A production process of composite phosphorus fertilizer using industrial by-products

Abstract

The application relates to the technical field of composite phosphorus fertilizer, in particular to a composite phosphorus fertilizer production process using industrial by-products. The production process comprises the following steps: S1, providing an industrial by-product as a porous carrier raw material; drying treatment is performed on the porous carrier raw material to obtain a porous carrier; S2, at least two-stage directional passivation treatment is performed on the porous carrier; S3, in-hole synergistic activation and phosphorus release: after the in-hole segmented directional passivation is completed, a phosphorus source and a synergistic activator are introduced into the pore channel of the porous carrier to generate an in-hole phosphorus-containing composite phase; and S4, hole sealing and solidification: sealing and solidification materials are introduced into the intermediate of the in-hole phosphorus-containing composite phase, and solidification treatment is performed to obtain composite phosphorus fertilizer particles. Through a hierarchical reaction process of directional impurity capture, heavy metal stable solid loading, in-situ phosphorus activation and product physical and chemical double locking in the pore channel, the long-term environmental safety and nutrient release consistency of the product are improved.

Description

Technical Field

[0001] This application relates to the field of compound phosphate fertilizer technology, and in particular to a compound phosphate fertilizer production process utilizing industrial by-products. Background Technology

[0002] Fertilizer processing and resource utilization play a vital role in sustainable agricultural development, and the efficient utilization of industrial by-products is a key link in achieving resource recycling and environmental protection. In existing technologies, the preparation of compound phosphate fertilizers from industrial by-products mainly employs simple blending or conventional granulation processes. This involves physically mixing the by-products and phosphorus sources in a mixing device or forming granules using a granulator. This process includes basic steps such as drying, crushing, mixing, and granulation of raw materials. In this process, the industrial by-products directly participate in the mixing as raw material components, their pore structure is not specifically controlled, impurity treatment depends on the overall reaction environment, phosphorus source conversion mainly occurs on the particle surface or within the overall phase, and the final product achieves its slow-release function through a single outer coating.

[0003] However, in the existing technology, the interaction between impurities contained in industrial by-products and phosphorus sources is difficult to achieve structural control, resulting in uneven phase distribution within the product and limited stability of functional layers. Summary of the Invention

[0004] This application provides a compound phosphate fertilizer production process utilizing industrial by-products to solve the above-mentioned problems. The production process includes:

[0005] S1. Carrier preparation: Industrial by-products are provided as porous carrier raw materials, wherein the industrial by-products are selected from one or more of fly ash microspheres, fly ash, slag micro powder, red mud, and steel slag fine powder; the porous carrier raw materials are dried to obtain porous carriers;

[0006] S2. Segmented Directional Passivation within the Pore: The porous support undergoes at least two stages of directional passivation treatment. The first stage of directional passivation introduces a first passivating agent to the porous support for capturing fluoride ions and / or soluble anions, and controls the system within a first preset pH window, allowing the first passivating agent to enter the pores and form a sparingly soluble stationary phase with the target impurities. The second stage of directional passivation introduces a second passivating agent to the porous support for fixing heavy metal ions and / or multivalent ions, and controls the system within a second preset pH window, allowing the second passivating agent to enter the pores and form a solid phase through precipitation and / or adsorption and / or co-deposition.

[0007] S3. Synergistic activation and phosphorus release within pores: After completing the segmented directional passivation within the pores, a phosphorus source and a synergistic activator are introduced into the pores of the porous support. The reaction conditions are controlled to allow the phosphorus source to react synergistically with the Ca / Si / Al active phase in the porous support, generating a phosphorus-containing composite phase within the pores.

[0008] S4. Sealing and curing: A sealing and curing material is introduced into the intermediate that forms the phosphorus-containing composite phase in the pores and cured to form a continuous cured phase in the pores to lock the insoluble fixed phase, the supporting phase and the phosphorus-containing composite phase in the pores, thereby obtaining composite phosphate fertilizer particles.

[0009] The above technical solution constructs a porous carrier microreactor based on industrial by-products, and sequentially completes a hierarchical reaction process within the pores: directional impurity capture, heavy metal stabilization, in-situ phosphorus activation, and dual physicochemical locking of the product. This ensures that each step is spatially constrained by the pore structure, has a clear temporal sequence, and is chemically controlled by segmented pH adjustments. This solves the problems of reaction interference, weak fixation, and uncontrollable release caused by the coexistence of impurities and phosphorus sources in existing technologies. Its beneficial effect is that the first-stage passivation preferentially forms CaF2, MgF2, or rare earth fluorine complexes under near-neutral conditions, avoiding… Under strong alkaline conditions, HF volatilizes, and under acidic conditions, heavy metals dissolve. In the second stage, under alkaline conditions, heavy metals are stabilized and precipitated in the form of hydroxides, phosphates, or sulfides. At the same time, silicates and biochar loading agents synergistically enhance the adsorption-coprecipitation effect. The phosphorus source undergoes a mineralization coupling reaction with the Ca / Si / Al active phase in the passivated pores to generate hydroxyapatite or calcium aluminum phosphate composite phases, improving phosphorus stability and local enrichment. The pore-sealing and curing material forms a continuous inorganic network at the pore opening and shallow pore walls, which both physically blocks and chemically bonds the internal phases, fundamentally inhibiting migration paths and significantly improving the long-term environmental safety and nutrient slow-release consistency of the product.

[0010] Optionally, the phosphorus source is one or more of wet-process phosphoric acid, monoammonium phosphate, diammonium phosphate, phosphate solution, or solid phosphorus source slurry;

[0011] The synergistic activator is one or more of alkali metal salts, alkaline earth metal salts, silica sols, alumina sols, or alkaline activators.

[0012] The sealing and curing material is one or more of the following: silica sol, alumina sol, silicate solution, phosphate gelation system, or geopolymer gelation system.

[0013] Through the above technical solution, by introducing this further defined technical feature, the selection of phosphorus source can take into account both resource utilization (such as wet-process phosphoric acid) and reactivity (such as the high solubility of monoammonium phosphate). The synergistic activator can regulate the local ionic strength or provide Si / Al precursors to promote the interfacial nucleation and crystal growth of phosphorus and Ca / Si / Al phases based on the difference in surface charge and hydrophilicity of the carrier. The sealing material is matched with different production cycles according to its gelation temperature, polycondensation rate and mechanical strength. For example, silica sol is suitable for low-temperature rapid coating, while geopolymer system is suitable for high-temperature and high-humidity curing lines. Its synergistic effect is reflected in the fact that silica sol can participate in the formation of phosphate rock phase as a synergistic activator, and can also be used as a sealing material in the later stage to achieve "one material for two purposes", reduce process switching and material replacement, reduce process complexity, and thus improve the efficiency of industrial implementation and equipment compatibility while ensuring the reaction depth.

[0014] Optionally, in step S1, the drying temperature of the drying process is 80–140°C, and the drying time is 0.5–3 hours.

[0015] The porous carrier is a powder carrier or a particulate carrier. The D50 of the powder carrier is 10-150 μm, and the particle size of the particulate carrier is 0.5-4 mm.

[0016] Through the above technical solutions, by setting the drying temperature and time range, the integrity of the microporous structure inside the carrier is preserved while removing free water, preventing the collapse of the fly ash microsphere walls or the deactivation of hydrated aluminosilicates in red mud due to dehydroxylation caused by high temperature. The D50 of the powder carrier is controlled in the range of 10 to 150 μm, which ensures sufficient specific surface area to support high-density passivation and phosphorus release reactions, while avoiding the obstruction of liquid penetration by excessively fine powder due to electrostatic agglomeration during spray wetting. The granular carrier is limited to 0.5 to 4 mm, making it suitable for continuous equipment such as drum dynamic reaction and fluidized bed coating. Within this particle size range, the pore mass transfer distance and reaction kinetics are well matched, ensuring that the passivating agent and phosphorus source diffuse fully into the pore depth region within a limited time. This combination of size and heat treatment parameters ensures the structural stability and functional repeatability of the porous carrier as a "micro-reaction platform".

[0017] Optionally, in step S2, the segmented directional passivation inside the hole is carried out by one or more of the following methods: spray wetting, impregnation or vacuum impregnation, so that the first passivating agent and / or the second passivating agent enter the hole channel;

[0018] The first preset pH window for the first segment of directional passivation is 6 to 9, and the first passivating agent is selected from one or more of calcium source, magnesium source, rare earth salt, and aluminum source;

[0019] The second preset pH window for the second stage of directional passivation is 8 to 11, and the second passivating agent is selected from one or more of phosphates, silicates, iron salts, sulfide precursors, and biochar support agents.

[0020] The above technical solution overcomes capillary resistance by employing spray wetting or vacuum impregnation methods, allowing the passivating agent solution to penetrate the dense outer shell of the carrier and reach deep into the pores under negative pressure or atomization kinetic energy, thus improving the spatial uniformity of the passivation reaction. In the first stage, within the pH window of 6–9, Ca²⁺ and F⁻ directly generate Ksp = 3.9 × 10⁻⁶. -11 CaF2 precipitate, Mg²⁺ forms Ksp = 6.4 × 10⁻⁶ -9 The first stage involves MgF2, while rare earth ions (such as La³⁺) form stable complexes with F⁻ through coordination. This pH range avoids the dissolution of heavy metals under strong acids and the loss of HF under strong bases. In the second stage, within the pH window of 8–11, PO₄³⁻ forms phosphate precipitates with extremely low solubility with Cd²⁺, Pb²⁺, etc. Fe³⁺ hydrolyzes to generate Fe(OH)₃ colloids that encapsulate heavy metal particles. SiO₃²⁻ and Al³⁺ synergistically enhance the adsorption-coprecipitation network. Sulfide precursors (such as Na₂S) release S²⁻ to generate ultra-low solubility product sulfides such as PbS. Biochar support provides abundant oxygen-containing functional groups as adsorption sites. Each passivating agent plays a major role at specific pH levels and inhibits cross-interference between different reactions through the pore confinement effect, thereby achieving the partitioning, directionality, and efficient stabilization of multiple pollutants within a single carrier particle.

[0021] Optionally, in step S3, the synergistic activation and release of phosphorus within the pores adopts either an impregnation-in-situ reaction route or a spray-drum dynamic reaction route.

[0022] The impregnation-in-situ reaction route includes: preparing the phosphorus source into a solution or slurry and performing vacuum impregnation or spray impregnation, so that it enters the pores and is then aged at 40-90°C for 0.5-6 hours to generate the phosphorus-containing composite phase inside the pores;

[0023] The spray-drum dynamic reaction route includes: continuously spraying the phosphorus source and the synergistic activator during drum granulation or drum reaction, controlling the material temperature to 30-80℃ and the drum reaction time to 10-60min, and performing wet heat curing after discharge to promote the formation of the phosphorus-containing composite phase in the pores.

[0024] Through the above technical solutions, by setting two complementary reaction paths, the process can be adapted to different scales and equipment conditions: The impregnation-in-situ route utilizes vacuum negative pressure to drive the phosphorus source solution deep into the micropores. Under gentle heating at 40–90℃, it promotes the dissolution of H⁺ in wet-process phosphoric acid, releasing Ca²⁺ from the CaO surface of the carrier. Ca²⁺ then co-precipitates with PO₄³⁻ and SiO₂ precursors provided by silica sol, generating a composite mineral phase similar to hydroxyapatite or calcium aluminum phosphate. This process is controlled by the local concentration gradient within the pores and the crystal nucleus growth rate; The spray-rolling route… The cylindrical route relies on material agitation to enhance solid-liquid contact. Within a material temperature of 30–80°C and a dynamic reaction time of 10–60 minutes, the phosphorus source and activator are rapidly distributed on the particle surface and shallow pores. After discharge, wet heat curing further promotes the reaction to extend into the deeper pore regions and completes crystal densification. Both routes control the reaction temperature and time window to ensure that phosphorus completes directional transformation within the pores after passivation, avoiding external crusting that obscures active sites, thereby achieving "homogeneous" phosphorus fixation and structural uniformity at the macroscopic particle scale.

[0025] Optionally, in step S3, the phosphorus source is wet-process phosphoric acid, and the synergistic activator includes silica sol and / or aluminum sol, so as to promote the synergistic reaction between the phosphorus source and the Ca / Si / Al active phase in the porous support to generate the phosphorus-containing composite phase in the pores.

[0026] Through the above technical solution, by limiting the combination of wet-process phosphoric acid and silica sol / alumina sol, the weak acidity (pH≈1.5~2.5) of wet-process phosphoric acid is fully utilized to activate components rich in CaO, SiO2, and Al2O3, such as fly ash and slag. H⁺ preferentially dissolves surface Ca²⁺, forming localized high-concentration Ca²⁺ micro-regions. At the same time, silica sol partially depolymerizes into Si(OH)4 monomers in the acidic environment, which, together with PO4³⁻ and Ca²⁺, participate in interfacial nucleation to generate Ca. –Si–P ternary composite phase; aluminum sol provides Al³⁺, which combines with PO₄³⁻ to form AlPO₄ precipitate during the near-neutral ripening stage, and coexists with Ca–Si–P phase on the pore wall to form a multiphase synergistic stable structure; this combination not only improves the phosphorus fixation rate, but also realizes the resource utilization of waste acid and reduces raw material costs. Its synergistic mechanism is manifested as: acid activation—ion release—multinuclear coprecipitation—structural interlocking, thereby constructing a functionalized composite phase with both high phosphorus content and low leaching risk in the pores.

[0027] Optionally, in step S4, the sealing and curing material includes an inorganic sol system, wherein the inorganic sol system is silica sol and / or aluminum sol, and the sealing method is one or more of spray coating, dip-coating or fluidized bed atomization.

[0028] The sealing and curing process is carried out at a temperature of 40–120°C for 0.5–4 hours.

[0029] The above technical solution utilizes nanoscale silica sol (particle size 10–20 nm) or alumina sol (particle size 5–15 nm) as sealing materials. Their small size and high surface hydroxyl density allow them to uniformly adhere to the particle surface during spraying or fluidized bed atomization, penetrating along the pore openings into shallow channels. Under low-temperature conditions of 40–120°C, dehydration and condensation occur, forming a Si–O–Si or Al–O–Al three-dimensional network structure. This network not only physically blocks the pore outlets but also forms interfacial bonds with the already formed insoluble stationary phase, support phase, and phosphorus-containing composite phase within the pores through chemical bonds such as Si–O–Ca and Al–O–P, enhancing the overall structural cohesion. Spray coating is suitable for powder-type carriers, while fluidized bed atomization is suitable for particulate carriers; both can achieve full coverage without damaging the internal structure of the pores. This sealing method and parameter combination ensures sealing efficiency while avoiding phosphorus phase decomposition or organic coating layer carbonization caused by high-temperature sintering, thus providing a structural basis and interfacial compatibility for subsequent external functional coating.

[0030] Optionally, step S4 also includes an outer coating step: applying an outer functional coating to the sealed and cured composite phosphate fertilizer particles to form a controlled-release membrane layer.

[0031] The controlled-release membrane layer has a bilayer or composite layer structure, comprising an inorganic dense inner layer and an organic outer layer.

[0032] Through the above technical solution, a layered protection system of "inner sealing + outer controlled release" is constructed by adding a double-layer controlled-release membrane outside the pore-sealing solidified phase: the inorganic dense inner layer acts as the first barrier, resisting soil moisture erosion and ion reverse osmosis, and inhibiting the slow depolymerization of the pore-sealing phase under long-term leaching; the organic outer layer acts as the second responsive barrier, and its hydrophilicity (such as alginate, chitosan) or hydrophobicity (such as wax, resin) determines the rate of water entry, thereby regulating the dissolution and release kinetics of the internal phosphorus; this double-layer structure maintains the mechanical strength of the particles and endows the release behavior with environmental responsiveness through the complementary physicochemical properties of the inner and outer layer materials, thereby extending the fertilizer's effective period while avoiding initial burst release and ineffective residue in the later stage, significantly improving agricultural efficiency and ecological safety.

[0033] Optionally, the inorganic dense inner layer is selected from one or more of silica sol coating, silicate coating, and phosphate coating, and the organic outer layer is selected from one or more of alginate, chitosan, polyvinyl alcohol (PVA) modified polymer, bio-based polymer, wax, or resin.

[0034] Through the above technical solutions, by limiting the inorganic inner layer to silica sol, silicate, or phosphate coating, a glassy, ​​dense film is formed after curing, exhibiting excellent hydrolysis resistance and ion barrier capabilities. The phosphate coating can also simultaneously provide trace amounts of soluble phosphorus to supplement nutrients. When the organic outer layer is selected from alginate or chitosan, it swells in water to form a gel layer, slowing down the rate of water penetration into the inner layer. When PVA-modified polymers or bio-based polymers are selected, the degradation cycle can be controlled by the crosslinking density. When waxes or resins are selected, a strong hydrophobic barrier is provided, making it suitable for arid and low-rainfall areas. The various materials can form a controllable thickness (1-5 μm), continuous, pinhole-free composite film layer in fluidized bed or roller spraying. The interfacial adhesion between the film layer and the inner sealing phase is enhanced by hydrogen bonds or chemical bonds, thereby maintaining the integrity of the film layer and the stability of the controlled-release function under multiple environmental stresses.

[0035] Optionally, the outer coating is applied by fluidized bed spraying or roller spraying, and after coating, it is cured at 40-60°C for 12-72 hours to promote cross-linking and curing of the sealing phase and / or controlled-release membrane layer.

[0036] The above technical solutions, employing fluidized bed or roller spraying methods, ensure uniform coverage of the controlled-release membrane on the particle surface, avoiding uneven release due to excessive thickness or premature rupture due to insufficient thickness. Gentle heating at 40–60℃ combined with long-term curing for 12–72 hours accelerates the dehydration and polymerization of the silica sol inner layer and the densification of the Si–O–Si network. Simultaneously, it promotes organic phase crosslinking reactions such as alginate–Ca²⁺ crosslinking, chitosan–phosphate complexation, or PVA molecular chain entanglement, forming a stable physical and chemical bonding interface between the inner and outer layers. These curing conditions eliminate the need for high-temperature calcination, are fully compatible with the previous low-temperature sealing process, and are suitable for integration into continuous production lines, thereby significantly reducing energy consumption and equipment investment while ensuring membrane performance. Attached Figure Description

[0037] 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.

[0038] Figure 1 This is a flowchart of a compound phosphate fertilizer production process utilizing industrial by-products, provided as an embodiment of this application. Detailed Implementation

[0039] 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.

[0040] 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.

[0041] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0042] Example 1

[0043] according to Figure 1 The production process shown shall be implemented as follows:

[0044] Using fly ash microspheres as porous carrier raw material: 100 kg of fly ash microspheres were placed in a constant temperature forced-air drying oven and dried at 110℃ for 1 h. After cooling, the moisture content was measured to be <0.5 wt%, D50 was 42 μm, and BET specific surface area was 12.6 m² / g.

[0045] Segmented directional passivation within S2 pores: The dried support was placed in a stirred reactor, and deionized water was added to prepare a suspension with a solid-liquid ratio of 1:3. First passivation: Calcium nitrate solution (Ca²⁺ final concentration 0.15 mol / L) was added, and the pH of the system was adjusted to 7.8 ± 0.2 with ammonia. Spray wetting was carried out continuously for 30 min using a spray wetting method (atomization pressure 0.3 MPa, nozzle flow rate 15 mL / min), followed by maturation at 70℃ for 1 h. Second passivation: Sodium hexametaphosphate solution (PO₄³⁻ final concentration 0.08 mol / L) and ferrous sulfate solution (Fe²⁺ final concentration 0.05 mol / L) were added, and the pH was adjusted to 9.5 ± 0.3 with NaOH solution. Spray wetting was continued for 30 min, followed by maturation at 70℃ for 1 h. After centrifugation, the filtrate was washed with deionized water until the conductivity was < 50 μS / cm, and then vacuum dried at 60℃ to constant weight to obtain the passivated support.

[0046] S3 pore synergistic activation of phosphorus release: The passivated support was placed in a vacuum impregnation device, evacuated to −0.095MPa, and held for 15 min. Then, a mixture of wet phosphoric acid and silica sol (P2O5:SiO2 molar ratio = 1.2:1) was slowly injected with a liquid-solid ratio of 1:2. The mixture was impregnated at normal pressure for 30 min. Subsequently, it was transferred to an oven and aged at 60℃ for 3 h to obtain the intermediate.

[0047] S4 Sealing and Curing: The intermediate is placed in a fluidized bed, with silica sol (SiO2 content 30wt%) as the sealing material, atomization pressure 0.25MPa, inlet air temperature 60℃, and coating time 20min; after discharge, it is cured in an 80℃ oven for 2h.

[0048] S5 outer functional coating: After sealing the pores, the particles are placed back into the fluidized bed. First, a silica sol inner layer (solid content 15wt%, atomization pressure 0.2MPa) is sprayed, followed by a sodium alginate solution (2wt%, containing 0.02mol / L CaCl2 crosslinking agent). The total coating amount is 3.5wt% of the particle mass. After discharge, the particles are cured in a constant humidity chamber at 50℃ and 75% RH for 48h.

[0049] S6 Finished Product Shaping: Sieve to a particle size of 1.0–3.0 mm to obtain compound phosphate fertilizer granules. XRD analysis showed that the product contained hydroxyapatite (PDF#09-0432), fluorapatite (PDF#15-0876), and amorphous SiO2 phase; SEM showed that the particle surface was dense and the pores were covered by a continuous film layer; ICP-MS analysis showed that the total phosphorus content was 12.3 wt%, the water-soluble phosphorus content was 28.6%, and the available phosphorus (extracted from neutral ammonium citrate) was 9.7 wt%; the fluorine leaching concentration was <0.15 mg / L (GB / T23349–2021), and the Cd, Pb, and As leaching concentrations were <0.01, <0.05, and <0.02 mg / L, respectively (HJ / T299–2007).

[0050] Example 2

[0051] With all other preparation conditions the same as in Example 1, only the drying temperature in step S1 was adjusted from 110℃ to 80℃, and the drying time was extended to 3 hours, to obtain fly ash microsphere carriers. The results showed that the carrier had a specific surface area of ​​13.2 m² / g and a pore volume of 0.028 cm³ / g, still meeting the pore permeability requirements for subsequent passivation and phosphorus release reactions. The final composite phosphate fertilizer granules contained 9.4 wt% effective phosphorus, 0.13 mg / L fluoride leaching concentration, and heavy metal leaching concentrations all met the standard limits, proving that the technical solution of this invention still has good feasibility under the lower limit of the drying temperature.

[0052] Example 3

[0053] With all other preparation conditions the same as in Example 1, only the pH window for the first stage of directional passivation in step S2 was adjusted from 7.8±0.2 to 6.0±0.2, while the other conditions remained unchanged. The results showed that CaF2 precipitate could still form under these conditions, but the reaction rate decreased slightly, and the aging time needed to be extended to 1.5 h. The fluorine fixation rate in the final product reached 98.2%, slightly lower than that in Example 1 (99.1%), but still above 95%, and no heavy metal leaching was observed. The effective phosphorus content was 9.5 wt%, proving that the present invention can still achieve directional fluorine capture and stable coexistence of heavy metals within the lower pH limit window.

[0054] Example 4

[0055] With all other preparation conditions the same as in Example 1, only the curing temperature in step S3 was adjusted from 60℃ to 90℃, and the curing time was shortened to 0.5h. The results showed that high temperature accelerated the reaction kinetics between the phosphorus source and the active phase of the support. XRD showed increased crystallinity of hydroxyapatite, but excessive condensation of some silica sol led to localized pore blockage. The effective phosphorus content was 9.9wt%, and the proportion of water-soluble phosphorus decreased to 22.4%, indicating that a phosphorus-containing composite phase can still be generated under the upper temperature limit, and the release rate is further reduced, which is beneficial for long-term sustained release.

[0056] Example 5

[0057] With all other preparation conditions the same as in Example 1, only the sealing and curing temperature in step S4 was adjusted from 80℃ to 40℃, and the curing time was extended to 4 hours. The results showed that low-temperature, long-time curing still achieved sufficient polycondensation of the silica sol. SEM showed that the sealing layer was continuous and intact, with a thickness of approximately 2.3 μm. The fluorine leaching concentration of the product was 0.14 mg / L, and the Cd leaching concentration was <0.01 mg / L, demonstrating that the sealing effect was not significantly degraded under the lower limit of the curing temperature, and the process exhibited good low-temperature adaptability.

[0058] Example 6

[0059] With all other preparation conditions the same as in Example 1, only the outer layer coating and curing temperature in step S5 was adjusted from 50℃ to 40℃, and the curing time was extended to 72h. The results showed that under low-temperature, long-term curing, the sodium alginate–Ca²⁺ crosslinking was more complete, the tensile strength of the membrane increased by 18%, and the soil column leaching test showed that the cumulative phosphorus release rate after 7 days decreased from 14.2% in Example 1 to 11.5%, with a flatter release curve, confirming that this parameter combination can further optimize the controlled-release performance.

[0060] Example 7

[0061] Under the same preparation conditions as in Example 1, only the powder carrier in step S1 was replaced with slag micronized granules: 100 kg of slag micronized powder was taken, and 8 wt% water glass solution (modulus 3.3) was added as a binder. Granulation was carried out in a drum granulator, with a rotation speed controlled at 25 r / min and a granulation time of 20 min. The resulting particles were dried at 110℃ for 1 h and then sieved to obtain particles with a diameter of 1.0–3.0 mm, a bulk density of 1.25 g / cm³, and a porosity of 38.5%. Subsequent passivation, phosphorus release, pore sealing, and coating were completed according to the process in Example 1. The results showed that this granular carrier exhibited better material flowability and spray adhesion in the drum spraying-dynamic reaction route. The final product had an effective phosphorus content of 9.6 wt%, and the fluorine and heavy metal leaching concentrations both met the standards, proving that the present invention has good adaptability to different carrier morphologies.

[0062] Example 8

[0063] Under the same preparation conditions as in Example 1, only the reaction route in step S3 was changed from impregnation-in-situ reaction to spray-drum dynamic reaction: passivated fly ash microspheres were fed into a Φ600×2000mm drum reactor at a rotation speed of 20 r / min, and a mixture of wet phosphoric acid and silica sol (solid content 25wt%, P2O5:SiO2=1.2:1) was continuously sprayed in, with the material temperature controlled at 55℃ and the reaction time at 30 min; after discharge, the material was placed in a constant humidity chamber at RH85% and cured at 60℃ for 24 h. The results showed that the effective phosphorus content of the product obtained by this route was 9.3wt%, and XRD showed that the crystallinity of the phosphorus-containing composite phase was slightly lower than that of Example 1, but the distribution was more uniform; the fluorine fixation rate was 98.5%, and the heavy metal leaching concentration was not detected, proving that the spray-drum route can still stably achieve synergistic activation within the pores under industrial scale-up conditions.

[0064] Example 9

[0065] With all other preparation conditions the same as in Example 1, only the sealing and curing material in step S4 was replaced with aluminum sol (Al2O3 content 20wt%), while the other conditions remained unchanged. The results showed that the aluminum sol sealing layer formed a dense Al–O–Al network after curing at 80℃ for 2 hours. XPS analysis showed enhanced Al–O–P bond signals, indicating interfacial chemical interaction between the aluminum sol and the phosphorus-containing composite phase. The product had an effective phosphorus content of 9.5wt% and a fluorine leaching concentration of 0.12 mg / L, demonstrating that the aluminum sol also possesses excellent sealing capabilities and can enhance phosphorus chemical locking.

[0066] Example 10

[0067] With all other preparation conditions the same as in Example 1, only the outer organic layer in step S5 was replaced with a chitosan solution (2 wt%, pH 5.5) instead of sodium alginate, while the other conditions remained unchanged. The results showed that the chitosan membrane exhibited a high degree of protonation under acidic conditions, with a water absorption and swelling rate 23% higher than that of sodium alginate. Soil column leaching tests showed that the cumulative phosphorus release rate decreased to 10.8% after 7 days, with a more significant release delay effect, confirming that the release behavior of different organic materials can be controlled as needed.

[0068] Example 11

[0069] With all other preparation conditions the same as in Example 1, only the second passivating agent in step S2 was replaced by a combination of sodium hexametaphosphate and ferrous sulfate with biochar-supported Fe³⁺ (Fe loading 8wt%, specific surface area 850m² / g), while the other conditions remained unchanged. The results showed that the biochar-supported agent exhibited adsorption capacities of 42.6 mg / g for Cd²⁺ and 68.3 mg / g for Pb²⁺ at pH 9.5. XRD did not detect any new crystalline phases, but XPS showed a positive shift of 0.8 eV in the binding energy of the Cd 3d peak, indicating strong coordination. The final product showed heavy metal leaching concentrations of <0.01 mg / L, demonstrating that the biochar-supported agent can replace traditional chemical passivating agents and improve environmental properties.

[0070] Example 12

[0071] With all other preparation conditions the same as in Example 1, only the drying time in step S1 was adjusted from 1 h to 0.5 h, and the drying temperature was maintained at 110 °C. The results showed that the residual moisture content of the carrier was about 1.2 wt%, which did not affect the subsequent spray wetting and penetration. The performance indicators of the final product were basically the same as those in Example 1, with an effective phosphorus content of 12.2 wt% and a fluorine leaching concentration of 0.14 mg / L, proving that the process remained robust under the lower limit of the drying time.

[0072] Example 13

[0073] With all other preparation conditions the same as in Example 1, only the curing time in step S4 was adjusted from 2 h to 0.5 h, and the curing temperature was maintained at 80 °C. The results showed that the silica sol initially condensed to form a continuous film, but the thickness was slightly thin (approximately 1.6 μm), and SEM revealed the presence of micropores in certain areas. The fluorine leaching concentration increased to 0.18 mg / L (still < 0.2 mg / L limit), and the effective phosphorus content was 12.1 wt%, demonstrating that short-time curing can still meet basic sealing requirements, providing a parameter window for energy-saving operation.

[0074] Example 14

[0075] With all other preparation conditions the same as in Example 1, only the curing time after coating in step S5 was adjusted from 48 h to 12 h, and the temperature was maintained at 50 °C. The results showed that the degree of cross-linking of the membrane was slightly lower, and the tensile strength decreased by 12%, but the cumulative phosphorus release rate after 7 days of leaching from the soil column was still 13.5%, and no burst release phenomenon occurred; the leaching concentrations of all heavy metals met the standards, proving that the lower limit of the curing time can still guarantee the basic controlled release function.

[0076] Example 15:

[0077] To verify the technical effect of the present invention, the following comparative examples were set up and system tests were carried out:

[0078] Comparative Example 1: Compound phosphate fertilizer was prepared by a simple mixing method of fly ash and phosphate rock powder (unpassivated, unsealed, and uncoated).

[0079] Comparative Example 2: Commercially available slow-release phosphate fertilizer (polyurethane-coated diammonium phosphate).

[0080] Comparative Example 3: Only S1+S3+S4 is performed (i.e., skipping the segmented passivation within the S2 hole), the rest is the same as in Example 1;

[0081] Comparative Example 4: Perform S1+S2+S3, but skip S4 sealing and curing and S5 outer layer coating, the rest is the same as Example 1;

[0082] The test items include: (1) Physicochemical properties: total phosphorus (GB / T8573–2020), water-soluble phosphorus (GB / T8573–2020), available phosphorus (NY / T797–2004), particle compressive strength (GB / T24482–2009), and bulk density (GB / T24482–2009); (2) Environmental safety: leaching concentrations of fluorine, Cd, Pb, and As (HJ / T299–2007); (3) Slow-release performance: soil column leaching test (simulated rainfall, 0.5 mL / min, continuous leaching for 21 days, and determination of cumulative phosphorus release rate at each time period); (4) Field fertilizer effect: potted Chinese cabbage test (sandy loam soil, application rate 300 kg P2O5 / ha), determination of plant height, biomass, and total phosphorus content of plants (HNO3-HF digestion, ICP-OES determination).

[0083] The test results are shown in Table 1. As shown in Table 1, the total phosphorus content of the compound phosphate fertilizer granules obtained in Examples 1-14 was 12.1-12.5 wt%, the water-soluble phosphorus content was 22.4-28.6%, the available phosphorus content was 9.3-9.9 wt%, and the granule compressive strength was 22.5-28.7 N / granule. All of these were superior to Comparative Example 1 (total phosphorus 10.8 wt%, available phosphorus 5.2 wt%, compressive strength 12.3 N / granule) and Comparative Example 3 (total phosphorus 12.3 wt%, but available phosphorus only 6.4 wt%, fluorine leaching 1.8 mg / L). Regarding environmental safety, the fluorine leaching concentration in Examples 1-14 was 0.12-0.18 mg / L, and Cd, Pb, and As were not detected (<0.01 mg / L), significantly better than... Comparative Example 1 (fluorine 3.2 mg / L, Cd 0.45 mg / L, Pb 1.2 mg / L) and Comparative Example 3 (fluorine 1.8 mg / L, Cd 0.12 mg / L); In terms of slow-release performance, the cumulative phosphorus release rate of Example 1 from day 14 to 21 was 62.3% to 71.5%, with a flat release curve, while Comparative Example 1 reached 94.2% (severe burst release), Comparative Example 4 was 85.6%, and Comparative Example 2 was 58.7%; Field trials showed that the plant height, biomass, and total phosphorus content of the Chinese cabbage treated in Example 1 were increased by 38.2%, 42.7%, and 35.6% respectively compared with Comparative Example 1, and by 12.4%, 9.8%, and 11.3% respectively compared with Comparative Example 2, and no seedling burn was observed. The results show that the present invention, through the synergistic effect of segmented passivation in the pores, synergistic activation and phosphorus release, pore sealing and solidification, and outer coating, not only significantly improves the impurity fixation rate and phosphorus stability, but also achieves precise control of release behavior, producing unexpected technical effects—that is, under the same phosphorus content, its environmental safety and agronomic utilization rate are improved simultaneously, breaking through the technical bottleneck of "high nutrient content equals high risk" in traditional compound phosphate fertilizers.

[0084] Table 1 Results of the effect test

[0085] Sample number Total phosphorus (wt%) Available phosphorus (wt%) Fluorine leaching (mg / L) Cd leaching (mg / L) 21-day cumulative phosphorus release rate (%) Bok choy biomass (g / pot) Example 1 12.3 9.7 0.15 ＜0.01 66.8 128.4 Example 2 12.1 9.4 0.13 ＜0.01 65.2 125.6 Example 3 12.2 9.5 0.14 ＜0.01 64.9 124.8 Example 4 12.5 9.9 0.16 ＜0.01 62.3 129.7 Example 5 12.2 9.5 0.14 ＜0.01 63.7 126.2 Comparative Example 1 10.8 5.2 3.2 0.45 94.2 93.2 Comparative Example 2 18.2 15.6 ＜0.01 ＜0.01 58.7 114.3 Comparative Example 3 12.3 6.4 1.8 0.12 89.4 98.5 Comparative Example 4 12.4 9.6 0.21 ＜0.01 85.6 118.9

[0086] Example 16:

[0087] Experimental results show that the compound phosphate fertilizer granules prepared in this invention exhibit good slow-release phosphorus effect and crop growth-promoting effect in a potted model of Chinese cabbage in alluvial soil in North China. Therefore, it can be used to prepare fertilizers for the prevention and / or treatment of low soil phosphorus availability and phosphorus deficiency in crops. Specifically, the products prepared in Examples 1-14, under the same phosphorus application rate, increased the biomass of Chinese cabbage by 38.2-42.7% and the total phosphorus content of plants by 35.6-40.1% compared with Comparative Example 1, and extended the maintenance time of available phosphorus in the soil to 42 days (compared to 18 days in Comparative Example 1), proving its clear application value in the field of phosphorus-deficient soil improvement and green agricultural inputs. All test samples were from the aforementioned Examples 1-14, and their performance data are shown in Table 1, verifying that the technical solution of this invention can stably achieve the dual technical effects of efficient phosphorus utilization and controllable environmental risks under different parameter combinations.

[0088] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A process for producing compound phosphate fertilizer using industrial by-products, characterized in that, include: S1. Carrier preparation: Industrial by-products are provided as porous carrier raw materials, wherein the industrial by-products are selected from one or more of fly ash microspheres, fly ash, slag micro powder, red mud, and steel slag fine powder; the porous carrier raw materials are dried to obtain porous carriers; S2. Segmented directional passivation within the pores: The porous support is subjected to at least two segments of directional passivation treatment, wherein the first segment of directional passivation introduces a first passivating agent for capturing fluoride ions and / or soluble anions into the porous support and controls the system at a first preset pH window, so that the first passivating agent enters the pores and forms a sparingly soluble stationary phase with the target impurities; The second stage of directional passivation introduces a second passivating agent for fixing heavy metal ions and / or multivalent ions into the porous support and controls the system at a second preset pH window, so that the second passivating agent enters the pores and forms a solid phase through precipitation and / or adsorption and / or co-deposition; S3. Synergistic activation and phosphorus release within pores: After completing the segmented directional passivation within the pores, a phosphorus source and a synergistic activator are introduced into the pores of the porous support. The reaction conditions are controlled to allow the phosphorus source to react synergistically with the Ca / Si / Al active phase in the porous support, generating a phosphorus-containing composite phase within the pores. S4. Sealing and curing: A sealing and curing material is introduced into the intermediate that forms the phosphorus-containing composite phase in the pores and cured to form a continuous cured phase in the pores to lock the insoluble fixed phase, the supporting phase and the phosphorus-containing composite phase in the pores, thereby obtaining composite phosphate fertilizer particles.

2. The production process according to claim 1, characterized in that, The phosphorus source is one or more of the following: wet phosphoric acid, monoammonium phosphate, diammonium phosphate, phosphate solution, or solid phosphorus source slurry. The synergistic activator is one or more of alkali metal salts, alkaline earth metal salts, silica sols, alumina sols, or alkaline activators. The sealing and curing material is one or more of the following: silica sol, alumina sol, silicate solution, phosphate gelation system, or geopolymer gelation system.

3. The production process according to claim 1, characterized in that, In step S1, the drying temperature of the drying process is 80-140℃, and the drying time is 0.5-3h. The porous carrier is a powder carrier or a particulate carrier. The D50 of the powder carrier is 10-150 μm, and the particle size of the particulate carrier is 0.5-4 mm.

4. The production process according to claim 1, characterized in that, In step S2, the segmented directional passivation inside the hole is carried out by one or more of the following methods: spray wetting, impregnation or vacuum impregnation, so that the first passivating agent and / or the second passivating agent enter the hole channel; The first preset pH window for the first segment of directional passivation is 6 to 9, and the first passivating agent is selected from one or more of calcium source, magnesium source, rare earth salt, and aluminum source; The second preset pH window for the second stage of directional passivation is 8 to 11, and the second passivating agent is selected from one or more of phosphates, silicates, iron salts, sulfide precursors, and biochar support agents.

5. The production process according to claim 1, characterized in that, In step S3, the synergistic activation and phosphorus release within the pores adopts either an impregnation-in-situ reaction route or a spray-drum dynamic reaction route. The impregnation-in-situ reaction route includes: preparing the phosphorus source into a solution or slurry and performing vacuum impregnation or spray impregnation, so that it enters the pores and is then aged at 40-90°C for 0.5-6 hours to generate the phosphorus-containing composite phase inside the pores; The spray-drum dynamic reaction route includes: continuously spraying the phosphorus source and the synergistic activator during drum granulation or drum reaction, controlling the material temperature to 30-80℃ and the drum reaction time to 10-60min, and performing wet heat curing after discharge to promote the formation of the phosphorus-containing composite phase in the pores.

6. The production process according to claim 1, characterized in that, In step S3, the phosphorus source is wet-process phosphoric acid, and the synergistic activator includes silica sol and / or aluminum sol, so as to promote the synergistic reaction between the phosphorus source and the Ca / Si / Al active phase in the porous support to generate the phosphorus-containing composite phase in the pores.

7. The production process according to claim 1, characterized in that, In step S4, the sealing and curing material includes an inorganic sol system, which is silica sol and / or aluminum sol, and the sealing method is one or more of spray coating, dip-coating or fluidized bed atomization. The sealing and curing process is carried out at a temperature of 40–120°C for 0.5–4 hours.

8. The production process according to claim 1, characterized in that, Step S4 also includes an outer coating step: applying an outer functional coating to the sealed and cured composite phosphate fertilizer particles to form a controlled-release membrane layer; The controlled-release membrane layer has a bilayer or composite layer structure, comprising an inorganic dense inner layer and an organic outer layer.

9. The production process according to claim 8, characterized in that, The inorganic dense inner layer is selected from one or more of silica sol coating, silicate coating, and phosphate coating, and the organic outer layer is selected from one or more of alginate, chitosan, polyvinyl alcohol (PVA) modified polymer, bio-based polymer, wax, or resin.

10. The production process according to claim 8, characterized in that, The outer coating is applied by fluidized bed spraying or roller spraying, and after coating, it is cured at 40-60°C for 12-72 hours to promote cross-linking and curing of the sealing phase and / or controlled-release membrane layer.

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