Controlled release phosphorus fertilizer and its preparation and application

Abstract

The application belongs to the field of phosphate fertilizer, and particularly relates to a controlled-release phosphate fertilizer, which comprises a core, an intermediate layer for coating the core, and a shell layer for coating the intermediate layer; the core contains phosphate fertilizer; the intermediate layer material is biochar and a phosphorus source adsorbed by the biochar; and the shell layer material is a material obtained by heat treatment of the intermediate layer material; the application further comprises preparation and application of the controlled-release phosphate fertilizer; in the application, biochar mixed with phosphate fertilizer is used as the core, biochar adsorbed with a phosphorus source is used as the intermediate transition layer, and a heat-treated material of biochar adsorbed with a phosphorus source is used as the shell layer material; through joint control of the materials and the special hierarchical structure, the composition and structure are synergized, the phosphorus is transferred from inside to outside in stages, the slow-release effect of the phosphorus is effectively improved, and in addition, the slow-release behavior of the phosphorus can be conveniently regulated based on the use requirements of different plants, so that the controlled slow release of the phosphate fertilizer is realized.

Description

Technical Field

[0001] This invention belongs to the field of fertilizers, specifically relating to a controlled-release phosphate fertilizer. Background Technology

[0002] Phosphorus is a fundamental element in all living organisms, playing a unique and crucial role in the food-energy-water cycle. More importantly, the rapidly growing population's demand for food is heavily reliant on agricultural phosphate fertilizers, and the increasing proportion of biofuel production in the agricultural sector further fuels this demand. These anthropogenically used phosphate fertilizers primarily come from geological phosphate mining, whose locations and total reserves are finite globally. In fact, recent studies predict that at the current rate of extraction, currently exploitable geological phosphate reserves will be depleted within 50 to 100 years. This imbalance between phosphorus demand and resources has raised significant concerns about phosphorus depletion and food security. On the other hand, human utilization of phosphorus is quite inefficient. Large amounts of phosphorus are lost during fertilizer production and soil application, food production and processing, and waste disposal. Phosphorus runoff from these processes is a major contributor to eutrophication, posing a significant threat to ecosystems and human health. Summary of the Invention

[0003] In view of the problems that existing phosphate fertilizers have unsatisfactory slow-release effects and are difficult to control, the primary objective of this invention is to provide a controlled-release phosphate fertilizer that aims to improve the slow-release effect and facilitate its controllable slow release.

[0004] The second objective of this invention is to provide the preparation and application of the aforementioned slow-release phosphate fertilizer.

[0005] A controlled-release phosphate fertilizer includes a core, an intermediate layer covering the core, and a shell covering the intermediate layer;

[0006] The core contains phosphate fertilizer;

[0007] The intermediate layer material is biochar and the phosphorus source it adsorbs;

[0008] The shell material is the intermediate layer material after heat treatment.

[0009] In this invention, biochar mixed with phosphate fertilizer is used as the core, biochar adsorbed with phosphorus source is used as the intermediate transition layer, and heat-treated biochar adsorbed with phosphorus source is used as the shell material. Through the combined control of the aforementioned materials and the special hierarchical structure, the synergy of components and structure can be achieved, promoting the stepwise transfer of phosphorus from the inside to the outside, which can effectively improve the slow-release effect of phosphorus. In addition, the slow-release behavior of phosphorus can be conveniently controlled according to the different plant usage needs, realizing the controllable slow release of phosphate fertilizer.

[0010] In this invention, the control of each layer of components and the hierarchical composite structure is the key to synergistically improving the slow-release effect of phosphate fertilizer.

[0011] In this invention, the phosphate fertilizer in the core is a conventional phosphate fertilizer known in the industry, such as at least one of superphosphate, triple superphosphate, and calcium magnesium phosphate.

[0012] Preferably, the core further comprises biochar; preferably, the biochar and the phosphate fertilizer are uniformly mixed. This invention has found that adding biochar to the core can further improve the synergy of the components and the hierarchical structure, contributing to a further improvement in the slow-release effect of phosphorus.

[0013] In this invention, the mass mixing ratio of biochar and phosphate fertilizer in the core is 1:(0.5-1).

[0014] In this invention, the biochar (e.g., biochar added to the core and biochar in the intermediate layer before adsorbing phosphorus sources) can be prepared using existing methods, such as conventional biomass carbonization.

[0015] In this invention, the biomass is a biomass raw material known in the industry. Considering waste utilization and resource recovery, the biomass can be at least one of sawdust, peanut shells, and livestock manure.

[0016] Preferably, the carbonization temperature is 300–600°C, and more preferably 300–500°C;

[0017] Preferably, the carbonization time is 50 to 500 minutes.

[0018] In this invention, the intermediate layer is biochar with a phosphorus source adsorbed on its surface. In this invention, the phosphorus-loaded biochar formed by adsorption binds to the biochar surface through physical processes (physical adsorption, etc.) or weak chemical processes (surface complexation, mineral bonding, etc.). Using this material as the intermediate layer can synergistically construct a phosphorus-releasing delivery channel, thus helping to improve the sustained-release effect.

[0019] The phosphorus source is a water-soluble phosphorus-containing compound, preferably at least one of phosphate, hydrogen phosphate, and dihydrogen phosphate.

[0020] Preferably, the biochar is placed in an aqueous solution containing a phosphorus source for adsorption, and then subjected to solid-liquid separation and drying to obtain the intermediate layer material.

[0021] In this invention, considering pollution treatment and resource utilization, the aqueous solution containing phosphorus source is phosphorus-containing wastewater.

[0022] In this invention, the phosphorus content in the intermediate layer material can be adjusted according to the controlled release requirements. For example, the phosphorus content in each gram of biochar in the intermediate layer material is 4 to 80 mg / g.

[0023] In this invention, a novel approach is to use biochar heat-treated material with adsorbed phosphorus source as the shell material. This allows it to work synergistically with the core-intermediate layer material and structure to improve the slow-release effect of phosphorus.

[0024] In this invention, the atmosphere for heat treatment is a protective atmosphere, such as nitrogen or an inert gas.

[0025] In this invention, the heat treatment temperature is 300–600°C, preferably 300–500°C; the preferred heat treatment time is 50–500 min.

[0026] Preferably, in the shell material, phosphorus is chemically bonded to the carbon material via COP covalent bonds.

[0027] In this invention, the weight ratio of the core, intermediate layer and shell is (1.5-2):(1-2):(1-2).

[0028] In this invention, the core has a D50 particle size of 0.074–0.1 mm; the intermediate layer has a thickness of 0.1–0.3 mm; and the shell has a thickness of 0.1–0.9 mm.

[0029] In this invention, the layers can be sequentially coated and composited using existing methods. For example, the core, intermediate layer, and shell layer are composited with an adhesive, which is at least one of polyvinyl alcohol aqueous solution, povidone aqueous solution, hydroxypropyl methylcellulose aqueous solution, starch paste, etc.

[0030] Preferably, the total content of binder in the controlled-release phosphate fertilizer is 0.5-1 wt%.

[0031] The present invention also provides a method for preparing the controlled-release phosphate fertilizer, wherein an intermediate layer is coated on the shell surface, and then a shell layer is coated on top to obtain the fertilizer.

[0032] In this invention, the materials in each layer can be prepared by layer-by-layer coating using conventional granulation processes.

[0033] This invention provides a preferred method for preparing controlled-release phosphate fertilizer, comprising the following steps:

[0034] Step 1: Crush agricultural waste biomass; agricultural waste biomass can be at least one of sawdust, peanut shells, and livestock manure, and the particle size of the crushed waste biomass is less than or equal to 0.18 mm;

[0035] Step 2: The crushed agricultural waste biomass is pyrolyzed and carbonized to obtain biochar I; the pyrolysis atmosphere is nitrogen or argon, the pyrolysis carbonization temperature is 300-600℃, and the heating rate is 2-10℃·min. -1 The pyrolysis carbonization time is 50–500 min;

[0036] Step 3: The biochar from Step 2 is used to recycle phosphorus resources from phosphorus-containing wastewater through adsorption to obtain phosphorus-loaded biochar II (intermediate layer material); for example, the phosphorus content in the phosphorus-containing wastewater is 20-100 mg / L, the solid-liquid ratio of biochar to phosphorus-containing wastewater is 0.1:20-80 (g:mL), the adsorption time is 5-24 h, and after filtration and separation, it is dried for 10-24 h at a drying temperature of 50-80 °C to obtain phosphorus-loaded biochar II, with a phosphorus content of 4-80 mg per g of biochar;

[0037] Step 4: The phosphorus-loaded biochar from Step 3 is pyrolyzed and carbonized again (heat treatment) to obtain secondary pyrolyzed phosphorus-containing biochar III (shell material); the phosphorus-loaded biochar II is then pyrolyzed and carbonized again at a carbonization temperature of 400–500℃, under a nitrogen or argon atmosphere, with a heating rate of 2–10℃·min. -1 The pyrolysis carbonization time is 60-300 min to obtain secondary pyrolysis phosphorus-containing biochar III, with a phosphorus content of 3.6-72 mg per g of biochar;

[0038] Step 5: Granulate conventional phosphate fertilizer / biochar I, biochar II, and biochar III in a certain ratio through an inner layer, interlayer (middle layer), and outer layer (shell layer) to obtain multilayer controlled-release phosphate fertilizer. The ratio of conventional phosphate fertilizer, biochar I, biochar II, and biochar III is 1:(0.5~1):(1~2):(1~2).

[0039] The granulation step in step 5 includes:

[0040] Step i: Add the conventional phosphate fertilizer / biochar I mixture into the granulator as the inner layer structure of the controlled-release phosphate fertilizer; the conventional phosphate fertilizer is any one or more of superphosphate, triple superphosphate, and calcium magnesium phosphate, with a particle size of 0.074-0.1 mm;

[0041] Step ii: Spray a certain amount of binder onto the granulator, then add a certain amount of biochar II, so that it coats the outer layer of the phosphate fertilizer as a sandwich structure of the controlled-release phosphate fertilizer. Repeat this step as needed, and finally coat the phosphorus-loaded biochar evenly on the surface of the phosphate fertilizer according to the formula; the binder is a polyvinyl alcohol aqueous solution (concentration 3.0-10.0%); the diameter of the granules after molding is 0.2-0.4 mm;

[0042] Step iii: Continue to spray a certain amount of binder into the granulator, then add a certain amount of biochar III, and granulate it to form an outer layer of phosphorus-loaded biochar as the outer structure of the controlled-release phosphate fertilizer. Repeat this step as needed to finally form a multi-layered granular controlled-release phosphate fertilizer with inner, sandwich, and outer layers of conventional phosphate fertilizer / biochar I, biochar II, and biochar III according to the formula. The diameter of the granules after molding is 0.5-1.0 mm.

[0043] The present invention also provides the application of the controlled-release phosphate fertilizer as a slow-release fertilizer for phosphorus.

[0044] Beneficial effects:

[0045] This invention uses different types of phosphorus materials arranged in a special core-intermediate layer and shell structure as described in this invention, which enables synergy between materials and structure and allows for the unexpected and controlled slow release of phosphorus.

[0046] Furthermore, the technical solution of this invention can use agricultural waste and phosphorus-containing wastewater as raw materials, thus realizing the resource utilization of waste. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the multilayer controlled-release phosphate fertilizer structure of the present invention (① represents a conventional phosphate fertilizer / biochar I mixture; ② represents biochar II; ③ represents biochar III).

[0048] Figure 2 The phosphorus dissolution rate in the soil environment of the controlled-release phosphate fertilizers in Comparative Examples 1, 2, and 5 and Example 3 of this invention is shown. Detailed Implementation

[0049] The technical solution of the present invention will be further described and illustrated below through specific embodiments, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, various changes or modifications to the material composition and dosage in these embodiments without departing from the essence and scope of the present invention also fall within the scope of protection of the present invention.

[0050] In this embodiment of the invention, the biomass raw materials include sawdust from mixed wood chips from a wood processing plant and cow dung from cattle manure from farms that primarily feed on herbaceous plants.

[0051] In this embodiment of the invention, the phosphorus-containing wastewater is all from wastewater discharged from phosphate fertilizer plants, with a pH value between 2.6 and 4 and a phosphorus content between 10 and 100 mg / L.

[0052] In the embodiments of the present invention, the controlled-release phosphate fertilizer granulation method is selected as disc granulation.

[0053] The specific steps for determining the available phosphorus content using the Olsen method in this embodiment of the invention are as follows: Weigh a quantitative amount of the sample to be tested, place it in a sample bottle, and extract it using a 0.5 mol / L NaHCO3 solution (pH 8.5) with a solid-liquid ratio of 1:20 (g:mL). After shaking for 24 hours, centrifuge and filter the sample, and then determine the available phosphorus content using the molybdate colorimetric method.

[0054] In the following cases, unless otherwise stated, the weight and weight ratio of each component refer to dry weight.

[0055] In the following examples, the adhesives used are all aqueous solutions of polyvinyl alcohol.

[0056] Example 1

[0057] Step 1: Take sawdust, crush it, pass it through an 80-mesh sieve, and place it in a tube furnace filled with nitrogen gas. Heat it at 10°C / min. -1 The heating rate was increased to 500℃ and held for 120 min to obtain the pyrolysis product, which was labeled as biochar I.

[0058] Step 2: Take 40 mL of phosphorus-containing wastewater with a concentration of 50 mg / L, add 0.1 g of biochar I, place it in a centrifuge tube, and shake it on a shaker for 24 h to obtain phosphorus-loaded biochar. Take a sample for analysis. The adsorption capacity of phosphorus is 15 mg / g. After the adsorption reaches equilibrium, perform solid-liquid separation. Dry the solid phase at 70℃ for 24 h and label it as biochar II.

[0059] Step 3: Place the biochar II back into the tube furnace purged with nitrogen and heat at 10°C / min. -1 The temperature was increased to 500℃ and held for 150 min to obtain a secondary pyrolysis product. The phosphorus content in the pyrolysis product was 13.5 mg / g. This product was labeled as biochar III.

[0060] Step 4: Mix biochar I with conventional superphosphate fertilizer at a mass ratio of 1:1. After grinding and passing through a 100-mesh sieve, add a 5.0% polyvinyl alcohol aqueous solution binder to the mixture and granulate it through a disc granulator to finally achieve a particle size of 0.15mm, thus obtaining inner layer particles.

[0061] Step 5: Continue to spray about 0.1 to 0.3 wt% binder on the surface of the inner layer particles, and then add a certain amount of biochar II (the mass ratio of inner layer particles to biochar II is 2:1.5) to coat the surface of the inner layer particles. Then, granulate the particles using a disc granulator to achieve a particle size of 0.35 mm, thus obtaining double-layer particles.

[0062] Step 6: Continue spraying approximately 0.3–0.5 wt% binder onto the surface of the double-layer granules, then add a certain amount of biochar III (inner layer granules: biochar II: biochar III = 2:1.5:1.8) to coat the surface of the double-layer granules. Granulate using a disc granulator until the final particle size reaches 0.55 mm, resulting in three-layer granules. These three-layer granules constitute the final multilayer controlled-release phosphate fertilizer. The available phosphorus content in this multilayer controlled-release phosphate fertilizer was determined using the Olsen method to be 16.0 mg / g.

[0063] Example 2

[0064] Step 1: Crush peanut shells and pass them through an 80-mesh sieve. Place the crushed shells in a tube furnace filled with nitrogen and heat at 5°C / min. -1 The temperature was increased to 300℃ at a heating rate and held for 100 min to obtain the pyrolysis product, which was labeled as biochar I.

[0065] Step 2: Take 50 mL of phosphorus-containing wastewater with a concentration of 60 mg / L, add 0.1 g of biochar I, place it in a centrifuge tube, and shake it on a shaker for 24 h to obtain phosphorus-loaded biochar. Take a sample for analysis. The adsorption capacity of phosphorus is 25 mg / g. After the adsorption reaches equilibrium, perform solid-liquid separation. Dry the solid phase at 70℃ for 24 h and label it as biochar II.

[0066] Step 3: Place the biochar II back into a tube furnace filled with nitrogen and heat at 5°C / min. -1 The temperature was increased to 300℃ and held for 100 min to obtain a secondary pyrolysis product. The phosphorus content in the pyrolysis product was 23 mg / g. This product was labeled as biochar III.

[0067] Step 4: Mix biochar I with conventional superphosphate fertilizer at a mass ratio of 1:0.8. After grinding and passing through a 100-mesh sieve, add 5.0% polyvinyl alcohol aqueous solution binder to the mixture and granulate it through a disc granulator to finally achieve a particle size of 0.09 mm, thus obtaining inner layer particles.

[0068] Step 5: Continue to spray about 0.1 to 0.3 wt% binder on the surface of the inner layer particles, and then add a certain amount of biochar II (the mass ratio of inner layer particles to biochar II is 1.8:1.2) to coat the surface of the inner layer particles. Then, granulate the particles using a disc granulator to achieve a particle size of 0.25 mm, thus obtaining double-layer particles.

[0069] Step 6: Continue spraying approximately 0.3–0.5 wt% binder onto the surface of the double-layer granules, then add a certain amount of biochar III (inner layer granules: biochar II: biochar III = 1.8:1.2:1.5) to coat the surface of the double-layer granules. Granulate using a disc granulator until the final particle size reaches 0.45 mm, resulting in triple-layer granules, which constitute the final controlled-release phosphate fertilizer. The available phosphorus content in this multi-layer controlled-release phosphate fertilizer was determined using the Olsen method to be 18.5 mg / g.

[0070] Example 3

[0071] Step 1: Take cow dung, crush it, pass it through an 80-mesh sieve, and put it into a tube furnace filled with nitrogen gas. Heat it at 10°C / min. -1 The temperature was increased to 400℃ at a heating rate and held for 200 min to obtain the pyrolysis product, which was labeled as biochar I.

[0072] Step 2: Take 80 mL of phosphorus-containing wastewater with a concentration of 80 mg / L, add 0.1 g of biochar I, place it in a centrifuge tube, shake it on a shaker, and react for 24 h to obtain phosphorus-loaded biochar. Take a sample for analysis. The adsorption capacity of phosphorus is 60 mg / g. After the adsorption reaches equilibrium, perform solid-liquid separation. Dry the solid phase at 70 °C for 24 h and label it as biochar II.

[0073] Step 3: Place the biochar II back into the tube furnace purged with nitrogen and heat at 10°C / min. -1 The temperature was increased to 400℃ and held for 200 min to obtain a secondary pyrolysis product with a phosphorus content of 55 mg / g. This product was labeled as biochar III.

[0074] Step 4: Mix biochar I with conventional superphosphate fertilizer at a mass ratio of 1:0.5. After grinding and passing through a 100-mesh sieve, add 5.0% polyvinyl alcohol aqueous solution binder to the mixture and granulate it through a disc granulator to finally achieve a particle size of 0.08 mm, thus obtaining inner layer particles.

[0075] Step 5: Continue to spray about 0.1 to 0.3 wt% binder on the surface of the inner layer particles, and then add a certain amount of biochar II (the mass ratio of inner layer particles to biochar II is 1.5:1) to coat the surface of the inner layer particles. Then, granulate the particles using a disc granulator to achieve a particle size of 0.2 mm, thus obtaining double-layer particles.

[0076] Step 6: Continue spraying approximately 0.3–0.5 wt% binder onto the surface of the double-layer granules, then add a certain amount of biochar III (inner layer granules: biochar II: biochar III = 1.5:1:1) to coat the surface of the double-layer granules. Granulate using a disc granulator until the final particle size reaches 0.4 mm, resulting in triple-layer granules. These triple-layer granules constitute the final controlled-release phosphate fertilizer. The available phosphorus content in this multi-layer controlled-release phosphate fertilizer was determined to be 26.4 mg / g using the Olsen method.

[0077] Example 4

[0078] Compared with Example 3, the main difference is that biochar I was not added to the core, and the main steps are as follows:

[0079] Step 1: Obtain biochar I as in Example 3.

[0080] Step 2: Obtain biochar II as in Example 3.

[0081] Step 3: Obtain biochar III as in Example 3.

[0082] Step 4: After passing the conventional superphosphate fertilizer through a 100-mesh sieve, add a 5.0% polyvinyl alcohol aqueous solution as a binder, and granulate it through a disc granulator to finally achieve a particle size of 0.1mm, thus obtaining inner layer particles.

[0083] Step 5: Continue to spray about 0.1 to 0.3 wt% binder on the surface of the inner layer particles, and then add a certain amount of biochar II (the mass ratio of inner layer particles to biochar II is 1.5:1) to coat the surface of the inner layer particles. Then, granulate the particles using a disc granulator to achieve a particle size of 0.2 mm, thus obtaining double-layer particles.

[0084] Step 6: Continue spraying approximately 0.3–0.5 wt% binder onto the surface of the double-layer granules, then add a certain amount of biochar III (inner layer granules: biochar II: biochar III = 1:1.5:1.8) to coat the surface of the double-layer granules. Granulate using a disc granulator until the granule size reaches 0.5 mm, resulting in three-layer granules. These three-layer granules constitute the final multilayer controlled-release phosphate fertilizer. The available phosphorus content in this multilayer controlled-release phosphate fertilizer was determined using the Olsen method to be 30.6 mg / g.

[0085] Comparative Example 1:

[0086] Compared with Example 3, the only difference is that a single-layer material is used as the phosphate fertilizer, for example, superphosphate is used as the single material.

[0087] Comparative Example 2:

[0088] Compared with Example 3, the only difference is that a single-layer material is used as the phosphate fertilizer. For example, the intermediate layer and shell layer are missing, and the inner layer particles (prepared in step 4 of Example 3) are used as the phosphate fertilizer.

[0089] Comparative Example 3:

[0090] Compared with Example 3, the only difference is that a single-layer material is used as the phosphate fertilizer, for example, lacking a core and shell, and the biochar II from step 2 of Example 3 is used as the phosphate fertilizer.

[0091] Comparative Example 4:

[0092] Compared with Example 3, the only difference is that a double-layer material is used as the phosphate fertilizer, for example, the biochar III in step 3 of Example 3 is used as the phosphate fertilizer.

[0093] Comparative Example 5:

[0094] Compared with Example 3, the only difference is that a double-layer material is used as phosphate fertilizer, for example, the shell layer is missing, and the double-layer particles obtained in step 5 of Example 3 are used as phosphate fertilizer.

[0095] Comparative Example 6:

[0096] Compared with Example 3, the only difference is that a double-layer material is used as the phosphate fertilizer. For example, the middle layer is missing, and the shell is directly composited on the core to form a core@shell double-layer phosphate fertilizer material.

[0097] Comparative Example 7:

[0098] Compared with Example 3, the only difference is that a double-layer material is used as the phosphate fertilizer. For example, the core is missing, and the shell is directly composited on the middle layer to form a middle layer@shell double-layer phosphate fertilizer.

[0099] Comparative Example 8:

[0100] Compared with Example 3, the only difference is that a three-layer material is used, but the layering is not set up according to the requirements of this invention. For example, biochar III (the outer shell layer of Example 1) is used as the core, biochar II (the middle layer of Example 1) is used as the middle layer, and a mixture of conventional superphosphate and biochar I (the core layer of Example 1) is used as the outer shell layer. Three-layer particles are obtained by granulation.

[0101] Comparative Example 9:

[0102] Compared with Example 3, the only difference is that the required hierarchical structure was not set up. Instead, conventional mixing was used. For example, phosphate fertilizer, biochar I, biochar II (the middle layer of Example 1), and biochar III (the outer shell layer of Example 1) were simply physically mixed (without layer-by-layer granulation) and mechanically stirred to ensure complete mixing of the raw materials to obtain a mixture.

[0103] The available phosphorus content, phosphorus dissolution rate, and phosphorus dissolution equilibrium time in the soil solution of different multilayer controlled-release phosphate fertilizers prepared in Examples 1-4 and phosphate fertilizers obtained from different treatments in Comparative Examples 1-10 were monitored. The results are shown in Table 1. The phosphorus dissolution rate was fitted using a pseudo-first-order kinetic equation, and the fitted equation is as follows:

[0104] Q t =Q e (1-e -kt )

[0105] Among them, Q t Let Q be the amount of phosphorus dissolved at time t (mg / g). e Let k be the amount of phosphorus dissolved at equilibrium (mg / g), and k be the phosphorus dissolution rate (d). -1 ), where t is the reaction time (d).

[0106] Table 1. Phosphorus dissolution rates in soil environment for controlled-release phosphate fertilizers of Examples 1-4 and Comparative Examples 1-9:

[0107]

[0108]

[0109] As shown in Table 1, the phosphorus dissolution rates of different treatment groups in the soil solution vary due to differences in biochar raw materials and the preparation process of the multilayer controlled-release phosphate fertilizer in this study. The dissolution rates range from 0.3 to 0.9 days. -1 The phosphorus dissolution rate varied within a certain range, but the multi-layer controlled-release phosphate fertilizer in Example 4, lacking biochar, exhibited the highest phosphorus dissolution rate, resulting in a weaker slow-release phosphorus effect compared to Examples 1-3. Comparative Examples 1, 2, 5, 6, 8, and 9 showed a larger fluctuation range in phosphorus dissolution rate, ranging from 1.0 to 65.1 days. -1 The phosphorus dissolution rates varied within the range and were all higher than those in Examples 1-4, indicating that phosphorus was released into the soil too quickly in these comparative groups, resulting in poor slow-release effects and reduced phosphorus utilization. In Comparative Examples 3, 4, and 7, the phosphorus dissolution rate was extremely low, which rendered the resulting controlled-release phosphate fertilizer almost worthless in supplying phosphorus to the soil, thus greatly reducing phosphorus utilization efficiency.

[0110] This invention further compares the phosphorus dissolution patterns of several typical comparative examples (Comparative Example 1, Comparative Example 2, and Comparative Example 5) with those in Example 3. Example 3 is a multilayer controlled-release phosphate fertilizer; Comparative Example 1 is conventional superphosphate granules; Comparative Example 2 is single-layer core particles composed of superphosphate and biochar I; and Comparative Example 5 is double-layer particles obtained by encapsulating the single-layer core particles of Comparative Example 2 with biochar II. The phosphorus dissolution rate in soil solution was monitored in each example, and the results are as follows: Figure 2 As shown.

[0111] Depend on Figure 2 The phosphorus dissolution curves show that the phosphorus dissolution rate in Comparative Example 1 reached 100% instantly; in Comparative Example 2, the phosphorus dissolution rate decreased under the action of biochar I, and the phosphorus dissolution rate could reach more than 90% in 5-10 days; in Comparative Example 5, the phosphorus dissolution rate was lower than that in Comparative Example 1, and the phosphorus dissolution rate could reach more than 85% in about 150 days; Example 3 is the multilayer controlled-release phosphate fertilizer prepared by this invention, which has the slowest phosphorus dissolution rate, and the phosphorus dissolution rate reaches more than 50% in about 250 days. It can continuously provide phosphorus to the soil for a longer period of time, has the best slow-release effect, and greatly improves the utilization rate of phosphate fertilizer.

Claims

1. A controlled-release phosphate fertilizer, characterized in that, It includes the core, the intermediate layer covering the core, and the shell covering the intermediate layer; The core contains phosphate fertilizer and biochar; the phosphate fertilizer is at least one of superphosphate, triple superphosphate, and calcium magnesium phosphate; the mass ratio of biochar to phosphate fertilizer in the core is 1:(0.5~1). The intermediate layer material is biochar and its adsorbed phosphorus source; the phosphorus source is a water-soluble phosphorus-containing compound; the phosphorus content in each gram of biochar in the intermediate layer material is 4~80 mg / g. The shell material is the intermediate layer material after heat treatment; the atmosphere during the heat treatment stage is a protective atmosphere, and the heat treatment temperature is 300~600℃. o C, heat treatment time is 50~500min; in the shell material, phosphorus is chemically bonded to the carbon material by COP covalent bonds; the phosphorus content in each gram of biochar is 3.6~72mg; The weight ratio of the core, intermediate layer and shell is (1.5~2):(1~2):(1~2).

2. The controlled-release phosphate fertilizer as described in claim 1, characterized in that, In the core, biochar and phosphate fertilizer are uniformly mixed.

3. The controlled-release phosphate fertilizer as described in claim 1, characterized in that, The biochar is obtained by carbonizing biomass.

4. The controlled-release phosphate fertilizer as described in claim 3, characterized in that, The biomass mentioned is organic waste.

5. The controlled-release phosphate fertilizer as described in claim 4, characterized in that, The biomass mentioned is at least one of sawdust, peanut shells, and livestock manure.

6. The controlled-release phosphate fertilizer as described in claim 3, characterized in that, The carbonization temperature is 300~600℃. o C; The carbonization time is 50~500min.

7. The controlled-release phosphate fertilizer as described in claim 1, characterized in that, The phosphorus source is at least one of phosphate, hydrogen phosphate, and dihydrogen phosphate; Biochar was placed in an aqueous solution containing a phosphorus source and adsorbed under ultrasonic vibration. Subsequently, the intermediate layer material was obtained through solid-liquid separation and drying.

8. The controlled-release phosphate fertilizer as described in claim 7, characterized in that, The aqueous solution containing the phosphorus source is phosphorus-containing wastewater.

9. The controlled-release phosphate fertilizer as described in claim 1, characterized in that, The core has a D50 particle size of 0.074~0.1mm; the intermediate layer has a thickness of 0.1~0.3mm; and the shell has a thickness of 0.1~0.9mm.

10. The controlled-release phosphate fertilizer according to any one of claims 1 to 9, characterized in that, The core, intermediate layer, and shell are composited together by an adhesive, which is at least one of polyvinyl alcohol aqueous solution, polyvinyl ketone aqueous solution, hydroxypropyl methylcellulose aqueous solution, and starch paste.

11. The controlled-release phosphate fertilizer as described in claim 10, characterized in that, In the controlled-release phosphate fertilizer, the total content of binder is 0.5~1wt%.

12. A method for preparing controlled-release phosphate fertilizer according to any one of claims 1 to 11, characterized in that, The core is coated with an intermediate layer material, and then coated with a shell material.

13. The application of a controlled-release phosphate fertilizer according to any one of claims 1 to 11, characterized in that, It is used as a slow-release fertilizer for phosphorus.

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