Composite slow / controlled-release fertilizer taking steel slag as base material and preparation process of composite slow / controlled-release fertilizer

Abstract

The invention discloses a preparation process of a controlled-release fertilizer taking steel slag as a base material, which comprises the following steps: grinding and screening the steel slag to obtain 40-400-mesh steel slag powder; dissolving half of the solid nutrient in water to obtain a nutrient solution, adding the steel slag powder and the bio-based auxiliary material, fully mixing and adsorbing, and drying to obtain a nutrient-loaded substrate material; continuously mixing the residual solid nutrients with the nutrient-loaded substrate material, adding the coating material A as a binder, carrying out blending granulation in a granulator, and drying to obtain an inner core; adding the inner core into a coating machine, adding a coating material B for coating, and drying to obtain the slow / controlled-release fertilizer. According to the invention, the steel slag powder and the bio-based auxiliary material are used as carriers, heavy metal pollutants are stabilized while the soil agglomeration structure is improved through double mechanisms of physical adsorption and chemical immobilization, and the effects of slow / controlled-release fertilizer and soil improvement are realized.

Description

Technical Field

[0001] This invention belongs to the field of composite controlled-release fertilizer technology, specifically relating to a composite controlled-release fertilizer based on steel slag and its preparation process. Background Technology

[0002] The widespread use of traditional chemical fertilizers in agriculture is accompanied by a significant drawback: low fertilizer utilization efficiency. Nutrients (such as nitrogen, phosphorus, and potassium) are released into the soil too rapidly, making it difficult for crops to absorb them simultaneously, resulting in substantial nutrient loss. This explosive release not only wastes resources but also leads to serious environmental consequences—nutrients enter water bodies with rainwater, causing eutrophication and harming aquatic ecosystems; long-term accumulation of residual fertilizers also disrupts soil microbial balance, damages soil health, and hinders sustainable agricultural development. Therefore, improving fertilizer utilization efficiency and reducing pollution emissions are key to the green transformation of agriculture. Controlled-release fertilizers, as an innovative solution, successfully address the three major pain points of traditional chemical fertilizers—low utilization efficiency, explosive nutrient release, and environmental pollution—by reducing the nutrient release rate.

[0003] Steel is a crucial pillar and fundamental industry in my country's industrial system. During steel production, steel slag, a typical industrial solid waste, poses environmental risks such as heavy metal leaching.

[0004] CN115583855A discloses a method for preparing controlled-release fertilizer, including mixing biochar and nutrient solution for thorough adsorption; then evaporating the solution to dryness to obtain nutrient-loaded biochar; further blending and granulating the nutrient-loaded biochar with solid nutrients to obtain biochar-nutrient blended particles; and finally coating the biochar-nutrient blended particles with polyurethane material. The dual slow-release mechanism of biochar carrier and polymer coating technology has clear value in nutrient controlled release: biochar relies on its porous structure to complete initial loading and slow release, while the outer coating constitutes a physical barrier to prolong the release period. However, this system has significant ecological risks. Biochar has a slow effect on soil improvement, and the physically adsorbed heavy metals are difficult to stabilize due to reversible action, resulting in limited actual remediation efficiency. More seriously, the polymer coating is difficult to degrade in the environment, continuously forming microplastic pollution, and the remaining unreacted monomers pose a potential threat to the soil ecosystem. This technology urgently needs breakthroughs in environmental safety and ecological compatibility. Summary of the Invention

[0005] The purpose of this invention is to provide a composite slow-release fertilizer based on steel slag, using steel slag powder and bio-based additives as carriers. It improves the soil aggregate structure and stabilizes heavy metal pollutants through a dual mechanism of physical adsorption and chemical fixation, thereby achieving the effects of slow-release fertilizer and soil improvement.

[0006] To achieve the above objectives, the following technical solution is adopted: A process for preparing a controlled-release fertilizer based on steel slag includes the following steps: S1. Grind and sieve the steel slag to obtain steel slag powder with a mesh size of 40-400. S2. Dissolve half of the solid nutrients in water to obtain a nutrient solution, add steel slag powder and bio-based auxiliary materials, mix thoroughly for adsorption, and dry to obtain a nutrient-loaded substrate material. S3. Continue to mix the remaining solid nutrients with the substrate material loaded with nutrients, and add coating material A as a binder. Co-mix and granulate in a granulator, and dry to obtain the core. S4. Add the kernel to the coating machine and add coating material B for coating. After drying, the slow-release fertilizer is obtained.

[0007] According to the above scheme, the steel slag mentioned in step S1 is an inorganic mineral solid residue produced during the iron and steel smelting process; its main components are SiO2, CaO, and Fe2O3.

[0008] According to the above plan, the raw materials used are as follows by weight: 50-90 parts solid nutrients, 10-50 parts steel slag powder, and 5-20 parts bio-based auxiliary materials.

[0009] According to the above scheme, the solid nutrients mentioned in step S2 are any one or a mixture of urea, ammonium sulfate, ammonium bicarbonate, ammonium chloride, superphosphate, ammonium phosphate, phosphorus nitrate, potassium dihydrogen phosphate, potassium nitrate, and trace element fertilizer.

[0010] According to the above scheme, the bio-based auxiliary material in step S2 is one or more of biochar, straw powder, and sawdust, with a size of 20-180 mesh. The straw powder can be selected from rice, corn, or reed straw.

[0011] According to the above scheme, the drying temperature in step S2 is 30-100℃.

[0012] According to the above scheme, the amount of coating material A added in step S3 is 1-20 wt% of the total mass; the coating material A is any one or a mixture of several bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, or their modified materials. The coating material can be formulated into a suitable single or mixed liquid as needed.

[0013] According to the above scheme, the amount of coating material B added in step S4 is 1-10 wt% of the core; the coating material B is any one or a mixture of several bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, etc., or their modified materials. The coating material can be formulated into a suitable single or mixed liquid as needed.

[0014] According to the above scheme, the drying temperature in steps S3 and S4 is 30-100℃.

[0015] The present invention also provides a controlled-release fertilizer based on steel slag, which is prepared by the above preparation process.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: Bio-based additives, with their high specific surface area and abundant porous structure, can efficiently adsorb and load water-soluble phosphorus nutrients, constructing a physical barrier to achieve slow nutrient release, prolonging the fertilizer's effective period and improving phosphorus utilization. Simultaneously, their abundant oxygen-containing functional groups can regulate soil pH and improve the soil microenvironment through ion exchange and protonation / deprotonation. Steel slag, rich in calcium, iron, magnesium, and other metal oxides, fixes phosphorus through chemical precipitation and surface complexation, forming highly stable phosphate complexes that significantly reduce phosphorus leaching loss. Its inherent silicon, calcium, magnesium, and other essential plant nutrients synergistically promote crop growth, and its alkaline properties neutralize acidic soils, enhancing soil phosphorus availability. The combination of these two materials as a base material can synergistically optimize the particle structure of slow-release phosphate fertilizers, enhance physical strength, provide stable support for subsequent bio-based oil coating, and simultaneously achieve high-value utilization of agricultural and industrial waste.

[0017] Bio-based oil coating materials play a dual role in fertilizer granulation and coating processes, achieving synergistic structural effects. As a granulation binder, the coating material penetrates and fills the pore structure of the steel slag-bio-based auxiliary material composite, forming a dense core matrix that significantly enhances the mechanical strength of the particles. In the coating stage, the continuous hydrophobic film formed by the bio-based oil coating material and the binder network in the core form a gradient barrier structure, establishing a dual controlled-release mechanism through precise regulation of molecular diffusion paths. This "matrix-coating" composite barrier system effectively extends the nutrient release cycle, and its fully bio-based characteristics thoroughly solve the microplastic contamination problem of synthetic coating materials. Attached Figure Description

[0018] Figure 1 This invention presents a schematic diagram of the controlled-release fertilizer release mechanism based on steel slag.

[0019] Figure 2 : Cumulative release rate graph of controlled-release fertilizer obtained from the examples and comparative examples.

[0020] Figure 3 Photograph of the controlled-release fertilizer product obtained in Example 1.

[0021] Figure 4 Photograph of the controlled-release fertilizer product obtained in Example 2.

[0022] Figure 5 Photo of the controlled-release fertilizer product obtained in Comparative Example 1. Detailed Implementation

[0023] The following embodiments further illustrate the technical solution of the present invention, but are not intended to limit the scope of protection of the present invention.

[0024] A specific implementation provides a process for preparing a controlled-release fertilizer based on steel slag: S1. Grind and sieve the steel slag to obtain steel slag powder with a mesh size of 40-400. S2. Dissolve half of the solid nutrients in water to obtain a nutrient solution, add steel slag powder and bio-based auxiliary materials, mix thoroughly for adsorption, and dry to obtain a nutrient-loaded substrate material. S3. Continue to mix the remaining solid nutrients with the substrate material loaded with nutrients, and add coating material A as a binder. Co-mix and granulate in a granulator, and dry to obtain the core. S4. The core is added to a coating machine, and coating material B is added for coating. After drying, the controlled-release fertilizer is obtained. A schematic diagram of the release mechanism of the controlled-release fertilizer based on steel slag is attached. Figure 1 As shown.

[0025] Specifically, the raw materials used, by weight, are as follows: 50-90 parts solid nutrients, 10-50 parts steel slag powder, and 5-20 parts bio-based auxiliary materials. The solid nutrients can be any one or a mixture of urea, ammonium sulfate, ammonium bicarbonate, ammonium chloride, superphosphate, ammonium phosphate, phosphorus nitrate, potassium dihydrogen phosphate, potassium nitrate, and micronutrient fertilizers. The bio-based auxiliary materials are one or a mixture of biochar, straw powder, and sawdust, with a mesh size of 20-180 mesh. The straw powder can be selected from rice, corn, or reed straw. In the optimized scheme, the drying temperature is 30-100℃.

[0026] Specifically, in step S3, the amount of coating material added is 1-20 wt% of the total mass of the remaining solid nutrients and the substrate material carrying the nutrients; the coating material is any one or a mixture of several bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, etc., or their modified materials. The coating material can be formulated into a suitable single or mixed liquid as needed.

[0027] Specifically, the amount of coating material added in step S4 is 1-10 wt% of the core; the coating material is any one or a mixture of several bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, or their modified forms. The coating material can be formulated as a suitable single or mixed liquid as needed.

[0028] In the optimized solution, the drying temperature in steps S3 and S4 is 30-100℃. A specific embodiment provides a steel slag, the composition of which is shown in Table 1 after X-ray fluorescence spectroscopy analysis.

[0029] Table 1

[0030] The following specific embodiments used the aforementioned steel slag samples. All other raw materials, unless otherwise specified, were obtained commercially.

[0031] Example 1 S1. Grind and sieve the steel slag to obtain 100-mesh steel slag; S2. Take 70 parts of potassium dihydrogen phosphate, dissolve 35 parts of potassium dihydrogen phosphate in an appropriate amount of water, and add 10 parts of 100-mesh steel slag and 10 parts of 100-mesh biochar. The weight of the fertilizer does not include the weight of the water used for dissolution. Add the above mixture to a beaker, mechanically stir for 10 minutes to fully dissolve and mix, let stand for 10 minutes to allow the substrate to fully absorb the nutrient, and then dry the entire mixture in a 100℃ oven to obtain the nutrient-loaded substrate material. S3. Add the remaining 35 parts of potassium dihydrogen phosphate from S2 and the nutrient-loaded substrate material obtained in S2 to a round pot granulator, add 10 parts of beeswax as a binder, and wet granulate at 65-70℃. Let it stand and cool naturally to obtain the core material. S4. Add the kernel material obtained in S3 to the rotary drum coating machine, and spray the kernel material with beeswax at 65-70℃. The amount of beeswax added is 5wt% of the kernel material. After coating, let it stand and cool naturally to obtain the slow-release fertilizer.

[0032] A photograph of the controlled-release fertilizer product obtained in this embodiment is attached. Figure 3 As shown.

[0033] Example 2 S1. Grind and sieve the steel slag to obtain 100-mesh steel slag; S2. Take 70 parts of potassium dihydrogen phosphate, dissolve 35 parts of potassium dihydrogen phosphate in an appropriate amount of water, and add 10 parts of 100-mesh steel slag and 10 parts of 40-mesh rice straw powder. The weight of the fertilizer does not include the weight of the water used for dissolution. Add the above mixture to a beaker, mechanically stir for 10 minutes to fully dissolve and mix, let stand for 10 minutes to allow the substrate to fully absorb the nutrient, and then dry the whole thing in a 100℃ oven to obtain the nutrient-loaded substrate material. S3. Add the remaining 35 parts of potassium dihydrogen phosphate from S2 and the nutrient-loaded substrate material obtained in S2 to a round pot granulator, add 10 parts of soybean wax as a binder, and wet granulate at 55-60℃. Let it stand and cool naturally to obtain the core material. S4. Add the kernel material obtained in S3 to the rotary drum coating machine, and spray the kernel material with soybean wax at 55-60℃. The amount of soybean wax added is 5wt% of the kernel material. After coating, let it stand and cool naturally to obtain slow-release fertilizer.

[0034] A photograph of the controlled-release fertilizer product obtained in this embodiment is attached. Figure 4 As shown.

[0035] Example 3 S1. Grind and sieve the steel slag to obtain 200-mesh steel slag; S2. Take 70 parts of potassium dihydrogen phosphate, dissolve 35 parts of potassium dihydrogen phosphate in an appropriate amount of water, and add 10 parts of 100-mesh steel slag and 10 parts of 100-mesh biochar. The weight of the fertilizer does not include the weight of the water used for dissolution. Add the above mixture to a beaker, mechanically stir for 10 minutes to fully dissolve and mix, let stand for 10 minutes to allow the substrate to fully absorb the nutrient, and then dry the entire mixture in a 100℃ oven to obtain the nutrient-loaded substrate material. S3. Add the remaining 35 parts of potassium dihydrogen phosphate from S2 and the nutrient-loaded substrate material obtained in S2 to a round pot granulator, add 10 parts of beeswax as a binder, and wet granulate at 65-70℃. Let it stand and cool naturally to obtain the core material. S4. Add the kernel material obtained in S3 to the rotary drum coating machine, and spray the kernel material with beeswax at 65-70℃. The amount of beeswax added is 5wt% of the kernel material. After coating, let it stand and cool naturally to obtain the slow-release fertilizer.

[0036] Comparative Example 1 In Example 1, the wet granulation method in step S3 is changed to tableting, while the other steps remain unchanged.

[0037] A photograph of the actual product obtained in Comparative Example 1 (sheet form) is attached. Figure 5 As shown.

[0038] Comparative Example 2 In Example 2, replace the 40-mesh rice straw powder added in step S2 with 200-mesh rice straw, and keep the other steps unchanged.

[0039] Comparative Example 3 The added material in Example 3 is removed, while the remaining steps remain unchanged.

[0040] 10g of each of the coated slow-release fertilizers obtained in the examples and comparative examples were weighed out and soaked in 200g of purified water and placed in a constant temperature incubator at 25℃. The effective nutrient release rate of the fertilizers was measured at 1, 3, 5, 10, 28, 56, and 84 days according to the GB / T23348-2009 standard. The test results are shown in the appendix. Figure 2 As shown.

[0041] According to the GB / T 23348-2009 standard, the initial nutrient release rate (1 day) of slow-release fertilizer should be ≤15%, and the cumulative release rate after 28 days should be ≤80%. Comparing the data of Example 1 (spherical) and Comparative Example 1 (flake), it was found that the slow-release effect of spherical fertilizer was slightly better. The reasons are: (1) Under the same mass, the specific surface area of ​​spherical fertilizer is lower than that of flake fertilizer. The release rate is also relatively lower. (2) Flake has obvious edges at the bottom edge, and the water erosion and internal stress at the edges are greater, and the surface film is prone to cracks or ruptures; while the water erosion and internal stress of spherical is more dispersed, and it is less likely to crack. It can be seen that spherical is better at delaying the release rate than flake and other shapes, thereby prolonging the slow-release period.

[0042] Compared with Comparative Example 2, an excessively high mesh size of rice straw additive impairs the slow-release effect. The mechanism is that an excessively high mesh size, on the one hand, damages the porous structure of the straw, weakening its slow-release capacity; on the other hand, it causes the straw to become powdery, losing its anchoring effect in the core, resulting in core instability and accelerated disintegration.

[0043] Compared with Comparative Example 3, without the addition of bio-based excipients, the slow-release effect is significantly worse mainly due to a sharp decrease in adsorption capacity and weakened particle structure strength. On the one hand, without the support of the high specific surface area and porous structure of bio-based excipients, the fertilizer core has insufficient capacity to retain nutrients, making it impossible to achieve internal slow release through adsorption-desorption mechanisms, thus making nutrients more easily and rapidly lost. On the other hand, the particle interior becomes loose due to the lack of skeletal support from bio-based excipients, making it prone to rapid disintegration upon entering water. This causes the solid nutrients in the core to be directly exposed and quickly dissolved, greatly reducing the slow-release effect of the outer coating and ultimately disrupting the stable release of nutrients.

Claims

1. A preparation process for a controlled-release fertilizer based on steel slag, characterized in that... Includes the following steps: S1. Grind and sieve the steel slag to obtain steel slag powder with a mesh size of 40-400. S2. Dissolve half of the solid nutrients in water to obtain a nutrient solution, and add steel slag powder and bio-based auxiliary materials to fully mix and adsorb, then dry to obtain a nutrient-loaded substrate material. S3. Continue to mix the remaining solid nutrients with the substrate material loaded with nutrients, and add coating material A as a binder. Co-mix and granulate in a granulator, and dry to obtain the core. S4. Add the kernel to the coating machine and add coating material B for coating. After drying, the slow-release fertilizer is obtained.

2. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The steel slag mentioned in step S1 is an inorganic mineral solid residue produced during the iron and steel smelting process; its main components are SiO2, CaO, and Fe2O3.

3. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The raw materials used are as follows by weight: 50-90 parts solid nutrients, 10-50 parts steel slag powder, and 5-20 parts bio-based auxiliary materials.

4. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The solid nutrients mentioned in step S2 are any one or a mixture of urea, ammonium sulfate, ammonium bicarbonate, ammonium chloride, superphosphate, ammonium phosphate, phosphorus nitrate, potassium dihydrogen phosphate, potassium nitrate, and trace element fertilizers.

5. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The bio-based auxiliary material mentioned in step S2 is one or more of biochar, straw powder, and wood chips, with a size of 20-180 mesh.

6. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The drying temperature in step S2 is 30-100℃.

7. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The amount of coating material A added in step S3 is 1-20 wt% of the total mass; the coating material A is any one or a mixture of several of bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, or their modified materials.

8. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The amount of coating material B added in step S4 is 1-10 wt% of the core; the coating material B is any one or a mixture of several of bio-based oils such as beeswax, tung oil, wood wax, soybean wax, shellac, or their modified materials.

9. The preparation process of the controlled-release fertilizer based on steel slag as described in claim 1, characterized in that... The drying temperature in steps S3 and S4 is 30-100℃.

10. A controlled-release fertilizer based on steel slag, characterized in that... The controlled-release fertilizer based on steel slag is prepared using the preparation process described in any one of claims 1-9.

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