Multi-layer coated fertilizer granules and preparation method thereof

Abstract

The present invention discloses a multi-layer coated fertilizer particle, which has a fertilizer particle at its center and a coal-based mineral coating layer at its outer layer. The particle comprises the following structural layers from the center to the outer surface: a fertilizer particle, a binder layer, a small-particle-size coal-based mineral coating layer, and a large-particle-size coal-based mineral coating layer. The small-particle-size coal-based mineral coating layer comprises a binder and a small-particle-size coal-based mineral; the large-particle-size coal-based mineral coating layer comprises a curing agent, humic acid, and a large-particle-size coal-based mineral. The particle size of the small-particle-size coal-based mineral is 0 to 100 μm and is not zero, and the particle size of the large-particle-size coal-based mineral is 100 to 200 μm. The surface of the fertilizer particle of the present invention is coated with two layers of coal-based mineral coating layers. The particle size of the coal-based mineral near the fertilizer particle is smaller than that of the outer layer. After the fertilizer particle absorbs water and expands, the coal-based mineral near the fertilizer particle squeezes into the gaps in the outer layer, so that the outer coating does not break and remains intact even after the fertilizer particle absorbs water and expands to a certain extent.

Description

Technical Field

[0001] The present invention belongs to the field of fertilizer industry, and in particular relates to a coated fertilizer particle, and more particularly relates to a multi-layer coated fertilizer particle and a preparation method thereof. Background Art

[0002] In the pursuit of high yields, the overuse of chemical fertilizers has become a common phenomenon in modern agriculture, with nitrogen fertilizer being a particularly serious problem. This excessive use of nitrogen fertilizer not only increases agricultural production costs but also easily causes excessive growth in early crop growth, compromising crop quality. Furthermore, the loss of nitrogen causes severe environmental pollution, particularly eutrophication of runoff water sources, a persistent problem plaguing most agricultural regions.

[0003] To effectively address the issue of excessive nitrogen application, developing slow-release urea has become an effective approach. Existing slow-release ureas are primarily classified into three categories: 1. Slowing nitrogen release by coating the urea surface, such as resin-coated urea and sulfur-coated urea; 2. Slowing nitrogen release by modifying the physicochemical properties of urea, such as urea-formaldehyde slow-release fertilizers; and 3. Slowing nitrogen release by inhibiting the conversion rate of urea, such as by adding nitrification inhibitors and urease inhibitors. Coated urea is the most widely used and easiest to prepare.

[0004] Common resin-coated urea can effectively slow the release rate of urea in soil, thereby increasing its utilization rate. However, resin production costs are high, reaching tens of thousands of yuan per ton. Furthermore, resin is difficult for microorganisms to decompose in soil, making it a potential new soil pollutant.

[0005] Chinese patent CN102653483A (Zhang Xichun, granted in 2014) discloses a controlled-release nitrogen-potassium fertilizer and its preparation method. This invention combines granular urea with potassium humate powder after coating it with granular fertilizer from monosodium glutamate waste in a certain proportion to produce a controlled-release nitrogen-potassium fertilizer. This invention, an earlier attempt to coat fertilizer granules, achieved a sustained-release effect, but the coating material used was simply humic acid, resulting in poor coating effectiveness and low stability.

[0006] Chinese patent CN102653483A (North University of China, authorized in 2020) discloses an inorganic clay-based multi-nutrient polymer slow-release fertilizer and its preparation method. The invention allows formaldehyde and urea to react at a certain temperature to obtain a hydroxymethyl urea solution; at the same time, a calculated amount of acrylic acid, acrylamide monomer and inorganic clay are added to another reactor, and a certain amount of KOH solution is added to adjust the neutralization degree of acrylic acid; then a calculated amount of initiator, potassium dihydrogen phosphate and the prepared hydroxymethyl urea solution are added in sequence; after mixing evenly, the mixture is reacted at a certain temperature to obtain a viscous product, and finally dried to obtain an inorganic clay-based water-absorbing and water-retaining biodegradable multi-nutrient polymer slow-release fertilizer. The patented production process involves a variety of acid and alkali solutions, and the production process generates a large amount of waste liquid, which is not environmentally friendly; and the fertilizer produced by this process must undergo a drying process, and the production cost is relatively high.

[0007] Chinese patent CN105085081A (Fujian Normal University, authorized in 2018) discloses a method for preparing slow-release fertilizers using potassium feldspar. The invention mixes acrylamide, potassium persulfate, and oil-tea polyphenols, raises the temperature to react for a period of time, cools to room temperature, and adds toluene diisocyanate to obtain a coating agent. The granular fertilizer is placed in a rotary coating machine, and the coating agent is sprayed on the surface of the fertilizer particles with a spray gun. After ventilation and drying, a slow-release fertilizer is obtained. The fertilizer produced by this patent contains only potassium and cannot meet the needs of crops for trace elements. In addition, this patent has a problem common to the above patents, that is, the outer coating has only one layer of membrane structure, the barrier effect is not enough, and after the fertilizer particles absorb water and expand, the volume becomes larger, the outer coating will break, the slow-release effect is lost, and the stability is low.

[0008] Therefore, the key to developing coated urea is to find a coating material that is low-cost, does not cause secondary pollution to the soil, and has high stability of coated fertilizer particles.

[0009] In order to solve the above problems, the present invention is proposed. Summary of the Invention

[0010] A first aspect of the present invention provides a multi-layer coated fertilizer particle, wherein the center is a fertilizer particle and the outer layer is a coal-based mineral coating layer, and the multi-layer coated fertilizer particle comprises the following structural layers from the center to the outer surface: a fertilizer particle, a binder layer, a small-particle-size coal-based mineral coating layer, and a large-particle-size coal-based mineral coating layer;

[0011] The small-particle-size coal-based mineral coating layer includes a binder and small-particle-size coal-based minerals dispersed in the binder;

[0012] The large-particle-size coal-based mineral coating layer includes a curing agent and humic acid, and large-particle-size coal-based minerals dispersed in the curing agent and humic acid;

[0013] The particle size of the small-particle coal-based mineral is 0 to 100 μm, but is not 0, and includes 100 μm; the particle size of the large-particle coal-based mineral is 100 to 200 μm, but does not include 100 μm.

[0014] Preferably, the fertilizer granules are selected from urea granules and / or oxamide, the particle size of the urea granules is 2.00-4.75 mm; the particle size of the oxamide is 0.5-5.0 mm.

[0015] Preferably, based on the total weight of the multi-layer coated fertilizer particles, the weight ratio of the fertilizer particles is 60% to 80%, the weight ratio of the small-particle coal-based minerals is 1% to 30%, the weight ratio of the organic binder is 0% to 10%, and is not 0, the weight ratio of the large-particle coal-based minerals is 0.5% to 30%, the weight ratio of the curing agent is 0% to 10%, and is not 0, and the weight ratio of humic acid is 0% to 5%, and is not 0.

[0016] A second aspect of the present invention provides a method for preparing the multi-layer coated fertilizer granules according to the first aspect of the present invention, comprising the following steps:

[0017] (1) Separating underflow minerals from a carbonaceous material source using flotation or micro-mineral separation technology, crushing and screening the underflow minerals to obtain small-particle-size coal-based minerals and large-particle-size coal-based minerals;

[0018] (2) Weighing a certain proportion of binder according to a weight ratio, adding water to adjust to a certain viscosity to obtain an organic adhesive;

[0019] (3) Weighing a certain proportion of fertilizer granules according to a weight ratio, adding them into a disc granulator, starting the machine, first adding the organic binder obtained in step (2), rotating for a certain time, so that the organic binder is wrapped around the outer surface of the fertilizer granules to form a binder layer; then adding a certain proportion of the small-particle size coal-based mineral obtained in step (1), rotating for a certain time, so that the small-particle size coal-based mineral is dispersed in the organic binder and wrapped around the outer surface of the binder layer to form a small-particle size coal-based mineral coating layer;

[0020] (4) adding a curing agent and large-particle coal-based minerals in batches, and finally adding powdered humic acid, and rotating for a certain period of time so that the large-particle coal-based minerals are dispersed in the curing agent and humic acid, and are wrapped around the outer surface of the small-particle coal-based mineral coating layer to form a large-particle coal-based mineral coating layer;

[0021] (5) The molded sample obtained in step (4) is placed in a cool and ventilated place for bonding and curing, and multi-layer coated fertilizer particles are obtained after a certain period of time.

[0022] Preferably, in step (1), the carbonaceous material source includes coal or coal gangue, and the flotation is carried out by separating carbon-hydrogen combustibles and non-combustible minerals through surfactants and mechanical adjustment based on the differences in the surface properties of the minerals, and the obtained non-combustible minerals are ground to obtain coal-based minerals; the micro-mineral separation technology is used to separate the carbonaceous material source before combustion / chemical conversion to obtain a micro-mineral separation bottom flow, and the micro-mineral separation bottom flow is ground to obtain coal-based minerals; the chemical conversion includes coal pyrolysis, coal gasification, coal liquefaction, coal dry distillation or coal coking; the coal-based minerals contain the following trace elements necessary for plant growth: B, Ca, Cl, Cu, Fe, Mg, Mn, Mo, S, Se or Zn.

[0023] Preferably, the chemical conversion includes, for example, coal pyrolysis, coal gasification, coal liquefaction, coal dry distillation, or coal coking. The coal or gangue is not burned, or the chemical conversion mentioned above means that the mineral particles therein are not sintered at high temperatures. The high temperature refers to above 500°C.

[0024] Preferably, the coal-based minerals contain one or more of the following trace elements essential for plant growth: B, Ca, Cl, Cu, Fe, Mg, Mn, Mo, S, Se, or Zn. Of course, the mineral particles may also include various other trace elements. These trace elements may be naturally present in the mineral particles or artificially added to the mineral particles.

[0025] Preferably, step (1), wherein the coal-based minerals are obtained by grinding the bottom flow obtained by micro-mineral separation technology, comprises the following steps:

[0026] a. Wet-grinding a carbonaceous material source comprising non-combustible minerals and carbon-hydrogen-containing combustibles in water until the average particle size of the particles is less than 500 microns; adding a hydrophobic surfactant to the coal-water slurry during the wet-grinding process to mix and disperse the particles thoroughly to obtain a micro-nano coal-water slurry containing the hydrophobic surfactant;

[0027] b, into the micro-nano coal slurry containing the hydrophobic surfactant, a microbubble having a diameter less than 200 microns is passed, wherein the combustible particles containing carbon-hydrogen float with the bubbles to become a floating stream, the mineral particles having wherein adhered to the hydrophobic surfactant agglomerate and sink as an underflow, obtaining the micro-mineral separation underflow, adding a dispersant and grinding the micro-mineral separation underflow to obtain a coal-based mineral substance (or directly grinding). The hydrophobic surfactant includes but is not limited to the natural non-toxic surfactants such as pine oil, pine oil, camphor oil or eucalyptus oil.

[0028] For a detailed preparation method of the mineral particles, please refer to another patent of the present applicant, 2017105027146, the full text of which is incorporated herein. Since after the above-mentioned steps a and b, the non-combustible minerals and carbon-hydrogen-containing combustibles contained in the carbonaceous material source can be almost completely separated from each other in the form of ultrafine particles, the steps a and b are collectively referred to as trace element mineral separation technology (referred to as "micro-mineral separation technology"). For an explanation of the "micro-mineral separation technology", please refer to the authorized invention patent applied for by the present applicant on September 20, 2017, application number: 201710853445.8, and the patent name is "A method for increasing the energy density of liquid fuel or gas fuel".

[0029] The minerals in the present invention can be obtained, for example, by the process method described in another patent application of the present applicant entitled "A process for producing high calorific value water-coal slurry using coal or coal gangue and a coal gasification process using the same" (application number 201710502714.6). The full text of the patent is incorporated herein. For the sake of brevity, the detailed preparation process will not be repeated. Of course, the mineral particles in this application can also be separated by other known or potential separation processes, and are not limited to the above-mentioned process methods, as long as these mineral particles derived from coal or coal gangue are not sintered at high temperature. Wherein the high temperature refers to above 500°C.

[0030] Preferably, in step (2), the organic binder used includes one or more of guar gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil; the viscosity is 25 to 60 mm 2 / s.

[0031] Preferably, in step (3), the fertilizer granules are selected from urea granules and / or oxalamide, the particle size of the urea granules is 2.00-4.75 mm; the particle size of the oxalamide is 0.5-5.0 mm; the weight ratio of the fertilizer granules is 60% to 80%, the weight ratio of the small-particle coal-based minerals is 1% to 30%; the weight ratio of the organic binder is 0% to 10%; the first rotation time is 1 min to 10 min; and the second rotation time is 5 min to 20 min.

[0032] Preferably, in step (4), the curing agent includes one or more of 4,4'-diphenylmethane diisocyanate MDI, liquefied MDI, tetramethyl-m-xylylene diisocyanate, polyphenyl polymethylene polyisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, methylcyclohexyl diisocyanate, toluene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, diethyl fumarate diisocyanate, and dimer fatty acid diisocyanate; the weight ratio of large-particle coal-based minerals is 0.5% to 30%, the weight ratio of the curing agent is 0% to 10%, and the weight ratio of humic acid is 0% to 5%; the rotation time is 15 min to 30 min; and the curing agent and large-particle coal-based minerals are added in 3-5 times.

[0033] Preferably, in step (5), the certain time is 2 hours to 12 hours.

[0034] The third aspect of the present invention provides a method for improving the stability of coated fertilizer particles, wherein the stability refers to the ability of the outer coating to remain intact after the fertilizer particles absorb water and swell. The specific method is: at least two layers of coal-based mineral coating are coated on the surface of the fertilizer particles, wherein the particle size of the coal-based mineral in the coating layer close to the fertilizer particle side is smaller than the particle size of the coal-based mineral in the coating layer away from the fertilizer particle side. After the fertilizer particles absorb water and swell, their volume increases, and the coal-based mineral in the coating layer close to the fertilizer particle side squeezes into the gap of the coal-based mineral in the coating layer away from the fertilizer particle side, so that when the fertilizer particles absorb water and swell to a certain extent, the outer coating will not break and remain intact.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The present invention adopts flotation or micro-mineral separation technology to prepare micro-nano minerals as the main coating material, and then forms a coating layer on the outer layer of the fertilizer particles with the assistance of a binder, a curing agent and humic acid. The above coating material preparation method is simple and will not cause secondary pollution to the soil, thereby simplifying the coated fertilizer production process.

[0037] 2. Coal-based minerals contain the following trace elements necessary for plant growth: calcium, magnesium, sulfur, boron, iron, zinc, manganese, and selenium. While coating, they also enrich the types of nutrients in the coated fertilizer and increase the nutrient content of the coated fertilizer.

[0038] 3. The surface of the fertilizer particles of the present invention is coated with at least two layers of coal-based mineral coating layers, wherein the particle size of the coal-based minerals in the coating layer close to the fertilizer particles is smaller than the particle size of the coal-based minerals in the coating layer away from the fertilizer particles. After the fertilizer particles absorb water and swell, their volume increases, and the coal-based minerals in the coating layer close to the fertilizer particles squeeze into the gaps of the coal-based minerals in the coating layer away from the fertilizer particles, so that the fertilizer particles absorb water and swell to a certain extent, and the outer coating will not break and remain intact. In other words, the present invention greatly improves the stability of the coated fertilizer through the multi-layer coating structure. After the stability of the coated fertilizer is improved, it can not only maintain good slow-release characteristics in dry land, but also have good slow-release characteristics in paddy fields, and can adapt to a variety of application environments, thereby improving the applicability of the coated fertilizer.

[0039] 4. The examples of the present invention have shown that the multi-layer coated fertilizer granules of the present invention greatly slow down the release rate of urea and improve the utilization rate of urea. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Diagram of the structure of the multi-layer coated fertilizer granules of the present invention, in which 1-fertilizer granules, 2-binder layer, 3-small-particle-size coal-based mineral coating layer, 4-large-particle-size coal-based mineral coating layer, 31-small-particle-size coal-based mineral, 41-large-particle-size coal-based mineral;

[0041] Figure 2 The present invention is a process flow chart for preparing multi-layer coated fertilizer particles. DETAILED DESCRIPTION

[0042] The present invention is described below with reference to specific examples, but the embodiments of the present invention are not limited thereto. Experimental methods in the examples where specific conditions are not specified generally follow conventional conditions and those described in manuals, or according to conditions recommended by the manufacturer. The general equipment, materials, and reagents used are all commercially available unless otherwise specified. The raw materials required in the following examples and comparative examples are all commercially available.

[0043] Example 1:

[0044] Minerals separated from coal are crushed and sieved to produce coal-based micro- and nano-scale minerals in two different particle size ranges: 0-100 μm and 100-200 μm. Large-particle urea (2.00-4.75 mm), coal-based micro- and nano-scale minerals, and powdered humic acid are used as the raw materials for the coated fertilizer. Guardo gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil are used as organic binders. Isocyanate PM-200 and tetramethyl-m-xylylene diisocyanate (TMXDI) are used as curing agents. The materials are prepared in the following weight ratios: large-particle urea: small-particle coal-based micro- and nano-scale minerals: large-particle coal-based micro- and nano-scale minerals: humic acid: organic binder: curing agent = 60:5:25:3:2:5. Large-particle urea is first added to a disc granulator, which is then started. The organic binder is then added to coat the urea surface. Then, 5% of the small-particle coal-based micro- and nano-scale minerals is added and the mixture is rotated for 5 minutes to mix thoroughly. 1% curing agent and 5% large-particle, micro- and nano-sized minerals were added in five separate batches, followed by 2% powdered humic acid. The mixture was rotated for a period of time to completely coat the urea particles. The formed sample was placed in a cool, well-ventilated area for bonding and curing. After 2 hours, coal-based mineral-coated urea was obtained.

[0045] This mineral slow-release urea is rich in nitrogen, humic acid, and various trace elements. Potato field trials have shown that, at equal fertilizer rates, mineral slow-release nitrogen fertilizer increases potato yield by 15.76% compared to conventional urea. When the mineral slow-release urea fertilizer rate is reduced by 20%, potato yield increases by 7.15%. And when the mineral slow-release urea fertilizer rate is reduced by 30%, potato yield increases by 5.58%.

[0046] Example 2:

[0047] Minerals separated from coal are crushed and sieved to produce coal-based micro- and nano-scale minerals in two different particle size ranges: 0-100 μm and 100-200 μm. Large-particle urea (2.00-4.75 mm), coal-based micro- and nano-scale minerals, and powdered humic acid are used as the raw materials for the coated fertilizer. Guardo gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil are used as organic binders. Isocyanate PM-200 and tetramethyl-m-xylylene diisocyanate (TMXDI) are used as curing agents. The materials are prepared in the following weight ratio: large-particle urea: small-particle coal-based micro- and nano-scale minerals: large-particle coal-based micro- and nano-scale minerals: humic acid: organic binder: curing agent = 75:5:15:1:2:2. Large-particle urea is first added to a disc granulator, which is then started. The organic binder is then added to coat the urea surface. Then, 5% of the small-particle coal-based micro- and nano-scale minerals is added and the mixture is rotated for 5 minutes to mix thoroughly. 0.5% curing agent and 3.75% large-particle, micro- and nano-scale minerals were added in four separate batches, followed by 1% powdered humic acid. The mixture was then rotated for a period of time to completely coat the urea particles. The formed sample was placed in a cool, well-ventilated area for bonding and curing. After 2 hours, coal-based mineral-coated urea was obtained.

[0048] This mineral slow-release urea is rich in nitrogen, humic acid, and various trace elements. Potato field trials have shown that, at equal fertilizer rates, mineral slow-release nitrogen fertilizer increases potato yield by 21.35% compared to conventional urea. When the mineral slow-release urea fertilizer rate is reduced by 20%, potato yield increases by 8.46%. And when the mineral slow-release urea fertilizer rate is reduced by 30%, potato yield increases by 6.73%.

[0049] Example 3:

[0050] Minerals separated from coal are crushed and sieved to produce coal-based micro- and nano-scale minerals in two different particle size ranges: 0-100 μm and 100-200 μm. Large-particle urea (2.00-4.75 mm), coal-based micro- and nano-scale minerals, and powdered humic acid are used as the raw materials for the coated fertilizer. Guardo gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil are used as organic binders. Isocyanate PM-200 and tetramethyl-m-xylylene diisocyanate (TMXDI) are used as curing agents. The materials are prepared in the following weight ratio: large-particle urea: small-particle coal-based micro- and nano-scale minerals: large-particle coal-based micro- and nano-scale minerals: humic acid: organic binder: curing agent = 80:5:10:1:2:2. Large-particle urea is first added to a disc granulator, which is then started. The organic binder is then added to coat the urea surface. Then, 5% of the small-particle coal-based micro- and nano-scale minerals is added and the mixture is rotated for 5 minutes to mix thoroughly. Add 0.5% curing agent and 2.5% large-particle, micro- and nano-scale minerals in four portions, and finally add 1% powdered humic acid. Rotate for a certain period of time to completely coat the urea particles. Place the formed sample in a cool, ventilated place for bonding and curing. After 2-5 hours, coal-based mineral-coated urea is obtained.

[0051] This mineral slow-release urea is rich in nitrogen, humic acid, and various trace elements. Potato field trials have shown that, at equal fertilizer rates, mineral slow-release nitrogen fertilizer increases potato yield by 25.32% compared to conventional urea. When the mineral slow-release urea fertilizer rate is reduced by 20%, potato yield increases by 9.58%. When the mineral slow-release urea fertilizer rate is reduced by 30%, potato yield increases by 7.11%.

[0052] Example 4:

[0053] The minerals separated from the coal are crushed and sieved to obtain coal-based micro- and nano-scale minerals with a particle size range of 0 to 200 μm (i.e., large and small particle sizes are not distinguished, and batch addition is not performed). Large-particle urea (2.00-4.75 mm), coal-based micro- and nano-scale minerals, and powdered humic acid are used as the raw materials for the coated fertilizer. Guardo gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil are used as organic binders. Isocyanate PM-200 and tetramethyl-m-xylylene diisocyanate (TMXDI) are used as curing agents. The materials are prepared in the following weight ratio: large-particle urea: coal-based micro- and nano-scale minerals: humic acid: organic binder: curing agent = 80:15:1:2:2. First, large urea granules are added to a disc granulator and the machine is started. An organic binder is then added to coat the urea surface. Coal-based micro- and nano-scale minerals with a particle size range of 0 to 200 μm and a curing agent are then added. Finally, 1% powdered humic acid is added and the machine is rotated for a period of time to completely coat the urea granules. The formed sample is placed in a cool, ventilated area for bonding and curing. After 2-5 hours, coal-based mineral-coated urea is obtained.

[0054] This mineral slow-release urea is rich in nitrogen, humic acid, and various trace elements. Potato field experiments have shown that, when applied at equal fertilizer rates, the mineral slow-release nitrogen fertilizer increased potato yield by 10.76% compared to conventional urea. When the mineral slow-release urea fertilizer was reduced by 20%, potato yield increased by 4.82%. And when the mineral slow-release urea fertilizer was reduced by 30%, potato yield increased by 3.11%. While this yield-increasing effect is modest, it is not as pronounced as in Examples 1-3, demonstrating that double-layer coating can indeed improve the slow-release effect and stability of coated fertilizers.

[0055] The above embodiments illustrate the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention and are not intended to limit the scope of the present invention in any way. Various changes and modifications may be made to the present invention without departing from the scope of the present invention, and all such changes and modifications fall within the scope of the claims.

Claims

1. A method for improving the stability of coated fertilizer particles, characterized in that: The center of the coated fertilizer particle is a fertilizer particle, and the outer layer is a coal-based mineral coating layer, which includes the following structural layers from the center to the outer surface: Fertilizer particles, binder layer, small-particle coal-based mineral coating layer, large-particle coal-based mineral coating layer; The small-particle-size coal-based mineral coating layer includes a binder and small-particle-size coal-based minerals dispersed in the binder; The large-particle-size coal-based mineral coating layer includes a curing agent and humic acid, and large-particle-size coal-based minerals dispersed in the curing agent and humic acid; The particle size of the small-particle coal-based mineral is 0-100 μm, and is not 0, and includes 100 μm; the particle size of the large-particle coal-based mineral is 100-200 μm, and does not include 100 μm; Based on the total weight of the coated fertilizer particles, the weight ratio of the fertilizer particles is 60% to 80%, the weight ratio of the small-particle coal-based mineral is 1% to 30%, the weight ratio of the binder is 0% to 10%, and is not 0, the weight ratio of the large-particle coal-based mineral is 0.5% to 30%, the weight ratio of the curing agent is 0% to 10%, and is not 0, and the weight ratio of humic acid is 0% to 5%, and is not 0; The stability is the ability of the large-particle coal-based mineral coating layer on the outer surface of the fertilizer particles to remain intact after absorbing water and swelling. The specific method is as follows: After the coated fertilizer particles absorb water and expand, their volume increases, and the coal-based minerals in the small-particle coal-based mineral coating layer close to one side of the fertilizer particles squeeze into the gaps in the coal-based minerals in the large-particle coal-based mineral coating layer away from the fertilizer particles, so that the fertilizer particles absorb water and expand to a certain extent, and the outer large-particle coal-based mineral coating layer will not break and remains intact.

2. The method according to claim 1, characterized in that The fertilizer granules are selected from urea granules and / or oxalamide, the particle size of the urea granules is 2.00-4.75 mm; the particle size of the oxalamide is 0.5-5.0 mm.

3. The method according to claim 1, characterized in that The method for preparing the coated fertilizer granules comprises the following steps: (1) Separating underflow minerals from carbonaceous material sources using flotation or micro-ore separation technology, crushing and screening them to obtain small-particle coal-based minerals and large-particle coal-based minerals; (2) Weigh a certain proportion of binder according to the weight ratio, add water and adjust to a certain viscosity to obtain an organic adhesive; (3) Weigh a certain proportion of fertilizer granules according to a weight ratio, add them into a disc granulator, start the machine, first add the organic binder obtained in step (2), rotate for a certain time, so that the organic binder is wrapped around the outer surface of the fertilizer granules to form a binder layer; then add a certain proportion of the small-particle size coal-based mineral obtained in step (1), rotate for a certain time, so that the small-particle size coal-based mineral is dispersed in the organic binder and wrapped around the outer surface of the binder layer to form a small-particle size coal-based mineral coating layer; (4) Adding the curing agent and large-particle coal-based minerals in batches, and finally adding powdered humic acid, rotating for a certain period of time, so that the large-particle coal-based minerals are dispersed in the curing agent and humic acid, and wrapped on the outer surface of the small-particle coal-based mineral coating layer to form a large-particle coal-based mineral coating layer; (5) The molded sample obtained in step (4) is placed in a cool and ventilated place for bonding and curing. After a certain period of time, multi-layer coated fertilizer particles are obtained.

4. The method according to claim 3, characterized in that In step (1), the carbonaceous material source includes coal or coal gangue, and the flotation is carried out by separating carbon-hydrogen combustibles and non-combustible minerals through surfactants and mechanical adjustment according to the difference in mineral surface properties, and the obtained non-combustible minerals are ground to obtain coal-based minerals; the micro-mineral separation technology is used to separate the carbonaceous material source before combustion / chemical conversion to obtain a micro-mineral separation bottom flow, and the micro-mineral separation bottom flow is ground to obtain coal-based minerals; the chemical conversion includes coal pyrolysis, coal gasification, coal liquefaction, coal dry distillation or coal coking; the coal-based minerals contain the following trace elements necessary for plant growth: B, Ca, Cl, Cu, Fe, Mg, Mn, Mo, S, Se or Zn.

5. The method according to claim 3, characterized in that In step (2), the organic binder used includes one or more of guar gum, sodium lignin sulfonate, sodium carboxymethyl cellulose, and olive oil; the viscosity is 25~60mm 2 / s.

6. The method according to claim 3, characterized in that In step (3), the fertilizer particles are selected from urea particles and / or oxalamide, the particle size of the urea particles is 2.00-4.75 mm; the particle size of the oxalamide is 0.5-5.0 mm; the weight ratio of the fertilizer particles is 60%-80%, the weight ratio of the small-particle coal-based minerals is 1%-30%; the weight ratio of the organic binder is 0%-10%; the first rotation time is 1 min-10 min; and the second rotation time is 5 min-20 min.

7. The method according to claim 3, characterized in that In step (4), the curing agent includes one or more of 4,4'-diphenylmethane diisocyanate, tetramethyl-m-xylylene diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, methylcyclohexyl diisocyanate, toluene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, and dimer fatty acid diisocyanate; the weight ratio of large-particle coal-based minerals is 0.5% to 30%, the weight ratio of the curing agent is 0% to 10%, and the weight ratio of humic acid is 0% to 5%; the rotation time is 15 minutes to 30 minutes; and the curing agent and large-particle coal-based minerals are added in 3-5 batches.

8. The method according to claim 3, characterized in that In step (5), the certain time is 2 h to 12 h.

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