation, as provided in one embodiment of this application. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0048] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0049] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0050] Example 1

[0051] according to Figure 1 The process flow is implemented as follows:

[0052] Weigh out 70.0 kg of phosphate rock powder (D50≈60μm), 8.0 kg of ammonium dihydrogen phosphate (MAP), 12.0 kg of dolomite powder, 1.0 kg of potassium humate, and 0.5 kg of sodium lignosulfonate, and mix them thoroughly to obtain a mixture. Place the mixture in a high-speed mixer, spray with a 2.0 wt% citric acid aqueous solution (material-to-liquid ratio 0.12 kg / kg), mix for 15 min, and adjust the pH of the system to 4.8 to obtain a wet mixture. Spread the wet mixture evenly on a tray and pre-dry it in a 60℃ hot air drying oven for 20 min to obtain a leaching and activation material. Add a mixture of magnesium oxide and calcium hydroxide (to make the Mg / P molar ratio = 0.25 and the Ca / P molar ratio = 0.80) to the leaching and activation material, then add 0.35 kg of sodium polyacrylate (0.5% based on the mass of the sparingly soluble phosphorus source), mix for 15 min, and obtain a secondary activation mixture. The secondary activated mixture was fed into a rotary drum granulator, and an 8wt% dextrin aqueous solution was sprayed in as a binder. The final moisture content of the granulation was controlled at 10.2%, and the granulation temperature was 35℃. Fertilizer core granules with a particle size of 3.2-3.8 mm were obtained. After sieving, granules with a size of 3.5±0.3 mm were taken for later use. The fertilizer core granules were transferred to a rotary drum coating machine and sprayed with an ethanol-water mixture (volume ratio 3:1) containing KH-550 (1.5wt%) and sodium polyacrylate (0.8wt%). The weight gain rate of the interface activation layer was controlled at 0.62%. Then, it was cured at 70℃ hot air for 10 min to obtain interface-activated fertilizer cores. Three layers of coating were continuously applied in the same rotary drum coating machine: ① Inner fast-release layer – spraying MAP fine powder (D90 < 10 μm) water slurry (solid content 25%), controlling the weight gain rate at 2.05%, and air-drying at room temperature for 30 seconds; ② Intermediate reaction slow-release layer – first spraying an activation liquid containing Mg(NO3)2 and Ca(NO3)2 (Mg²⁺:Ca²⁺ = 1:2, total solid content 10%), then sprinkling dolomite fine powder (D90 < 45 μm), controlling the total weight gain rate at 4.12%, and then curing in a 90℃ constant temperature zone for 25 minutes; ③ Outer controlled-release layer – spraying PBAT / starch (7:3) chloroform solution (solid content 8%), controlling the weight gain rate at 5.03%, and then curing in an 80℃ hot air zone for 30 minutes. The coated particles were then transferred to a fluidized bed dryer and dried at 60℃ until the moisture content was ≤ 3.2%. After cooling, the particles were sieved to obtain a product with a target particle size of 2.8–4.2 mm. The finished product granules were tested and found to have a compressive strength of 44.8 ± 1.3 N / granule; the cumulative release rates after 7 days, 30 days, and 90 days were 20.3%, 49.7%, and 81.6%, respectively; XRD patterns showed characteristic peaks of the Ca–Mg–P–O composite phase (2θ = 25.9°, 31.8°) and PBAT crystallization peaks (2θ = 21.5°); SEM images showed a clear three-layer structure, with an interface activation layer thickness of approximately 220 nm, an intermediate reaction layer thickness of approximately 125 μm, and an outer coating thickness of approximately 180 μm.

[0053] Example 2

[0054] Under the same preparation conditions as in Example 1, only the powder refinement target of the phosphate rock powder in step S1 was adjusted from D50≈60μm to D50=20μm (obtained by air jet milling) to prepare a slow-release compound phosphate fertilizer. The results showed that the product still had good granulation performance (yield 94.2%) and coating suitability. The cumulative release rates after 7 days, 30 days, and 90 days were 18.7%, 47.5%, and 78.3%, respectively, and the particle compressive strength was 42.1±1.6 N / particle, proving that the technical solution of the present invention can still be implemented and maintain stable release performance under D50=20μm.

[0055] Example 3

[0056] Under the same preparation conditions as in Example 1, only the concentration of citric acid solution in step S2 was adjusted from 2.0 wt% to 0.5 wt%, while the other parameters remained unchanged, to obtain a slow-release compound phosphate fertilizer. The results showed that the product had a slightly lower degree of dissolution activation, but XRD still showed a significant broadening of the hydroxyapatite (002) peak. The cumulative release rates after 7 days, 30 days, and 90 days were 17.2%, 45.8%, and 76.9%, respectively, and the particle compressive strength was 41.5 ± 1.4 N / particle. This demonstrates that the technical solution of this invention is still feasible under the lower limit of the activation medium concentration.

[0057] Example 4

[0058] Under the same preparation conditions as in Example 1, only the Mg / P molar ratio in step S3 was adjusted from 0.25 to 0.05 (lower limit), while the Ca / P molar ratio remained unchanged at 0.80, to obtain a slow-release compound phosphate fertilizer. The results showed that the proportion of Mg phosphate phase decreased and the proportion of Ca phosphate phase increased in the intermediate reaction slow-release layer of this product. The cumulative release rates after 7 days, 30 days, and 90 days were 19.1%, 46.3%, and 77.4%, respectively, and the particle compressive strength was 43.0 ± 1.2 N / particle. This demonstrates that even under the boundary condition of Mg / P = 0.05, the technical solution of this invention can still achieve the expected slow-release effect.

[0059] Example 5

[0060] Under the same preparation conditions as in Example 1, only the granulation moisture content in step S4 was adjusted from 10.2% to 6.0% (lower limit) to obtain a slow-release compound phosphate fertilizer. The results showed that the sphericity of the fertilizer core particles decreased slightly (0.85), but after interface activation in S5, it still met the coating requirements. The cumulative release rates after 7 days, 30 days, and 90 days were 18.9%, 46.8%, and 78.1%, respectively, and the particle compressive strength was 40.3 ± 1.5 N / particle. This demonstrates that the technical solution of this invention still possesses process robustness under the lower limit granulation moisture condition.

[0061] Example 6

[0062] Under the same preparation conditions as in Example 1, only the weight gain rate of the interface activation layer in step S5 was adjusted from 0.62% to 0.10% (lower limit) to obtain a slow-release compound phosphate fertilizer. The results showed that the interfacial anchoring density of the product decreased, but the three-layer shell could still stably coat the surface. After 10 freeze-thaw cycles, the peel strength was 4.02 N / mm, and the cumulative release rates at 7 days, 30 days, and 90 days were 19.5%, 48.2%, and 79.3%, respectively. The particle compressive strength was 42.7 ± 1.3 N / particle, proving that the technical solution of this invention can still maintain its basic function under the lower limit condition of the interface activation layer weight gain rate.

[0063] Example 7

[0064] Under the same preparation conditions as in Example 1, only the weight gain rate of the inner fast-release layer in step S6 was adjusted from 2.05% to 0.50% (lower limit) to obtain a slow-release compound phosphate fertilizer. The results showed that the initial phosphorus supply capacity of the product was weakened, with the release rate dropping to 15.4% after 7 days. However, the release rates after 30 days and 90 days were 47.9% and 78.7%, respectively, and the particle compressive strength was 43.2 ± 1.1 N / particle. This demonstrates that even under the lower limit condition of the inner layer weight gain rate, the present invention can still achieve the basic function of segmented release.

[0065] Example 8

[0066] Under the same preparation conditions as in Example 1, only the weight gain rate of the intermediate reaction slow-release layer in step S6 was adjusted from 4.12% to 1.00% (lower limit) to obtain a slow-release compound phosphate fertilizer. The results showed that the thickness of the intermediate reaction layer was reduced, but the Ca–Mg–P–O composite phase was still detected by XRD. The cumulative release rates after 7 days, 30 days, and 90 days were 19.8%, 44.6%, and 75.2%, respectively, and the particle compressive strength was 41.9 ± 1.4 N / particle. This demonstrates that the technical solution of this invention still possesses a slow-release function under the lower limit condition of the intermediate layer weight gain rate.

[0067] Example 9

[0068] Under the same preparation conditions as in Example 1, only the weight gain rate of the outer controlled-release layer in step S6 was adjusted from 5.03% to 1.00% (lower limit) to obtain a slow-release compound phosphate fertilizer. The results showed that the outer layer barrier capacity of the product decreased, and the release rate after 90 days dropped to 74.8%, but the release rates after 7 days and 30 days were 20.1% and 48.5%, respectively, and the particle compressive strength was 42.4±1.2 N / particle, proving that the present invention can still maintain basic controlled-release performance under the lower limit condition of outer layer weight gain rate.

[0069] Example 10

[0070] With all other preparation conditions the same as in Example 1, only the curing temperature in step S6 was uniformly adjusted from 90℃ (intermediate layer) and 80℃ (outer layer) to 45℃ to obtain a slow-release composite phosphate fertilizer. The results showed that the intermediate layer reaction was insufficient, the peak intensity of the Ca–Mg–P–O composite phase in XRD decreased by 42%, and the cumulative release rates after 7 days, 30 days, and 90 days were 21.5%, 42.3%, and 70.6%, respectively. The particle compressive strength was 39.8±1.6 N / particle. This demonstrates that the technical solution of this invention can still be implemented under low-temperature curing conditions, although the release performance is slightly reduced, verifying the rationality of the 45℃ lower limit value in this application.

[0071] Example 11

[0072] With all other preparation conditions the same as in Example 1, only the gradient reaction shell coating order in step S6 was changed from "inner layer—middle layer—outer layer" to "outer layer—middle layer—inner layer," while all other parameters remained unchanged, to obtain a control sample. The results showed that because the outer layer acted as a barrier, the inner MAP layer could not contact moisture, resulting in a release rate of only 3.2% after 7 days, 38.7% after 30 days, and 65.4% after 90 days. Furthermore, the particles exhibited significant shell cracking during transport, demonstrating that the order specified in this application is irreplaceable for achieving the segmented release function.

[0073] Example 12

[0074] Under the same preparation conditions as in Example 1, only the interface activation solution in step S5 was replaced with pure water (without KH-550 and sodium polyacrylate), while all other parameters remained unchanged, to obtain a comparative sample. The results showed that the bonding force between the three-layer shell and the core of this sample was significantly reduced, and visible peeling occurred after 5 mild vibrations, with a peel strength of only 1.35 N / mm. The cumulative release rates at 7 days, 30 days, and 90 days were 22.1%, 40.5%, and 63.8%, respectively, and the particle compressive strength was 33.2 ± 2.1 N / particle, proving that interface activation plays a crucial role in ensuring the integrity of the overall structure.

[0075] Example 13

[0076] With all other preparation conditions the same as in Example 1, only the complexation regulating component in step S3 was omitted (i.e., potassium humate and sodium polyacrylate were not added), and the remaining parameters remained unchanged, a comparative sample was prepared. The results showed that the phosphorus fixation rate of this sample increased to 35.6% in simulated soil solution, the release rate decreased to 39.2% after 30 days, and the release rate was 64.1% after 90 days. XRD showed the formation of a large amount of amorphous CaHPO4 phase, confirming that the complexation regulating component plays a decisive role in inhibiting ineffective precipitation and guiding oriented phase formation.

[0077] Example 14

[0078] With all other preparation conditions the same as in Example 1, only the pre-drying temperature in step S2 was adjusted from 60℃ to 90℃ (the upper limit), and the pre-drying time was adjusted from 20 min to 10 min, to obtain a slow-release compound phosphate fertilizer. The results showed that the product experienced significant loss of surface-bound water, but an effective complex intermediate could still be formed in step S3. The cumulative release rates after 7 days, 30 days, and 90 days were 18.4%, 46.1%, and 77.0%, respectively, and the particle compressive strength was 41.0 ± 1.5 N / particle. This demonstrates that the technical solution of this invention remains feasible even under the upper limit of the pre-drying parameters.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A process for preparing a slow-release compound phosphate fertilizer based on multi-stage activation, characterized in that, include: S1, Raw Materials Pretreatment: Provide a phosphorus-based raw material containing a sparingly soluble phosphorus source, and perform powder refinement treatment on the phosphorus-based raw material to obtain pretreated phosphorus material; S2, Dissolution Activation: The pretreated phosphorus material is mixed with a weak acid or organic acid activation medium to cause local dissolution on the surface of the phosphorus-based raw material and form reactive active sites. After pre-drying, the dissolved activated material is obtained. S3, Complexation / Salt Formation Activation: Salt formation regulating components containing Ca and / or Mg and complexation regulating components are added to the leaching activation material to form ion-bridged or complexed intermediates and to construct a salt formation reaction system for subsequent shell formation, thereby obtaining a secondary activation mixture. S4. Granulation and nucleation: Granulate the secondary activated mixture to obtain fertilizer core particles; S5. Interface activation: The surface of the fertilizer core particles is subjected to interface activation treatment to form an interface activation layer on the surface of the fertilizer core particles for shell anchoring and nucleation. S6. Gradient reaction shell one-time molding and coating: An inner fast-release layer, an intermediate reaction slow-release layer and an outer controlled-release layer are sequentially constructed on the surface of the fertilizer core particles; S7. Drying and granulation: The coated granules are dried and sieved to obtain the slow-release compound phosphate fertilizer product. The multi-stage activation includes dissolution activation, complexation / salt formation activation, and interface activation, and the gradient reaction shell is used to achieve the segmented release of the finished product.

2. The preparation process according to claim 1, characterized in that, In step S1, the insoluble phosphorus source includes one or more of the following: phosphate rock powder, phosphate rock associated phosphorus, phosphate slag phosphorus, and insoluble phosphate. The goal of the powder refining process is to control the particle size D50 of the pretreated phosphate material within the range of 20–120 μm.

3. The preparation process according to claim 1, characterized in that, In step S2, the weak acid or organic acid activating medium is selected from one or more of citric acid, humic acid, lactic acid, tartaric acid and their salts; The mixing process of the pretreated phosphate material with the weak acid or organic acid activation medium is carried out by spraying and mixing. The ratio of the activation medium to the pretreated phosphate material is 0.05 to 0.25 kg / kg, and the pH of the activation system is controlled to be 3.0 to 6.

0. The pre-drying conditions include: a pre-drying temperature of 50–90°C and a pre-drying time of 10–40 min.

4. The preparation process according to claim 1, characterized in that, In step S3, the salt formation regulating component includes one or more of magnesium oxide, magnesium hydroxide, dolomite powder, limestone powder, and calcium hydroxide. The amount of the salt-forming regulating component added satisfies the following conditions: the Ca / P molar ratio is 0.3 to 2.0 and / or the Mg / P molar ratio is 0.05 to 0.8, wherein P is calculated based on the phosphorus content in the leaching activation material; The complexing regulating component includes humate and / or polycarboxylic acid polymer, and the amount of the complexing regulating component added is 0.2% to 3% of the mass of the insoluble phosphorus source.

5. The preparation process according to claim 1, characterized in that, In step S4, the granulation objectives include: a granulation moisture content of 6% to 14%, and a fertilizer core particle size of 2 to 5 mm.

6. The preparation process according to claim 1, characterized in that, In step S5, the interface activation process includes: An interface activation liquid is sprayed onto the surface of the fertilizer core particles and cured, wherein the interface activation liquid contains a silane coupling agent and / or a polycarboxyl polymer, and the weight gain of the interface activation layer is 0.1% to 1.5%. The curing temperature for the interface activation treatment is 40–90°C, and the curing time is 5–25 min.

7. The preparation process according to claim 1, characterized in that, In step S6, the inner fast-release layer contains a water-soluble phosphorus source, which is selected from one or more of ammonium dihydrogen phosphate, monoammonium phosphate, and potassium dihydrogen phosphate, and the weight gain rate of the inner fast-release layer is 0.5% to 6%.

8. The preparation process according to claim 1, characterized in that, In step S6, the intermediate reaction slow-release layer is generated by "spraying a salt-forming activation liquid containing Ca and / or Mg + sprinkling solid powder containing Ca and / or Mg" to form a slightly soluble phosphate multiphase, thereby forming a reaction shell, and is reacted and cured at 45-120℃ for 10-40 min, and the weight gain rate of the intermediate reaction slow-release layer is 1%-10%; The slightly soluble phosphate complex phase includes a Ca-Mg complex phosphate phase and / or a Ca phosphate phase and / or a Mg phosphate phase.

9. The preparation process according to claim 1, characterized in that, In step S6, the outer controlled-release layer comprises a biodegradable polymer coating material and / or a resin wax coating material and / or an inorganic-organic hybrid sol coating material, and the weight gain rate of the outer controlled-release layer is 1% to 12%. The curing temperature of the outer controlled-release layer is 50–110°C, and the curing time is 10–60 min.

10. The preparation process according to claim 1, characterized in that, In step S6, the process of one-time molding and coating of the gradient reaction shell is as follows: it is completed continuously in the same coating device in the order of "inner fast-release layer - intermediate reaction slow-release layer - outer controlled-release layer", and the interlayer mixing is controlled by spraying, powdering and curing rhythm.

Images