Seaweed source slow-release ecological fertilizer and preparation method thereof

Abstract

The application discloses a seaweed source slow-release ecological fertilizer and a preparation method thereof, and relates to the technical field of slow-release ecological fertilizers. The ecological fertilizer comprises a granular fertilizer core and a continuous alginate composite coating. The composite coating comprises a calcium-magnesium ion crosslinking area connected with the fertilizer core and a phosphate mineralization area located outside the calcium-magnesium ion crosslinking area. In the preparation, an alginate liquid film is first formed on the surface of the fertilizer core, a calcium-magnesium mixed salt is used for first crosslinking, intermediate dehydration is performed, phosphate is applied to the outer surface, phosphate ions are in-situ reacted with calcium-magnesium ions in the coating, a calcium-magnesium mixed salt is further applied for second fixation, and drying is performed. Through the sequential cooperation of crosslinking, dehydration, mineralization and second fixation, the application adjusts nutrient release and improves the stability of the coating under repeated wetting conditions, and is suitable for granular fertilizers containing at least two nutrients of nitrogen, phosphorus and potassium.

Description

Technical Field

[0001] This invention relates to the field of slow-release ecological fertilizer technology, specifically to a seaweed-derived slow-release ecological fertilizer and its preparation method. Background Technology

[0002] When granular fertilizer is applied to the soil, water enters the granules and dissolves nutrients. The dissolved nutrients then diffuse outward through the granule surface. Coated slow-release fertilizers typically regulate the rate of water entry and nutrient diffusion by forming a diffusion barrier layer on the outside of the fertilizer granules. Alginate, derived from seaweed, can form an ionic cross-linking network in the presence of polyvalent ions such as calcium ions, and is therefore used for fertilizer coating, gel carriers, and nutrient encapsulation.

[0003] Chinese patent CN105481557B discloses a seaweed-sealed slow-release eco-fertilizer, which uses compound fertilizer, porous minerals, seaweed, chitosan, and calcium carbonate as main raw materials. It utilizes porous inorganic materials to adsorb the fertilizer and seals the pores of the porous materials through a calcium alginate gel and a chitosan coating. The key technical aspect of this solution lies in the adsorption and sealing of the porous carrier.

[0004] Chinese patent CN104649806B discloses a sodium alginate oligosaccharide-coated slow-release fertilizer. The coating material includes sodium alginate oligosaccharides, auxiliary coating materials, additives, initiators, crosslinking agents, and blocking agents. The auxiliary coating materials may include inorganic materials such as calcium magnesium phosphate fertilizer and magnesium ammonium phosphate. This solution provides a technical approach for forming a coating with seaweed-derived oligosaccharides and various organic or inorganic auxiliary materials.

[0005] International patent WO2023105293A2 discloses various ways to incorporate alginate into fertilizers, including forming a protective layer of alginate cross-linked by divalent ions on the outside of fertilizer particles, co-granulating alginate with fertilizer, or forming microspheres; the scheme also mentions that a phosphate buffer system can be used to compete for calcium ions to regulate the gelation rate of alginate.

[0006] Existing technologies have demonstrated that alginate, calcium or magnesium ions, and phosphate can participate in fertilizer coating or alginate gel regulation, respectively. Further research in this field can explore how to utilize sequential treatment with an aqueous phase and lower temperatures to enable the region near the fertilizer core to undergo basic ionic cross-linking within the same continuous alginate coating, while allowing controlled phosphate mineralization to occur in the region near the outer surface. This would improve the stability of the coating in repeatedly wetting environments while controlling initial nutrient release. Summary of the Invention

[0007] The purpose of this invention is to provide a seaweed-derived slow-release ecological fertilizer and its preparation method. By controlling the sequence of crosslinking, dehydration, mineralization and re-fixation of the alginate liquid film, the technical problems of initial swelling and release fluctuations that may occur when only homogeneous alginate gel is used are reduced.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a seaweed-derived slow-release ecological fertilizer, comprising a granular fertilizer core and a composite coating covering the outer surface of the granular fertilizer core; the composite coating is an alginate coating continuously formed from the inside to the outside along the radial direction of the granular fertilizer core, and the composite coating includes an inner ion crosslinking region in contact with the granular fertilizer core and an outer mineralization region located outside the inner ion crosslinking region; The inner ionic crosslinking zone contains an alginate ionic crosslinking network formed by treatment with a mixed salt solution containing calcium and magnesium ions, wherein calcium ions are the main crosslinking ions, and magnesium ions are used to adjust the divalent ion composition of the ionic crosslinking network and the subsequent phosphate fixation process; the outer mineralization zone contains a calcium- and / or magnesium-containing phosphate mineralization phase dispersed in the alginate network; the phosphate mineralization phase is formed by applying phosphate ions from the outer surface of the alginate coating to the granular fertilizer core and pre-crosslinking with calcium and magnesium ions and intermediate dehydration treatment, so that the phosphate ions react in situ with the calcium and magnesium ions in the coating, and are fixed in the outer mineralization zone by applying calcium and magnesium ions again from the outer surface; the inner ionic crosslinking zone and the outer mineralization zone constitute the same continuous alginate coating.

[0009] Furthermore, the particle size of the granular fertilizer core is 2~5 mm; based on the total dry weight of the final dried product, the mass fraction of the composite coating is 1.5%~5.0%, and the corresponding dry weight ratio of the granular fertilizer core to the composite coating is 98.5:1.5~95:5.

[0010] Furthermore, the alginate is selected from one or two of sodium alginate and potassium alginate; the film-forming polymer in the composite coating is composed of the alginate.

[0011] Furthermore, the calcium and magnesium ions are provided by water-soluble calcium salts and water-soluble magnesium salts, with a molar ratio of calcium ions to magnesium ions of 3:1 to 8:1; the water-soluble calcium salt is selected from one or more of calcium chloride, calcium nitrate, and calcium acetate, and the water-soluble magnesium salt is selected from one or more of magnesium chloride, magnesium nitrate, and magnesium acetate; the phosphate used to form the phosphate mineralization phase is selected from one or more of potassium dihydrogen phosphate, dipotassium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0012] Furthermore, by weight, the granular fertilizer core comprises 35-60 parts of urea, 15-30 parts of monoammonium phosphate and / or diammonium phosphate, 10-25 parts of potassium sulfate and / or potassium chloride, and 1-8 parts of binder; the binder is selected from one or more of lignin sulfonate, starch, and humate.

[0013] This invention also provides a method for preparing the above-mentioned seaweed-derived slow-release ecological fertilizer, comprising the following steps: S1. Mix fertilizer raw materials containing at least two of the following nutrients: nitrogen, phosphorus, and potassium, and granulate them to obtain granular fertilizer cores; S2. While the granular fertilizer core is in a rolling state, spray an alginate aqueous solution onto the surface of the granular fertilizer core to form an alginate liquid film on the surface of the granular fertilizer core. The alginate aqueous solution does not contain pre-made calcium phosphate particles, magnesium phosphate particles, or calcium magnesium phosphate particles. S3. The alginate is mainly ionically crosslinked with calcium ions, and magnesium ions are introduced into the alginate liquid film to form an inner ion crosslinking region treated with calcium and magnesium mixed salt. S4. The particles that have completed step S3 are subjected to intermediate dehydration treatment for 2 to 10 minutes under hot air conditions of 30 to 45°C, so that the alginate coating is initially shaped in a state that is not completely dry. S5. Spray an aqueous phosphate solution onto the outer surface of the alginate coating to allow phosphate ions to diffuse from the outside to the inside and react in situ with calcium and magnesium ions in the coating, forming a calcium- and / or magnesium-containing phosphate mineralization phase outside the inner ion crosslinking zone; then spray a second mixed salt solution containing calcium and magnesium salts onto the outer surface of the alginate coating again to fix the phosphate mineralization phase a second time, forming an outer mineralization zone that is continuous with the inner ion crosslinking zone; S6. The particles obtained in step S5 are dried to obtain the seaweed-derived slow-release ecological fertilizer.

[0014] Further, in step S1, 35-60 parts of urea, 15-30 parts of monoammonium phosphate and / or diammonium phosphate, 10-25 parts of potassium sulfate and / or potassium chloride, and 1-8 parts of binder are mixed by weight and granulated under the conditions of material moisture content of 5%-12% and material temperature of 20-45℃ to obtain granular fertilizer cores with a particle size of 2-5mm.

[0015] Furthermore, in step S2, the mass concentration of the alginate aqueous solution is 1%~3% and the pH is 6.0~8.0; when spraying the alginate aqueous solution, the temperature of the granular fertilizer core is controlled at 30~45℃, and the alginate aqueous solution is adhered to the surface of the granular fertilizer core by alternating spraying and hot air treatment.

[0016] Furthermore, the first mixed salt solution in step S3 and the second mixed salt solution in step S5 contain calcium salt and magnesium salt, respectively, with a molar ratio of calcium ions to magnesium ions of 3:1 to 8:1, and a total mass concentration of calcium salt and magnesium salt of 0.5% to 2.0%; the ion crosslinking time in step S3 is 2 to 10 min, and the treatment time after spraying the second mixed salt solution in step S5 is 1 to 5 min.

[0017] Furthermore, in step S5, the mass concentration of the phosphate aqueous solution is 0.1%~0.8%, the pH is 6.5~8.0, and the treatment time after spraying the phosphate aqueous solution is 1~8 min; steps S2, S3 and S5 are carried out in a rotating drum with a drum speed of 20~50 r / min and a spraying atomization pressure of 0.15~0.50 MPa; the drying temperature in step S6 is 40~55℃ and the drying time is 1~6 h.

[0018] The first calcium-magnesium treatment of this invention enables the formation of a basic ionic cross-linking network on the surface of the fertilizer core using an alginate liquid film. Calcium ions can form coordination cross-links with the carboxyl groups of alginate, and the participation of magnesium ions can change the local cross-linking state of the single calcium cross-linking network. The intermediate dehydration reduces the free water in the liquid film and preliminarily shapes the film layer, thereby regulating the migration rate of phosphate ions in the film layer.

[0019] When the phosphate solution is applied to the outer surface of the pre-formed membrane, phosphate ions preferentially contact and react in situ with calcium and magnesium ions on the outer side of the membrane, resulting in mineralization in the outer region. The second calcium-magnesium treatment further provides divalent ions to the outer region and fixes the insufficiently cross-linked alginate segments and mineralized areas. Thus, basic cross-linking and outer mineralization work synergistically within the same continuous alginate network.

[0020] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. This invention does not simply premix alginate, calcium magnesium salt and phosphate, but rather uses a determined sequence to make the basic ion crosslinking and the outer in-situ mineralization occur at different stages, which is beneficial to reduce disordered precipitation and local defects; intermediate dehydration keeps the alginate coating in a preliminary shape and not completely dry before phosphate treatment, which is beneficial to take into account both the ability of phosphate to enter the membrane layer and its controlled reaction in the outer region.

[0021] 2. The second calcium-magnesium treatment of this invention helps to fix the outer mineralized area, reduce the loss of mineralized components during initial wetting, and improve the stability of release behavior under multiple wetting and drying conditions. Using aqueous spraying and lower temperature drying, the film-forming polymer can be made solely of alginate, without relying on polyurethane, urea-formaldehyde resin, or additional petrochemical film-forming polymers. By adjusting the coating amount, alginate concentration, calcium-magnesium ratio, intermediate dehydration conditions, and phosphate treatment conditions, the nutrient release process can be regulated, and it is applicable to granular fertilizer cores containing at least two of the following nutrients: nitrogen, phosphorus, and potassium. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of the seaweed-derived slow-release ecological fertilizer of the present invention. In the diagram, 1 is the granular fertilizer core, 2 is the inner ion cross-linking region, and 3 is the outer mineralization region.

[0024] Figure 2 This is a schematic diagram of the process flow for preparing seaweed-derived slow-release ecological fertilizer according to the present invention. S1 is granulation of the fertilizer core, S2 is formation of a sodium alginate liquid film, S3 is the first calcium-magnesium crosslinking, S4 is intermediate dehydration, S5 is in-situ phosphate mineralization and the second calcium-magnesium fixation, and S6 is final drying. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] The structure formation process of the present invention can be explained as follows: During the first crosslinking, calcium ions preferentially coordinate with carboxylate groups in the alginate chain segments, causing the alginate liquid film on the surface of the granular fertilizer core to change from a flowing state to an ion crosslinking film that can maintain a continuous shape; magnesium ions have a weaker crosslinking ability than calcium ions, and in the system of the present invention, they are mainly used to adjust the divalent ion composition in the coating and the subsequent phosphate fixation process.

[0027] If phosphate is applied directly after the first crosslinking, the abundant free water in the coating may cause the phosphate to diffuse rapidly into the coating, potentially leading to re-swelling of the coating. Conversely, if the coating after the first crosslinking is completely dried, it may shrink, reducing the uniform spreading ability of the phosphate solution. Therefore, this invention incorporates an intermediate dehydration step between the first crosslinking and phosphate application, reducing the free water on the coating surface while retaining a certain amount of moisture inside the coating.

[0028] Subsequently, phosphate is applied to the outer surface of the coating, and calcium and magnesium ions are provided through a second fixative, which facilitates phosphate fixation primarily occurring on the outer portion of the continuous coating. Thus, the inner portion of the continuous coating retains the flexibility of the alginate ion cross-linking structure, while the outer portion reduces direct swelling and surface erosion through phosphate fixation. This invention aims to reduce the tendency of the continuous coating to crack during repeated water absorption and drying processes by controlling nutrient release through the combined function of the inner and outer layers.

[0029] Figure 1 This is a schematic diagram of the structure of the seaweed-derived slow-release eco-fertilizer of the present invention. It illustrates the relative positions of the granular fertilizer core 1, the inner ion-crosslinked region 2, and the outer mineralization region 3. The inner ion-crosslinked region 2 and the outer mineralization region 3 belong to the same functional region within a continuous sodium alginate coating, rather than being two independent coating layers formed by two different film-forming polymers. The inner ion-crosslinked region and the outer mineralization region represent the relative functional regions formed by the application sequence; there is no abrupt interface or fixed thickness ratio required between them. The outer mineralization region refers to the relatively outer portion where the phosphate solution is applied from outside the coating and fixed by the second mixed salt solution, and does not indicate that phosphorus is completely absent from the inner portion of the coating.

[0030] Figure 2 This is a schematic diagram of the process flow for preparing seaweed-derived slow-release ecological fertilizer according to the present invention. S3 first establishes a basic ionic cross-linking network; S4 performs controlled intermediate dehydration before complete drying; S5 first applies phosphate ions to the outer surface of the coating for in-situ mineralization, followed by a second calcium and magnesium fixation. This sequence differs from adding phosphate to the sodium alginate solution beforehand, mixing all salt solutions at once, or applying phosphate before sodium alginate cross-linking.

[0031] Unless otherwise stated, all percentages below are by mass, and all parts by weight are on a dry basis. Alginate is selected from sodium alginate, potassium alginate, or combinations thereof. The molar ratio of calcium to magnesium ions is calculated based on the amount of dissociable calcium and magnesium ions in the calcium and magnesium salts used. The following examples preferably use CaCl₂·2H₂O and MgCl₂·6H₂O. The first mixed salt aqueous solution is used for the first ionic crosslinking, and the second mixed salt aqueous solution is used for secondary fixation after phosphate treatment.

[0032] The phosphate aqueous solution is preferably prepared from potassium dihydrogen phosphate and dipotassium hydrogen phosphate, and the pH is adjusted to 6.5-8.0. Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or a combination thereof may also be used.

[0033] The mass ratio of granular fertilizer core to composite coating is calculated based on the dry mass of the granular fertilizer core and the dry mass of the coating in the final dried product. The dry mass of the coating can be determined by the difference in constant weight between the samples before and after coating under the same drying conditions.

[0034] Intermediate dehydration refers to the hot air treatment of particles after the first calcium-magnesium cross-linking and before the spraying of phosphate aqueous solution, which initially shapes the coating but does not result in final complete drying. Secondary fixation refers to applying a mixed salt aqueous solution containing calcium and magnesium ions to the outer surface of the coating again after the spraying of phosphate aqueous solution. Example 1

[0035] The preparation method of seaweed-derived slow-release ecological fertilizer in this embodiment includes the following steps: S1, Granular fertilizer core granulation: Weigh 52 parts of granular urea, 24 parts of monoammonium phosphate, 20 parts of potassium sulfate and 4 parts of sodium lignosulfonate by dry basis weight, and mix them evenly; adjust the moisture content of the mixture to 8% with deionized water, and granulate in a rotary drum at 32℃. Screen out particles with a particle size of 3~4mm, and dry them at 45℃ to constant weight to obtain granular fertilizer cores; weigh 1000g of granular fertilizer cores for coating this batch.

[0036] S2, Sodium alginate liquid film formation: The granular fertilizer core is placed in a rotary drum coating machine, and the drum speed is controlled at 35 r / min, the particle temperature at 38℃, and the atomization pressure at 0.30 MPa; 18.0 g of sodium alginate is dissolved in 882.0 g of deionized water, and the pH is adjusted to 7.0 with dilute hydrochloric acid or dilute sodium hydroxide aqueous solution to obtain 900 g of sodium alginate aqueous solution with a mass concentration of 2.0%; the solution is divided into several equal portions and atomized and sprayed onto the surface of the rolling granular fertilizer core. After each portion is sprayed, 38℃ hot air is introduced until there is no obvious adhesion on the particle surface before spraying the next portion, forming a continuous sodium alginate liquid film.

[0037] S3, First calcium-magnesium crosslinking: 7.83 g CaCl2·2H2O and 2.17 g MgCl2·6H2O were dissolved in deionized water and brought to a final volume of 1000 g, yielding a total calcium and magnesium salt concentration of 1.0%. 2+ A mixed salt aqueous solution with a Mg²⁺ molar ratio of 5:1 was prepared. 320g of the mixed salt aqueous solution was uniformly sprayed onto the surface of the sodium alginate liquid film. The drum rotation speed was maintained at 35r / min, the particle temperature at 38℃, and the atomization pressure at 0.30MPa, and the mixture was continuously rolled for crosslinking for 6min.

[0038] S4, intermediate dehydration: Stop spraying liquid and continue rolling for 6 minutes under 38℃ hot air conditions to allow the coating after the first cross-linking to be initially shaped but not completely dried.

[0039] S5, In-situ phosphate mineralization and secondary calcium and magnesium fixation: Potassium dihydrogen phosphate and dipotassium hydrogen phosphate were mixed at a mass ratio of 45:55. 0.80 g of the mixture was dissolved in 199.20 g of deionized water, and the pH was adjusted to 7.2 with a dilute potassium hydroxide aqueous solution to obtain 200 g of a 0.4% phosphate aqueous solution. This 200 g of the phosphate aqueous solution was uniformly sprayed onto the surface of the intermediately dehydrated particles, and treated for 4 min while maintaining the same drum speed, particle temperature, and atomization pressure. Subsequently, 320 g of the mixed salt aqueous solution with the same composition as S3 was uniformly sprayed, and the mixture was continuously rolled for 3 min to complete the secondary calcium and magnesium fixation.

[0040] S6, Final Drying: The obtained granules were dried with hot air at 48℃ for 4 hours and continued to dry until the mass difference between two consecutive weighings did not exceed 0.1%. After cooling to room temperature, the adhering granules were sieved to obtain the seaweed-derived slow-release ecological fertilizer. Based on the constant weight difference before and after drying, the mass ratio of the granular fertilizer core to the composite coating was 97.5:2.5. Example 2

[0041] The preparation method of seaweed-derived slow-release ecological fertilizer in this embodiment includes the following steps: S1, Granular fertilizer core granulation: Weigh 52 parts of granular urea, 24 parts of monoammonium phosphate, 20 parts of potassium sulfate and 4 parts of sodium lignosulfonate by dry basis weight, and mix them evenly; adjust the moisture content of the mixture to 8% with deionized water, and granulate in a rotary drum at 32℃. Screen out particles with a particle size of 3~4mm, and dry them at 45℃ to constant weight to obtain granular fertilizer cores; weigh 1000g of granular fertilizer cores for coating this batch.

[0042] S2, Sodium alginate liquid film formation: The granular fertilizer core is placed in a rotary drum coating machine, and the drum speed is controlled at 40 r / min, the particle temperature at 40℃, and the atomization pressure at 0.35 MPa; 22.5 g of sodium alginate is dissolved in 877.5 g of deionized water and the pH is adjusted to 7.0 to obtain 900 g of sodium alginate aqueous solution with a mass concentration of 2.5%; the entire solution is applied to the surface of the granular fertilizer core by alternating between spraying in portions and hot air treatment at 40℃ to form a continuous sodium alginate liquid film.

[0043] S3, First calcium-magnesium crosslinking: 11.15g CaCl2·2H2O and 3.85g MgCl2·6H2O were dissolved in deionized water and the volume was adjusted to 1000g to obtain a total calcium-magnesium concentration of 1.5%. 2+ With Mg 2+ A mixed salt aqueous solution with a molar ratio of 4:1 was prepared; 250g of this solution was evenly sprayed onto the surface of the sodium alginate liquid film, and the above drum conditions were maintained to continue rolling and crosslinking for 7 minutes.

[0044] S4, intermediate dehydration: Stop spraying liquid and continue rolling for 7 minutes under 40℃ hot air conditions to allow the coating to initially set but not completely dry.

[0045] S5, in-situ phosphate mineralization and secondary calcium and magnesium fixation: Potassium dihydrogen phosphate and dipotassium hydrogen phosphate were mixed at a mass ratio of 45:55. 1.00 g of the mixture was dissolved in 199.00 g of deionized water and the pH was adjusted to 7.2 to obtain 200 g of phosphate aqueous solution with a mass concentration of 0.5%. This solution was evenly sprayed onto the surface of the intermediate dehydrated particles and treated for 5 min. Subsequently, 250 g of a mixed salt aqueous solution with the same composition as S3 was sprayed and the mixture was rolled for another 4 min.

[0046] S6, Final Drying: The obtained granules were dried with hot air at 50℃ for 4 hours and then dried to constant weight. After cooling and sieving, the seaweed-derived slow-release ecological fertilizer was obtained. The mass ratio of granular fertilizer core to composite coating was 97:3. Example 3

[0047] The preparation method of seaweed-derived slow-release ecological fertilizer in this embodiment includes the following steps: S1, Granular fertilizer core granulation: Weigh 52 parts of granular urea, 24 parts of monoammonium phosphate, 20 parts of potassium sulfate and 4 parts of sodium lignosulfonate by dry basis weight, and mix them evenly; adjust the moisture content of the mixture to 8% with deionized water, and granulate in a rotary drum at 32℃. Screen out particles with a particle size of 3~4mm, and dry them at 45℃ to constant weight to obtain granular fertilizer cores; weigh 1000g of granular fertilizer cores for coating this batch.

[0048] S2, Sodium alginate liquid film formation: The granular fertilizer core is placed in a rotary drum coating machine, and the drum speed is controlled at 20 r / min, the particle temperature at 30℃, and the atomization pressure at 0.15 MPa; 11.0 g of sodium alginate is dissolved in 1089.0 g of deionized water and the pH is adjusted to 6.0 to obtain 1100 g of sodium alginate aqueous solution with a mass concentration of 1.0%; the entire solution is applied by alternating between spraying in portions and hot air treatment at 30℃ to form a continuous sodium alginate liquid film.

[0049] S3, First calcium-magnesium crosslinking: 3.42g CaCl2·2H2O and 1.58g MgCl2·6H2O were dissolved in deionized water and the volume was adjusted to 1000g to obtain a total calcium-magnesium concentration of 0.5%. 2+ With Mg 2+ A mixed salt aqueous solution with a molar ratio of 3:1; take 400g of this solution and spray it evenly onto the surface of the liquid film and continue rolling for cross-linking for 2min.

[0050] S4, intermediate dehydration: Stop spraying liquid and continue rolling for 2 minutes under 30℃ hot air conditions.

[0051] S5, in-situ phosphate mineralization and second calcium and magnesium fixation: potassium dihydrogen phosphate and dipotassium hydrogen phosphate were mixed at a mass ratio of 80:20. 0.20g of the mixture was dissolved in 199.80g of deionized water and the pH was adjusted to 6.5 to obtain 200g of phosphate aqueous solution with a mass concentration of 0.1%. The solution was sprayed evenly and treated for 1min. Then, 400g of mixed salt aqueous solution with the same composition as S3 was sprayed and rolled for another 1min.

[0052] S6, Final Drying: The obtained granules were dried with hot air at 40℃ for 1 hour and continued to dry to constant weight. After cooling and sieving, the seaweed-derived slow-release ecological fertilizer was obtained. The mass ratio of granular fertilizer core to composite coating was 98.5:1.5. Example 4

[0053] The preparation method of seaweed-derived slow-release ecological fertilizer in this embodiment includes the following steps: S1, Granular fertilizer core granulation: Weigh 52 parts of granular urea, 24 parts of monoammonium phosphate, 20 parts of potassium sulfate and 4 parts of sodium lignosulfonate by dry basis weight, and mix them evenly; adjust the moisture content of the mixture to 8% with deionized water, and granulate in a rotary drum at 32℃. Screen out particles with a particle size of 3~4mm, and dry them at 45℃ to constant weight to obtain granular fertilizer cores; weigh 1000g of granular fertilizer cores for coating this batch.

[0054] S2, Sodium alginate liquid film formation: The granular fertilizer core is placed in a rotary drum coating machine, and the drum speed is controlled at 50 r / min, the particle temperature at 45℃, and the atomization pressure at 0.50 MPa; 36.0 g of sodium alginate is dissolved in 1164.0 g of deionized water and the pH is adjusted to 8.0 to obtain 1200 g of sodium alginate aqueous solution with a mass concentration of 3.0%; the entire solution is applied by alternating between spraying in portions and hot air treatment at 45℃ to form a continuous sodium alginate liquid film.

[0055] S3, First calcium-magnesium crosslinking: 17.05g CaCl2·2H2O and 2.95g MgCl2·6H2O were dissolved in deionized water and the volume was adjusted to 1000g to obtain a total calcium-magnesium concentration of 2.0%. 2+ With Mg 2+ A mixed salt aqueous solution with a molar ratio of 8:1; take 350g of this solution and spray it evenly onto the surface of the liquid film and continue rolling for cross-linking for 10min.

[0056] S4, intermediate dehydration: Stop spraying liquid and continue rolling for 10 minutes under 45℃ hot air conditions.

[0057] S5, in-situ phosphate mineralization and secondary calcium and magnesium fixation: Potassium dihydrogen phosphate and dipotassium hydrogen phosphate were mixed at a mass ratio of 11:89. 2.40g of the mixture was dissolved in 297.60g of deionized water and the pH was adjusted to 8.0 to obtain 300g of phosphate aqueous solution with a mass concentration of 0.8%. The solution was sprayed evenly and treated for 8min. Then, 350g of mixed salt aqueous solution with the same composition as S3 was sprayed and continued to roll for 5min.

[0058] S6, Final Drying: The obtained granules were dried with hot air at 55℃ for 6 hours and then dried to constant weight. After cooling and sieving, the seaweed-derived slow-release ecological fertilizer was obtained. The mass ratio of granular fertilizer core to composite coating was 95:5.

[0059] Comparative Example 1: This comparative example is used to verify the role of magnesium ions and calcium ions in cross-linking and mineralization fixation. Except for replacing the calcium-magnesium mixed salt with calcium chloride of equal total divalent ion content, the other formulations, feed amounts and processes are consistent with Example 1.

[0060] Comparative Example 2: This comparative example is used to verify the role of intermediate dehydration; except for the omission of S4, the rest of the formulation, feed amount and process are consistent with Example 1.

[0061] Comparative Example 3: This comparative example was used to verify the second calcium and magnesium fixation effect after phosphate treatment; except for the omission of the second mixed salt solution, the other formulations, feed amounts and processes were consistent with those in Example 1.

[0062] Comparative Example 4: This comparative example was used to verify the order of the treatment steps. The total amount of sodium alginate, calcium salt, magnesium salt, and phosphate used was the same as in Example 1, but the phosphate treatment was placed before the calcium-magnesium crosslinking, and the two calcium-magnesium treatments were combined into one. After the sodium alginate liquid film was formed, 200 g of a 0.4% phosphate aqueous solution with the same composition as in Example 1 was immediately sprayed and rolled for 4 min; then 640 g of a 1.0% calcium-magnesium mixed salt aqueous solution with the same composition as in Example 1 was sprayed and rolled for another 9 min; the intermediate dehydration in S4 and the second calcium-magnesium fixation were not performed separately, and the other conditions were the same as in Example 1.

[0063] Comparative Example 5: This comparative example is used to verify the effect when the phosphate concentration is below the lower limit; except that the phosphate aqueous solution mass concentration is 0.05%, the other formulations, feed amounts and processes are consistent with those in Example 1.

[0064] Comparative Example 6: This comparative example is used to verify the effect when the phosphate concentration is higher than the upper limit; except that the phosphate aqueous solution mass concentration is 1.2%, the other formulations, feed amounts and processes are consistent with those in Example 1.

[0065] Comparative Example 7: This comparative example is used to verify the effect of intermediate dehydration conditions being below the lower limit; except for S4 which uses 25°C and 1 min, the other formulations, feed amounts and processes are consistent with those in Example 1.

[0066] Comparative Example 8: This comparative example is used to verify the effect of intermediate dehydration conditions exceeding the upper limit; except for S4 which uses 50°C for 15 minutes, the other formulations, feed amounts and processes are consistent with those in Example 1.

[0067] Performance testing conditions: I. Sample batches and result calculation Each embodiment or comparative example prepared three batches of samples independently according to the corresponding preparation method. Each batch of samples was sampled and tested separately, and the independently prepared batches were used as statistical units.

[0068] For nutrient release tests and soil column leaching tests, one batch of results is obtained for each batch of samples; for wet-dry cycle integrity tests, 30 samples are randomly selected from each batch to complete the test, and the membrane breakage rate and membrane quality loss rate of that batch are obtained.

[0069] The results in the table are the arithmetic mean of the results from three valid batches, calculated using the following formula:

[0070] in, The arithmetic mean recorded in the table is... The results are for batches 1, 2, and 3, respectively.

[0071] Cumulative release rate, membrane rupture rate, membrane mass loss rate, cumulative nitrogen leaching rate, and cumulative phosphorus leaching rate are all retained to one decimal place; T 80 Retain to one decimal place.

[0072] II. Nutrient Release Test Nutrient release tests were conducted in accordance with the current GB / T23348-2009 standard, "Slow-Release Fertilizers." Three batches of samples were prepared independently for each example or comparative example. 10.00 g of sample was weighed from each batch, placed in a 100-mesh inert mesh bag, and then placed in a 250 mL covered container. 200 mL of deionized water was added, and the mixture was allowed to stand at 25 ± 0.5 °C.

[0073] Samples were taken at 24h, 7d, 14d, 28d, 42d, 60d, and 70d. At each sampling, the entire extract was transferred and brought to volume, followed by the addition of 200mL of fresh deionized water to the container. The sample mass, extract volume, container specifications, liquid exchange procedure, and testing method remained consistent across batches.

[0074] The nitrogen release from each extraction solution was determined based on the composition of the granular fertilizer core. The cumulative total nitrogen release rate was calculated as the percentage of the total nitrogen mass in all extraction solutions to the initial total nitrogen mass of the sample.

[0075] Calculate the time T required for each batch to reach a cumulative release rate of 80%. 80 When the cumulative release rate at two adjacent detection time points is below and above 80% respectively, the T value for this batch is calculated using linear interpolation. 80 ; Three batches obtained T respectively 80 Then, calculate their arithmetic mean.

[0076] If the sample has not reached 80% cumulative release rate by 60 days, testing should continue until 70 days, and interpolation should be performed based on the test results at 60 days and 70 days; if it still has not reached 80% by 70 days, T should not be extrapolated without basis. 80 The testing time should be extended until two actual testing points located on both sides of 80% are obtained.

[0077] III. Dry-wet cycle integrity test Three batches of samples were prepared independently for each embodiment or comparative example, with 30 intact particles randomly selected from each batch. The samples were immersed in a simulated soil solution for 12 hours, then dried at 35°C and 50%–60% relative humidity for 12 hours, forming one wet-dry cycle, for a total of 10 cycles.

[0078] After the cycle is completed, visual and conventional optical image inspections are performed under the same lighting conditions, shooting distance, magnification, and observation angle. Particles with penetrating membrane cracks or visible membrane detachment exceeding 10% of the particle surface area are identified as membrane-broken particles.

[0079] The membrane breakage rate of each batch of samples is calculated according to the following formula:

[0080] After calculating the membrane breakage rate for each batch of samples, the arithmetic mean of the membrane breakage rates for the three batches of samples is then calculated.

[0081] The coating mass loss rate was calculated by the mass change of the samples after drying to constant weight under the same conditions before and after cycling, and the loss of fertilizer core solubles determined by parallel release blanks was deducted. A coating mass loss rate was obtained for each batch, and the arithmetic mean of the results for the three batches was calculated.

[0082] IV. Soil Column Leakage Test PVC soil columns with an inner diameter of 8 cm and a height of 35 cm were used, and each column was filled with 3.0 kg of air-dried sandy loam. Three soil columns were set up for each embodiment or comparative example, and each soil column corresponded to an independent preparation batch.

[0083] Each treatment was fertilized with equal nitrogen content, and the fertilizer was buried 5 cm below the soil surface. Every 5 days, 250 mL of deionized water was slowly added to each soil column, and all leachate was collected. This process was repeated for 60 days.

[0084] The inorganic nitrogen and total phosphorus contents in each leachate were measured separately, and the cumulative leaching loss of each soil column was the sum of the corresponding nutrient masses in all leachates within 60 days.

[0085] The cumulative nitrogen leaching rate and cumulative phosphorus leaching rate are calculated as the percentage of the cumulative leached nitrogen or phosphorus mass relative to the initial nitrogen or phosphorus input of the soil column. Each soil column forms a batch result, and the arithmetic mean of the results of the three soil columns is then calculated.

[0086] The specific test results are shown in Tables 1, 2, and 3 below: Table 1. Cumulative Total Nitrogen Release Rate and T80 Example 1 9.7 29.8 45.9 65.3 80.6 91.4 95.0 41.8 Example 2 8.4 26.1 40.7 59.8 75.9 88.2 92.8 48.0 Example 3 13.7 38.4 56.2 74.7 87.1 95.2 97.2 33.9 Example 4 7.9 23.9 37.8 54.7 69.8 79.6 84.8 60.8 Comparative Example 1 11.8 34.2 52.0 70.8 85.4 94.0 96.8 36.1 Comparative Example 2 14.3 41.2 60.4 77.9 90.2 96.3 98.2 30.4 Comparative Example 3 12.9 38.3 57.0 75.2 88.1 95.0 97.4 33.1 Comparative Example 4 15.9 44.4 64.1 81.1 92.5 97.4 98.7 27.3 Comparative Example 5 12.8 37.0 55.4 73.8 87.0 94.2 96.8 34.0 Comparative Example 6 10.6 31.0 50.8 76.2 90.0 96.0 98.5 31.9 Comparative Example 7 14.0 40.0 59.0 77.0 89.7 95.8 98.1 31.2 Comparative Example 8 12.0 36.0 54.9 73.5 86.7 94.5 97.1 34.9 Table 2. Results of wet-dry cycle integrity Example 1 10.0 5.6 Example 2 6.7 4.3 Example 3 20.0 8.9 Example 4 16.7 7.5 Comparative Example 1 23.3 10.5 Comparative Example 2 30.0 13.1 Comparative Example 3 26.7 11.8 Comparative Example 4 36.7 16.0 Comparative Example 5 23.3 10.4 Comparative Example 6 30.0 14.3 Comparative Example 7 30.0 12.8 Comparative Example 8 26.7 11.3 Table 3. Cumulative leaching rate of soil column over 60 days Example 1 27.4 14.8 Example 2 23.8 12.1 Example 3 35.6 21.7 Example 4 21.9 10.9 Comparative Example 1 34.4 19.0 Comparative Example 2 39.8 25.7 Comparative Example 3 36.5 28.4 Comparative Example 4 44.1 31.5 Comparative Example 5 35.2 24.0 Comparative Example 6 33.2 17.8 Comparative Example 7 39.0 26.0 Comparative Example 8 35.0 20.1 Analysis of experimental results: Example 1 employed a complete process involving sodium alginate liquid film formation, first calcium-magnesium crosslinking, intermediate dehydration, in-situ phosphate mineralization, and second calcium-magnesium fixation. Its 24-hour cumulative total nitrogen release rate was 9.7%. 80 The duration was 41.8 days, the membrane rupture rate after 10 wet-dry cycles was 10.0%, the membrane mass loss rate was 5.6%, and the cumulative nitrogen and phosphorus leaching rates after 60 days were 27.4% and 14.8%, respectively.

[0087] The 24-hour cumulative total nitrogen release rate in Example 2 was 8.4%, lower than the 9.7% in Example 1; its T 80 The effective release time was 48.0 days, higher than the 41.8 days of Example 1. The membrane breakage rate and membrane mass loss rate of Example 2 were 6.7% and 4.3%, respectively, also lower than the 10.0% and 5.6% of Example 1. Its cumulative nitrogen and phosphorus leaching rates were 23.8% and 12.1%, respectively, both lower than those of Example 1. These results indicate that the membrane amount, sodium alginate concentration, calcium-magnesium crosslinking conditions, intermediate dehydration conditions, and phosphate mineralization conditions of Example 2 were optimally matched, maintaining good wetting cycle stability while reducing initial release.

[0088] The 24-hour cumulative total nitrogen release rate in Example 3 was 13.7%, T 80 The duration of action was 33.9 days, with a membrane breakage rate of 20.0% and a membrane mass loss rate of 8.9%, and cumulative nitrogen and phosphorus leaching rates of 35.6% and 21.7%, respectively. These results are all weaker than those of Examples 1 and 2, indicating that when the coating amount, sodium alginate concentration, calcium and magnesium salt concentration, and phosphate concentration are all at low levels, the diffusion barrier ability and cycling stability of continuous coatings are relatively weakened.

[0089] The 24-hour cumulative total nitrogen release rate in Example 4 was 7.9%, lower than that in Examples 1 and 2, but the 60-day cumulative total nitrogen release rate was only 79.6%, increasing to 84.8% by day 70, corresponding to T 80 The duration was 60.8 days. The membrane breakage rate of Example 4 was 16.7%, which was higher than 10.0% of Example 1 and 6.7% of Example 2. This indicates that increasing the amount of coating and improving the degree of mineralization can suppress the initial release, but may excessively prolong the release cycle, and that higher coating and mineralization are not necessarily more beneficial to the integrity of the wet-dry cycle.

[0090] Comparative Example 1, which used only calcium ions and not magnesium ions, had a 24-hour cumulative total nitrogen release rate of 11.8%, higher than the 9.7% of Example 1; T 80 The survival time was 36.1 days, shorter than 41.8 days in Example 1; the membrane rupture rate was 23.3%, significantly higher than 10.0% in Example 1. Its cumulative nitrogen and phosphorus leaching rates were 34.4% and 19.0%, respectively, also higher than in Example 1.

[0091] Comparative Example 2, without intermediate dehydration, had a 24-hour cumulative total nitrogen release rate of 14.3%. 80 The time was 30.4 days, with a membrane breakage rate of 30.0% and a membrane mass loss rate of 13.1%, and cumulative nitrogen and phosphorus leaching rates of 39.8% and 25.7%, respectively. Compared to Example 1, all of the above indicators showed significant deterioration, indicating that applying phosphate solution immediately after the first calcium-magnesium crosslinking may have disturbed the sodium alginate crosslinking network, which had not yet been preliminarily stabilized. Intermediate dehydration helps to preliminarily stabilize the basic network and regulate the degree of subsequent phosphate incorporation into the membrane.

[0092] Comparative Example 3, after the second calcium-magnesium fixation without phosphate treatment, showed a cumulative phosphorus leaching rate of 28.4%, nearly twice that of Example 1; its membrane breakage rate and coating quality loss rate were 26.7% and 11.8%, respectively, also higher than Example 1. These results primarily support the claim that the second calcium-magnesium fixation can re-fix the outer mineralized components and adjacent sodium alginate network, thereby reducing the loss of phosphate mineralized components during wetting.

[0093] Comparative Example 4 changed the key process sequence, placing the phosphate treatment before the basic calcium-magnesium crosslinking and combining the two calcium-magnesium treatments into one. Its 24-hour cumulative total nitrogen release rate was 15.9%, T 80 The treatment duration was 27.3 days, with a membrane breakage rate of 36.7% and a membrane quality loss rate of 16.0%, while the cumulative nitrogen and phosphorus leaching rates were 44.1% and 31.5%, respectively. Overall, this group performed the weakest among all the treatment schemes. This comparison demonstrates that there is a clear sequential relationship between phosphate treatment, basic calcium-magnesium crosslinking, intermediate dehydration, and secondary calcium-magnesium fixation, and the same effect cannot be obtained by arbitrarily changing the order.

[0094] The phosphate aqueous solution concentration of Comparative Example 5 was below the lower limit, and its 24-hour cumulative total nitrogen release rate was 12.8%. 80 The duration of treatment was 34.0 days, the membrane breakage rate was 23.3%, and the cumulative leaching rates of nitrogen and phosphorus were 35.2% and 24.0%, respectively. Compared with Example 1, the above results indicate that when the phosphate concentration is too low, the outer mineralization is insufficient, making it difficult to form an effective diffusion barrier and mineralization fixation zone.

[0095] The phosphate aqueous solution concentration of Comparative Example 6 was higher than the upper limit. Its 24-hour cumulative total nitrogen release rate was 10.6%, which was relatively small compared to 9.7% in Example 1, but its 28-day cumulative release rate reached 76.2%, higher than 65.3% in Example 1. 80 The time to release was shortened to 31.9 days. The membrane rupture rate and membrane mass loss rate were 30.0% and 14.3%, respectively. This indicates that excessively high phosphate concentrations may not directly result in a 24-hour burst release, but may lead to rapid deposition on the membrane surface, accelerating the release in the later stages and reducing the membrane stability under repeated wetting conditions.

[0096] Comparative Example 7 had an intermediate dehydration temperature and time below the lower limit, resulting in a 24-hour cumulative total nitrogen release rate of 14.0%. 80 The time was 31.2 days, with a film breakage rate and coating quality loss rate of 30.0% and 12.8%, respectively, and cumulative nitrogen and phosphorus leaching rates of 39.0% and 26.0%, respectively. This indicates that when intermediate dehydration is insufficient, the basic network formed by the first cross-linking is not yet fully stabilized, and subsequent spraying operations can easily change the basic cross-linking structure.

[0097] Comparative Example 8 had an intermediate dehydration temperature and time exceeding the upper limit, resulting in a 24-hour cumulative total nitrogen release rate of 12.0%. 80The effective period was 34.9 days, with a membrane breakage rate of 26.7% and a membrane mass loss rate of 11.3%, and cumulative nitrogen and phosphorus leaching rates of 35.0% and 20.1%, respectively. While its performance was better than Comparative Example 2, which completely eliminated intermediate dehydration, it was still weaker than Examples 1 and 2. This result indicates that excessive intermediate dehydration may reduce the ability of phosphate solution to penetrate the membrane, causing the mineralization reaction to occur more on the membrane surface, thereby reducing the degree of continuous bonding between the outer mineralization zone and the inner crosslinking zone.

[0098] Comparing Example 1 with Comparative Examples 1-4, the first calcium-magnesium crosslinking, intermediate dehydration, application of phosphate from the outer surface of the coating, and the second calcium-magnesium fixation each correspond to different action stages. The complete process is not a simple parallel use of sodium alginate, calcium-magnesium salt, and phosphate, but rather involves the sequential control of forming an inner ionic crosslinking zone and an outer mineralization zone within the same continuous sodium alginate coating.

[0099] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0100] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A seaweed-derived slow-release ecological fertilizer, characterized in that, It includes a granular fertilizer core and a composite coating covering the outer surface of the granular fertilizer core; the composite coating is an alginate coating continuously formed from the inside to the outside along the radial direction of the granular fertilizer core, and the composite coating includes an inner ion crosslinking region in contact with the granular fertilizer core and an outer mineralization region located outside the inner ion crosslinking region; The inner ionic crosslinking zone contains an alginate ionic crosslinking network formed by treatment with a mixed salt solution containing calcium and magnesium ions, wherein calcium ions are the main crosslinking ions, and magnesium ions are used to adjust the divalent ion composition of the ionic crosslinking network and the subsequent phosphate fixation process; the outer mineralization zone contains a calcium- and / or magnesium-containing phosphate mineralization phase dispersed in the alginate network; the phosphate mineralization phase is formed by applying phosphate ions from the outer surface of the alginate coating to the granular fertilizer core and pre-crosslinking with calcium and magnesium ions and intermediate dehydration treatment, so that the phosphate ions react in situ with the calcium and magnesium ions in the coating, and are fixed in the outer mineralization zone by applying calcium and magnesium ions again from the outer surface; the inner ionic crosslinking zone and the outer mineralization zone constitute the same continuous alginate coating.

2. The seaweed-derived slow-release ecological fertilizer according to claim 1, characterized in that, The particle size of the granular fertilizer core is 2-5 mm; the mass fraction of the composite coating is 1.5%-5.0% based on the total dry weight of the final dried product, and the corresponding dry weight ratio of the granular fertilizer core to the composite coating is 98.5:1.5-95:

5.

3. The seaweed-derived slow-release ecological fertilizer according to claim 1, characterized in that, The alginate is selected from one or two of sodium alginate and potassium alginate; the film-forming polymer in the composite coating is composed of the alginate.

4. The seaweed-derived slow-release ecological fertilizer according to claim 1, characterized in that, The calcium and magnesium ions are provided by water-soluble calcium salts and water-soluble magnesium salts, with a molar ratio of calcium ions to magnesium ions of 3:1 to 8:1; the water-soluble calcium salt is selected from one or more of calcium chloride, calcium nitrate, and calcium acetate, and the water-soluble magnesium salt is selected from one or more of magnesium chloride, magnesium nitrate, and magnesium acetate; the phosphate used to form the phosphate mineralization phase is selected from one or more of potassium dihydrogen phosphate, dipotassium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

5. The seaweed-derived slow-release ecological fertilizer according to claim 1, characterized in that, By weight, the granular fertilizer core comprises 35-60 parts of urea, 15-30 parts of monoammonium phosphate and / or diammonium phosphate, 10-25 parts of potassium sulfate and / or potassium chloride, and 1-8 parts of binder; the binder is selected from one or more of lignin sulfonate, starch, and humate.

6. A method for preparing a seaweed-derived slow-release ecological fertilizer, characterized in that, Includes the following steps: S1. Mix fertilizer raw materials containing at least two of the following nutrients: nitrogen, phosphorus, and potassium, and granulate them to obtain granular fertilizer cores; S2. While the granular fertilizer core is in a rolling state, spray an alginate aqueous solution onto the surface of the granular fertilizer core to form an alginate liquid film on the surface of the granular fertilizer core. The alginate aqueous solution does not contain pre-made calcium phosphate particles, magnesium phosphate particles, or calcium magnesium phosphate particles. S3. The alginate is mainly ionically crosslinked with calcium ions, and magnesium ions are introduced into the alginate liquid film to form an inner ion crosslinking region treated with calcium and magnesium mixed salt. S4. The particles that have completed step S3 are subjected to intermediate dehydration treatment for 2 to 10 minutes under hot air conditions of 30 to 45°C, so that the alginate coating is initially shaped in a state that is not completely dry. S5. Spray an aqueous phosphate solution onto the outer surface of the alginate coating to allow phosphate ions to diffuse from the outside to the inside and react in situ with calcium and magnesium ions in the coating, forming a calcium- and / or magnesium-containing phosphate mineralization phase outside the inner ion crosslinking zone; then spray a second mixed salt solution containing calcium and magnesium salts onto the outer surface of the alginate coating again to fix the phosphate mineralization phase a second time, forming an outer mineralization zone that is continuous with the inner ion crosslinking zone; S6. The particles obtained in step S5 are dried to obtain the seaweed-derived slow-release ecological fertilizer.

7. The method for preparing a seaweed-derived slow-release ecological fertilizer according to claim 6, characterized in that, In step S1, 35-60 parts of urea, 15-30 parts of monoammonium phosphate and / or diammonium phosphate, 10-25 parts of potassium sulfate and / or potassium chloride, and 1-8 parts of binder are mixed by weight and granulated under the conditions of material moisture content of 5%-12% and material temperature of 20-45℃ to obtain granular fertilizer cores with a particle size of 2-5mm.

8. The method for preparing a seaweed-derived slow-release ecological fertilizer according to claim 6, characterized in that, In step S2, the mass concentration of the alginate aqueous solution is 1%~3% and the pH is 6.0~8.0; when spraying the alginate aqueous solution, the temperature of the granular fertilizer core is controlled at 30~45℃, and the alginate aqueous solution is adhered to the surface of the granular fertilizer core by alternating spraying and hot air treatment.

9. The method for preparing a seaweed-derived slow-release ecological fertilizer according to claim 6, characterized in that, The first mixed salt solution in step S3 and the second mixed salt solution in step S5 contain calcium salt and magnesium salt, respectively. The molar ratio of calcium ions to magnesium ions is 3:1 to 8:1, and the total mass concentration of calcium salt and magnesium salt is 0.5% to 2.0%. The ion crosslinking time in step S3 is 2 to 10 min, and the treatment time after spraying the second mixed salt solution in step S5 is 1 to 5 min.

10. The method for preparing a seaweed-derived slow-release ecological fertilizer according to claim 6, characterized in that, In step S5, the mass concentration of the phosphate aqueous solution is 0.1%~0.8%, the pH is 6.5~8.0, and the treatment time after spraying the phosphate aqueous solution is 1~8 min; steps S2, S3 and S5 are carried out in a rotating drum with a drum speed of 20~50 r / min and a spraying atomization pressure of 0.15~0.50 MPa; the drying temperature in step S6 is 40~55℃ and the drying time is 1~6 h.

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