Straw carbon coated fertilizer and preparation method thereof

Abstract

The invention provides a straw carbon coated fertilizer and a preparation method thereof, and belongs to the technical field of agricultural waste utilization and fertilizer production, and the preparation method comprises the following steps: in a granulator, raising the temperature of fertilizer granules to 45-55 DEG C, spraying water on the surfaces of the fertilizer granules, and carrying out preliminary modification on the fertilizer granules; the temperature is raised to 50-65 DEG C, after the surfaces of the fertilizer particles are dried, bentonite bonding liquid is sprayed to the surfaces of the fertilizer particles, meanwhile, straw carbon powder is scattered to the surfaces of the fertilizer particles, and a compact biochar wrapping layer is formed on the surfaces of the fertilizer particles; and drying to obtain the straw carbon coated fertilizer. According to the preparation method of the straw carbon coated fertilizer, straw carbon prepared from agricultural waste straw is used as a biochar raw material and is mixed with fertilizer particles for granulation, and in the granulation process, through preliminary modification of the surfaces of the fertilizer particles and control of the use amount of a binding solution and the use amount of biochar, the fertilizer particles are coated with the straw carbon, so that the yield of the fertilizer is increased; the straw carbon coated fertilizer with good slow and controlled release characteristics and high utilization rate is prepared.

Description

Technical Field

[0001] The invention belongs to the technical field of agricultural waste utilization and fertilizer production, and more specifically, relates to a straw charcoal coated fertilizer and a preparation method thereof. Background Art

[0002] As an agricultural power, my country faces the dual challenges of population growth and limited land resources. According to data from 2023, the country's arable land area is about 1.94 billion mu, and the per capita arable land level is only about 1.4 mu. At the same time, my country's nitrogen fertilizer utilization rate is only 30% to 35%, which is significantly lower than the global level of more than 60%. This gap in fertilizer utilization efficiency not only affects crop growth and yield, but also puts pressure on the sustainable management of agricultural resources. Therefore, it is imperative to develop efficient and environmentally friendly fertilizer technology.

[0003] Slow-release fertilizers have attracted widespread attention because of their ability to slowly release nutrients. Their use can effectively reduce fertilizer loss, improve fertilizer utilization, and thus reduce environmental pollution. In this context, biochar, as a new type of fertilizer carrier, shows good application prospects due to its stability, large specific surface area, and good nutrient release performance. Biochar is produced by high-temperature pyrolysis of organic waste (such as straw), is rich in carbon, hydrogen, oxygen and other elements, and has strong biochemical stability.

[0004] In the agricultural field, biochar has significant application potential, including soil improvement, nutrient supply and pollutant removal. However, existing studies have shown that biochar has complex effects on plant nutrient absorption and growth, and the nutrient release and capture mechanisms of different biochars have not been fully elucidated. Therefore, how to combine biochar with traditional fertilizers to prepare straw charcoal-coated fertilizers with good slow-release properties and high utilization rate is a difficult problem in the development of biochar-based fertilizers. Summary of the invention

[0005] The object of the present invention is to provide a straw charcoal coated fertilizer and a preparation method thereof, so as to prepare the straw charcoal coated fertilizer with good slow-controlled release characteristics and high utilization rate.

[0006] To achieve the above object, the first aspect of the present invention provides a method for preparing straw charcoal coated fertilizer, comprising the following steps:

[0007] In the granulator, the fertilizer granules are heated to 45-55°C and water is sprayed on the surface of the fertilizer granules to perform preliminary modification on the fertilizer granules;

[0008] The temperature is raised to 50-65°C. After the surface of the fertilizer particles is dry, bentonite binding liquid is sprayed on the surface of the fertilizer particles. At the same time, straw charcoal powder is sprinkled on the surface of the fertilizer particles to form a tight biochar coating layer on the surface of the fertilizer particles.

[0009] After drying, the straw charcoal coated fertilizer is obtained.

[0010] Furthermore, during the preliminary modification process, the amount of water sprayed on the surface of the fertilizer particles is 8-12% of the mass of the fertilizer particles.

[0011] Furthermore, the spraying amount of the bentonite binder is 4-10% of the mass of the fertilizer particles.

[0012] Furthermore, in the bentonite binder, the mass concentration of bentonite is 10-30%.

[0013] Furthermore, the amount of the straw charcoal powder sprinkled is 10-30% of the mass of the fertilizer particles.

[0014] Furthermore, the straw charcoal powder is at least one of corn straw charcoal powder, wheat straw charcoal powder, and rice straw charcoal powder.

[0015] Furthermore, the fertilizer particles are at least one of diammonium phosphate, ammonium dihydrogen phosphate, urea, ammonium sulfate, and potassium chloride.

[0016] Furthermore, the bentonite binder is prepared by the following method:

[0017] Sieve the bentonite to remove impurities;

[0018] Adding water to bentonite to prepare a bentonite suspension, and centrifuging to separate a bentonite precipitate;

[0019] After drying, bentonite concentrate is obtained, and water is added to the bentonite concentrate to prepare the bentonite binding liquid.

[0020] Furthermore, the bentonite was sieved through a 200-mesh sieve to remove impurities.

[0021] The second aspect of the present invention provides a straw charcoal coated fertilizer obtained by any of the preparation methods described above.

[0022] Compared with the prior art, the present invention has the following technical effects:

[0023] The method for preparing a straw charcoal coated fertilizer of the present invention utilizes straw charcoal prepared from agricultural waste straw as a biochar raw material, and mixes the biochar with fertilizer particles for granulation. During the granulation process, the surface of the fertilizer particles is preliminarily modified and the amount of the binder and the amount of the biochar are controlled, so as to prepare the straw charcoal coated fertilizer with good slow-controlled release characteristics and high utilization rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0025] Figure 1 This is a comparison chart of the nutrient release rates of the straw charcoal coated fertilizer and DAP prepared in Examples 1-3 of the present invention;

[0026] Figure 2 Comparison chart of the cumulative dissolution rates of the straw charcoal coated fertilizers prepared in Examples 4-7 of the present invention;

[0027] Figure 3 Cumulative dissolution rate diagram of the straw charcoal coated fertilizer prepared in Example 6 of the present invention;

[0028] Figure 4 This is a comparison chart of the cumulative dissolution rates of the straw charcoal coated fertilizers prepared in Examples 1-3 of the present invention;

[0029] Figure 5 This is a SEM image of the straw charcoal coated fertilizer prepared in Example 6 of the present invention. DETAILED DESCRIPTION

[0030] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The singular forms "a", "said" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0032] The embodiment of the present invention provides a straw charcoal coated fertilizer and a preparation method thereof, comprising the following steps:

[0033] (1) In a granulator, the fertilizer granules are heated to 45-55°C, and water is sprayed on the surface of the fertilizer granules to perform preliminary modification on the fertilizer granules;

[0034] (2) Raising the temperature to 50-65° C., after the surface of the fertilizer particles is dry, spraying bentonite binding liquid on the surface of the fertilizer particles, and at the same time sprinkling straw charcoal powder on the surface of the fertilizer particles, so that a tight biochar coating layer is formed on the surface of the fertilizer particles;

[0035] (3) After drying, the straw charcoal coated fertilizer is obtained.

[0036] In the above step (1), the fertilizer particles used in the embodiment of the present invention are sieved particles with a particle size of about 4 mm. By spraying water evenly on the surface of the fertilizer particles, the surface of the fertilizer particles will slightly dissolve, forming a thin layer of water film, which is conducive to the initial bonding between the fertilizer particles. This can provide a better foundation for subsequent granulation, coating and other processes, and improve the strength and stability of the fertilizer particles. In addition, spraying water on the surface of the fertilizer particles can improve the surface activity of the fertilizer particles. After the water is sprayed, a wet layer is formed on the surface of the fertilizer particles, which can improve the surface activity of the particles, so that subsequent materials such as biochar coating agents can be more evenly attached to the surface of the fertilizer particles, thereby improving the adhesion effect of the biochar coating agent. Furthermore, the initial modification can adjust the dissolution rate of the fertilizer particles, so that it releases nutrients more evenly in the soil, avoids the problem of excessive or uneven fertilizer effect, and thus improves the utilization efficiency of the fertilizer.

[0037] Furthermore, in order to achieve a good initial modification effect, during the initial modification process, the amount of water sprayed on the surface of the fertilizer particles is 8-12% of the mass of the fertilizer particles.

[0038] In the above step (2), after the fertilizer particles are initially modified, the temperature is continued to rise to 50-65°C, so that the moisture on the surface of the fertilizer particles can be properly evaporated, so as to avoid the fertilizer particles being too wet and resulting in loose bonding, or causing subsequent uneven distribution of biochar on the surface of the fertilizer particles. The spraying amount of the bentonite binder used in the embodiment of the present invention is 4-10% of the mass of the fertilizer particles. If a thicker coating layer is required, the spraying amount of the bentonite binder can be increased to nearly 10%; if only a thin layer of protection is required, the spraying amount of the bentonite binder is more suitable at 5%-7%. In addition, when the fertilizer particles are small or the surface is rough, increasing the spraying amount of the bentonite binder can help the binder to cover evenly. When the amount of biochar used is high or the particles are coarse, the amount of bentonite binder can also be appropriately increased to ensure a firm bond between the biochar powder and the surface of the fertilizer particles.

[0039] The bentonite binder used in the embodiment of the present invention has a bentonite mass concentration of 10-30% and the amount of straw charcoal powder added is 10-30% of the mass of the fertilizer particles. The embodiment of the present invention prepares straw charcoal coated fertilizer with good slow-release properties and high utilization rate by controlling the amount of binder and biochar.

[0040] In the embodiment of the present invention, the straw charcoal powder is at least one of corn straw charcoal powder, wheat straw charcoal powder and rice straw charcoal powder. Specifically, the straw charcoal powder can be prepared by the following method:

[0041] 1) Preparation of straw powder: Cut the straw into small pieces, wash them, put them into an oven at 70-80°C for drying, put them into a grinder, grind them twice, and sieve the ground straw powder with a 100-mesh sieve.

[0042] 2) Carbonization: Weigh 10g of dry straw powder, pour the dried straw powder into a porcelain boat, then place it in a tube furnace, heat it to 600℃ at a rate of 5℃ / min under a nitrogen atmosphere, keep it warm for 2h, and take it out after cooling. Subsequently, dissolve the prepared powder in a 2mol / L hydrochloric acid solution, stir it in a magnetic stirrer for 0.5h, and then soak it for 12h to remove possible metal ions. Finally, wash it with distilled water until it is neutral, and dry it in an oven at 80℃ for 7h to obtain straw charcoal powder.

[0043] In order to obtain bentonite concentrate with finer particle size and higher purity, the uniformity and stability of the binder are improved, thereby improving the slow-release performance and utilization rate of the prepared straw charcoal coated fertilizer. Further, the bentonite binder of the embodiment of the present invention is prepared according to the following method:

[0044] 1) sieving the bentonite to remove impurities, specifically, sieving the bentonite through a 200-mesh sieve to remove large particles of impurities;

[0045] 2) adding an appropriate amount of water to the bentonite to prepare a bentonite suspension, and then subjecting the suspension to high-speed centrifugal separation to obtain a bentonite precipitate, which removes water-soluble impurities in the bentonite;

[0046] 3) Collect the bentonite precipitate with finer particle size separated by centrifugation, put it into a drying oven at 60° C. and dry it for 24 hours to obtain bentonite concentrate, and add water to the bentonite concentrate to prepare bentonite binder liquid.

[0047] The preparation method of the straw charcoal coated fertilizer of the embodiment of the present invention is applicable to a variety of conventional chemical fertilizers, and the optional fertilizer particles can be at least one of diammonium phosphate, ammonium dihydrogen phosphate, urea, ammonium sulfate, and potassium chloride. The embodiment of the present invention specifically takes diammonium phosphate (DAP for short) as an example for explanation. Most granular fertilizers (such as urea, potassium chloride, etc.) are similar to diammonium phosphate in shape, particle size, etc., and therefore can adapt to similar spraying and coating processes in terms of preparation methods. However, in the specific preparation process, according to the different physical properties of the selected fertilizer particles, parameters such as the water spraying amount, operating temperature, binder dosage, and biochar coating layer thickness can be adjusted to ensure the stability of the prepared straw charcoal coated fertilizer, and to ensure that it has good slow-release properties and high fertilizer utilization.

[0048] A straw charcoal coated fertilizer and a preparation method thereof according to an embodiment of the present invention are illustrated below by means of a plurality of specific embodiments.

[0049] Example 1

[0050] Embodiment 1 of the present invention provides a straw charcoal coated fertilizer and a preparation method thereof, comprising the following steps:

[0051] (1) Add a certain amount of sieved (about 4 mm) diammonium phosphate particles into the granulator. When the temperature of the fertilizer particles is raised to 50°C, use a watering can to evenly spray pure water on the surface of the fertilizer particles to perform preliminary modification on the surface of the fertilizer particles. The amount of water sprayed on the surface of the fertilizer particles is 10% of the mass of the fertilizer particles.

[0052] (2) Continue to raise the temperature to 65°C, wait for a while until the surface of the fertilizer particles is dry, and then spray a 20% by mass bentonite binder on the surface of the fertilizer particles in small amounts and multiple times, so that the spraying amount of the bentonite binder is 7% of the mass of the fertilizer particles; at the same time, slowly sprinkle corn straw charcoal powder accounting for 10% of the mass of the fertilizer particles, rotate for about five minutes, and form a tight coating layer on the surface of the fertilizer particles.

[0053] (3) The prepared fertilizer granules are placed in a dark place, dried for 24 hours, and then sealed in bags to obtain straw charcoal coated fertilizer, which is marked as R1.

[0054] Example 2

[0055] The difference from Example 1 is that corn straw charcoal powder accounting for 20% of the mass of the fertilizer particles is sprinkled in step (2), and the other steps are the same. The obtained straw charcoal coated fertilizer is marked as R2.

[0056] Example 3

[0057] The difference from Example 1 is that corn straw charcoal powder accounting for 30% of the mass of the fertilizer particles is sprinkled in step (2), and the other steps are the same. The obtained straw charcoal coated fertilizer is marked as R3.

[0058] Example 4

[0059] Embodiment 4 of the present invention provides a straw charcoal coated fertilizer and a preparation method thereof, comprising the following steps:

[0060] (1) Add a certain amount of sieved (about 4 mm) diammonium phosphate particles into the granulator. When the temperature of the fertilizer particles is raised to 50°C, use a watering can to evenly spray pure water on the surface of the fertilizer particles to perform preliminary modification on the surface of the fertilizer particles. The amount of water sprayed on the surface of the fertilizer particles is 10% of the mass of the fertilizer particles.

[0061] (2) Continue to raise the temperature to 65°C, wait for a while until the surface of the fertilizer particles is dry, and then spray a 10% by mass bentonite binder on the surface of the fertilizer particles in small amounts and multiple times, so that the spraying amount of the bentonite binder is 6% of the mass of the fertilizer particles; at the same time, slowly sprinkle corn straw charcoal powder accounting for 30% of the mass of the fertilizer particles, rotate for about five minutes, and form a tight coating layer on the surface of the fertilizer particles.

[0062] (3) The prepared fertilizer granules are placed in a dark place, dried for 24 hours, and then sealed in bags to obtain straw charcoal coated fertilizer, which is marked as S1.

[0063] Example 5

[0064] The difference from Example 4 is that in step (2), a bentonite binder having a mass fraction of 15% is sprayed on the surface of the fertilizer particles in small amounts and multiple times, so that the spraying amount of the bentonite binder is 6% of the mass of the fertilizer particles, and the other steps are the same. The obtained straw charcoal coated fertilizer is marked as S2.

[0065] Example 6

[0066] The difference from Example 4 is that in step (2), a bentonite binder having a mass fraction of 20% is sprayed on the surface of the fertilizer particles in small amounts and multiple times, so that the spraying amount of the bentonite binder is 6% of the mass of the fertilizer particles, and the other steps are the same. The obtained straw charcoal coated fertilizer is marked as S3.

[0067] Example 7

[0068] The difference from Example 4 is that in step (2), a bentonite binder having a mass fraction of 30% is sprayed on the surface of the fertilizer particles in small amounts and multiple times, so that the spraying amount of the bentonite binder is 6% of the mass of the fertilizer particles, and the other steps are the same. The obtained straw charcoal coated fertilizer is marked as S4.

[0069] Comparative Example 1

[0070] The difference between this method and Example 1 is that in step (1), pure water is not sprayed on the surface of the fertilizer particles, that is, the fertilizer particles are not preliminarily modified, and the other steps are the same.

[0071] The straw charcoal coated fertilizer and DAP prepared in Examples 1-3 of the present invention were subjected to a soil column leaching experiment. The specific experimental process was as follows: In this experiment, a soil column was constructed by a Buchner funnel and a 300-mesh screen to ensure that the soil particles were evenly distributed. First, naturally air-dried potted soil was screened, 100 g was filled in a glass tube, and the height was controlled to be 5 cm. Subsequently, a slow-release fertilizer was placed on the top of the soil column and covered with about 400 g of soil to ensure that the fertilizer was completely wrapped. In order to reduce the scouring force of the water flow, 25 g of quartz sand was evenly covered on the top. The operation was repeated twice for each fertilizer sample to improve accuracy. During the first leaching treatment, 150 mL of distilled water was slowly added, and 5 mL of 10% H2SO4 was added. The temperature was kept at 25 ° C and constant for 24 hours before leaching. Each time 100 mL of distilled water was used for leaching, the filtrate was collected and its total nitrogen content was determined, and a nutrient cumulative dissolution (i.e., nutrient release) curve was plotted.

[0072] The experimental results are as follows Figure 1 As shown, Figure 1The nutrient release rate on the vertical axis is the cumulative dissolution rate of ammonia nitrogen (total nitrogen) in water over time. Figure 1 It can be seen that the nutrient release rate of diammonium phosphate (DAP) on the first day was as high as 77.35%, while the nutrient release of biochar coated fertilizers in groups R1, R2 and R3 was significantly lower, at 9.86%, 7.03% and 3.87%, respectively. This comparison shows that the nutrient release rate of diammonium phosphate is much faster than that of groups R1, R2 and R3. This significant difference stems from the high solubility of diammonium phosphate particles, which makes its nutrient release extremely rapid. In contrast, groups R1, R2 and R3 showed a significant slow-release effect due to the biochar coating, and this effect became more obvious as the biochar content increased. By the 13th day, almost all the nutrients of diammonium phosphate were released, while the nutrient release of the other groups of coated fertilizers remained stable, once again verifying the significant slow-release effect of biochar-coated fertilizers. After 28 days of observation, the cumulative nutrient release of groups R1, R2 and R3 was 65.77%, 53.62% and 30.81%, respectively, all meeting the standards of slow-release fertilizers. It is worth noting that the cumulative nutrient release of group R3 was much lower than that of the other two groups, which may be due to the increase in the amount of biochar, which reduced the effective proportion of fertilizer in the coated fertilizer, making it more difficult for nutrients to dissolve.

[0073] The same method was used to conduct a soil column leaching experiment on the straw charcoal coated fertilizer prepared in Comparative Example 1 of the present invention, and it was found that the nutrient release rate of the biochar coated fertilizer prepared in Comparative Example 1 reached 24.86% on the first day, and on the 14th day, its nutrients were basically completely released. Compared with Example 1, the controlled release performance of the straw charcoal coated fertilizer in Comparative Example 1 was significantly reduced.

[0074] Using the same method, the straw charcoal coated fertilizer prepared in Examples 4-7 of the present invention was subjected to a soil column leaching experiment. The cumulative dissolution rate test results are as follows: Figure 2 As shown, Figure 2 The middle vertical axis is the cumulative dissolution rate of ammonia nitrogen (total nitrogen) in water over time.

[0075] from Figure 2 It can be seen that when the mass fraction of the binder is 20% (S3), the long-term effect of the slow-release fertilizer is better. The cumulative dissolution rate in 28 days reached 82.9%. After the 14th day, the curve tended to be stable and the release rate slowed down. The figure shows that the dissolution rate of the sample showed a sudden increase trend on the 8th day, the 10th day, the 12th day and the 14th day, respectively, and as the mass concentration of the binder increased, the sudden increase time was gradually delayed. This may be because the increase in the mass concentration of the binder enhances the adhesion of the coated fertilizer particles, making the particles less likely to break, thereby improving the slow-release effect.

[0076] The nutrients of the three groups of samples S1, S2 and S4 were completely released on the 16th day, 24th day and 20th day, respectively. The mass concentration of the binder in the S1 and S2 groups was small, the dosage was small, the bonding between biochar and diammonium phosphate was weak, and the nutrients were released quickly after contacting water. The capsule ruptured on the 8th and 10th days, and the nutrients were released rapidly. The initial dissolution rate of the S4 group was low, but it was completely released on the 20th day. The excessive amount of binder of 30% by mass affected the contact between biochar and diammonium phosphate, and the slow-release structure formed was poor, mainly manifested as a mixture of bentonite and diammonium phosphate. The overall slow-release effect of the S3 group was the best, with a maximum cumulative dissolution rate of 82.9% in 28 days, and the surface of the capsule ruptured on the 12th day.

[0077] The addition of binders forms a protective layer and narrow pores on the surface of fertilizer particles, which become a physical barrier to nutrient release. The higher the binder content, the thicker the film layer, and the slower the release of nutrients. As the binder increases, it is more evenly distributed inside the particles, and the bonding is enhanced after being wetted with water during granulation, making the fertilizer particles more compact, the polymerization effect better, not easy to break, the internal pores smaller, and the nutrient release slower.

[0078] The coated fertilizer prepared in Example 6 of the present invention was subjected to a dissolution rate experiment in static water, and the experimental process was as follows: 5.0 g of the coated fertilizer was accurately weighed, put into a 15 cm long gauze bag, and after sealing, it was placed in a plastic bottle containing 250 mL of deionized water, ensuring that the bottle mouth was well sealed, tightening the lid, and placing it in a constant temperature box at 25 ° C for cultivation. The dissolution rate of the fertilizer was determined by measuring the ammonia nitrogen content in the solution by Nessler's reagent photometry, Nessler's reagent and ammonium standard solution were prepared according to the steps, 50 g of potassium tartrate was taken to prepare the solution, the spectrophotometer was first calibrated with the ammonium standard solution, and then the absorbance of the solution in the bottle was measured at a wavelength of 420 nm with water as the control group, and a standard curve was drawn, and the release of total nitrogen was calculated according to the curve equation. The phosphate content in the solution is determined by the phosphomolybdenum blue spectrophotometry. In an acidic solution, active phosphate reacts with ammonium molybdate to generate soluble phosphomolybdenum yellow, which is reduced to phosphomolybdenum blue with ascorbic acid and measured at 700nm. The color intensity is proportional to the phosphate content.

[0079] The test results of the cumulative dissolution rate of total nitrogen and phosphate (P2O5) of the coated fertilizer prepared in Example 6 are as follows: Figure 3 shown.

[0080] from Figure 3 It can be seen that the total nitrogen and P2O5 of the coated fertilizer prepared in Example 6 of the present invention have low initial dissolution rates in static water, which are 5.6% and 7.7% respectively. Within 28 days, only 72.8% and 73.6% of the total nutrient content of the effective nutrients nitrogen and phosphorus respectively become small molecules and diffuse into the solution. The hydrolysis rate is slow, and most of the substances are not dissolved, which basically meets the requirements of "Slow-Release Fertilizer".

[0081] from Figure 3 It can be seen that the nutrient release rate of biochar-coated fertilizer in still water is relatively fast in the initial stage (the first 16 days). This may be because the surface layer or local areas of the fertilizer particles are not completely coated by biochar, causing these parts to dissolve rapidly when they come into contact with water. Further observations showed that the nutrient release rate of the biochar-coated fertilizer slowed down significantly from the 20th day, indicating that the small molecule nutrients that failed to form slow-release bodies between the layers have basically dissolved. At this time, the total nitrogen and P2O5 contents in the fertilizer are approximately 59% and 66%, showing the significant effect of biochar-coated fertilizer in slow-release nutrients. In summary, only a small amount of nutrients are dissolved in biochar-coated fertilizer in still water, and it is difficult to degrade it by water alone.

[0082] The same method was used to conduct a dissolution rate test in static water on the straw charcoal coated fertilizer prepared in Examples 1-3 of the present invention. The test results are as follows: Figure 4 shown.

[0083] from Figure 4 It can be seen that the initial dissolution rates of the three groups of coated fertilizers in still water were low, at 9.1%, 7.5% and 3.8% respectively. The dissolution rate curve of the slow-release fertilizer showed a gentle upward trend, gradually increasing with time. The cumulative dissolution rate of R3 was significantly lower than that of R2 and R1, with a longer release period and the best slow-release effect. At 28 days, the cumulative release rates of nutrients in each group were 73.8%, 58.1% and 31.4% respectively, all meeting the standards for slow-release fertilizers. The cumulative dissolution rate of the R3 group was much lower than that of the other groups, with slower nutrient release and longer duration. It can be seen from the three curves that the greater the amount of biochar used, the lower the cumulative dissolution rate and the stronger the slow-release ability of the fertilizer.

[0084] The biochar coated fertilizer prepared in Example 6 of the present invention was scanned using a scanning electron microscope (SEM) to reveal its microstructure. Figure 5 shown.

[0085] from Figure 5 It can be seen that the biochar layer on the surface of the biochar coated fertilizer prepared in Example 6 of the present invention is evenly distributed and dense, forming a protective barrier. At the same time, the porous structure of the straw charcoal can be seen, and these pores further increase the water retention of the fertilizer and play a role in slow release during the nutrient release process.

[0086] The straw charcoal coated fertilizer prepared by the embodiment of the present invention significantly improves the fertilizer utilization rate and reduces the waste of resources caused by excessive fertilization by controlling the slow release of nutrients. The embodiment of the present invention provides a green and environmentally friendly fertilizer solution for agriculture, supports farmers to achieve efficient and sustainable production, and at the same time improves the soil structure, enhances the water retention and fertility of the soil, and helps to increase crop yields. In addition, the straw charcoal coated fertilizer prepared by the embodiment of the present invention can also effectively reduce the eutrophication effect of fertilizer loss on water bodies, protect the ecological environment, and promote the coordinated development of agriculture and ecology.

[0087] The above embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the attached claims.

Claims

1. A method for preparing straw charcoal coated fertilizer, characterized in that: The following steps are involved: In the granulator, the fertilizer granules are heated to 45-55°C and water is sprayed on the surface of the fertilizer granules to perform preliminary modification on the fertilizer granules; The temperature is raised to 50-65°C. After the surface of the fertilizer particles is dry, bentonite binding liquid is sprayed on the surface of the fertilizer particles. At the same time, straw charcoal powder is sprinkled on the surface of the fertilizer particles to form a tight biochar coating layer on the surface of the fertilizer particles. After drying, the straw charcoal coated fertilizer is obtained.

2. The method for preparing a straw charcoal coated fertilizer according to claim 1, characterized in that: During the preliminary modification process, the amount of water sprayed on the surface of the fertilizer particles is 8-12% of the mass of the fertilizer particles.

3. The method for preparing a straw charcoal coated fertilizer according to claim 1, characterized in that: The spraying amount of the bentonite binder is 4-10% of the mass of the fertilizer particles.

4. The method for preparing a straw charcoal coated fertilizer according to claim 3, characterized in that: In the bentonite bonding liquid, the mass concentration of bentonite is 10-30%.

5. The method for preparing a straw charcoal coated fertilizer according to claim 1, characterized in that: The amount of the straw charcoal powder sprinkled is 10-30% of the mass of the fertilizer particles.

6. The method for preparing a straw charcoal coated fertilizer according to claim 5, characterized in that: The straw charcoal powder is at least one of corn straw charcoal powder, wheat straw charcoal powder and rice straw charcoal powder.

7. The method for preparing a straw charcoal coated fertilizer according to claim 1, characterized in that: The fertilizer particles are at least one of diammonium phosphate, ammonium dihydrogen phosphate, urea, ammonium sulfate, and potassium chloride.

8. A method for preparing a straw charcoal coated fertilizer according to any one of claims 1 to 7, characterized in that: The bentonite binding liquid is prepared as follows: Sieve the bentonite to remove impurities; Adding water to bentonite to prepare a bentonite suspension, and centrifuging to separate a bentonite precipitate; After drying, bentonite concentrate is obtained, and water is added to the bentonite concentrate to prepare the bentonite binding liquid.

9. The method for preparing a straw charcoal coated fertilizer according to claim 8, characterized in that: The bentonite was sieved through a 200-mesh sieve to remove impurities.

10. A straw charcoal coated fertilizer, characterized in that: The method is obtained by the preparation method according to any one of claims 1 to 9.

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