Patent Description:
The crop growth process, from seed to maturity, includes emergence of seedlings, tillering, jointing, booting, flowering, and seeding, mainly including two stages for nutrient absorption: nutrient growth and reproductive growth. Each stage requires different amounts of nutrients. However, the original nutrients in the soil generally cannot meet the nutritional needs of crops, so a certain proportion of fertilizers is required.

At present, most of fertilizers available on the markets contain fixed elements, for example, urea is a nitrogen fertilizer, calcium superphosphate contains phosphorus and calcium, potassium dihydrogen phosphate contains phosphorus and potassium, etc. Regardless of organic fertilizer, inorganic fertilizer, single fertilizer or compound fertilizer, the nutrients contained only meet the needs of a crop in one stage, but cannot meet the reasonable nutritional needs of a crop in the whole growth cycle.

In the actual cultivation, in order to ensure crops can absorb the relevant nutrients in different growth stages, it is necessary to calculate the required amount of elements, different types of fertilizers should be prepared to topdress the crops. This fertilization method is simply a combination of different types of fertilizers, and the nutrients often cannot be fully utilized, with low utilization rate of fertilizers. If it is required to enhance the yield and utilization rate of fertilizers, multiple topdressings should be carried out for the crops in different growth stages, with high labor intensity and high cost of fertilization.

With the improvement of planting techniques, soilless cultivation has emerged, and nutrients in the water can be detected at any time and the elements needed for the crop can be replenished in time. However, this method can only be applied in a small area, such as a greenhouse, and its investment and maintenance costs are high, so it cannot be applied to the large-scale agriculture.

The <CIT> discloses a controlled release granular fertilizer having water-soluble fertilizers as a nucleus and limited-soluble plant nutrient compounds as coating layers, wherein more than two coating layers of limited-soluble compounds are formed around the nucleus, by bonding the nucleus with the coating layers having various solubilities by means of an aqueous <NUM>-<NUM> wt % sulfuric acid or <NUM>-<NUM> wt % phosphoric acid (based on P<NUM>O<NUM>) solution mixed with monoammonium phosphate, diammonium phosphate, or ammonium bicarbonate as a binder, and the release rates of the nutrient compounds are controlled by regulating the solubilities of the limited-soluble compounds.

The purpose of the present invention is to provide a method for preparing the multi-layer coating structure slow/controlled-release fertilizer. The method is simple and feasible, and it can achieve mass production and be used in large-scale agriculture.

The present invention employs the following technical solution to solve the technical problem thereof.

The present invention proposes a method for preparing multi-layer coating structure slow/controlled-release fertilizer, the fertilizer comprising a layer-by-layer coated fertilizer layer and a coating layer, wherein the fertilizer layer is separated from the coating layer, the fertilizer layer has at least two layers and the coating layer has at least one layer, and one of the fertilizer layers is a fertilizer core. The fertilizer layer includes at least two of nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer, and the coating layer is polyurea formaldehyde. The raw materials of the polyurea formaldehyde are urea and formaldehyde, the molar ratio of the urea to the formaldehyde is <NUM>-<NUM>:<NUM>, and the polyurea formaldehyde contains at least a polyurea formaldehyde having two to five carbon atoms.

The present invention provides a method for preparing the multi-layer coating structure slow/controlled-release fertilizer, the multi-layer coating structure slow/controlled-release fertilizer comprising a layer-by-layer coated fertilizer layer and a coating layer, wherein the fertilizer layer is separated from the coating layer, the fertilizer layer has at least two layers, the coating layer has at least two layers, and an the innermost layer of the fertilizer layers is a fertilizer core; the fertilizer layer includes at least two of nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer, the coating layer is polyurea formaldehyde, wherein the raw materials of the polyurea formaldehyde are urea and formaldehyde, the molar ratio of the urea to the formaldehyde is <NUM>-<NUM>:<NUM>, and the polyurea formaldehyde includes at least a polyurea formaldehyde having two to five carbon atoms, wherein the method comprising steps of: applying the fertilizer layer and the fertilizer corresponding to the coating layer to the periphery of the fertilizer core according to a preset order, to form layer-by-layer coated fertilizer granules; Wherein polyurea formaldehyde is prepared by the following steps: mixing urea and formaldehyde at a molar ratio of <NUM>- <NUM>:<NUM>, adjusting the pH of the reaction system to <NUM>-<NUM> and the temperature to <NUM>°-<NUM>, and then adjusting the pH to <NUM>~<NUM>. The multi-layer coating structure slow/controlled-release fertilizer thus prepared consists of a fertilizer layer and a coating layer, and the fertilizer layer is separated from the coating layer, and layers are coated to form a spherical shape. The ratio of nutrients such as nitrogen, phosphorus, potassium and medium and trace elements in the fertilizer layer are designed according to the growth requirements of different crops under different soil structures and different climatic conditions. For the polyurea formaldehyde as a coating layer, the coated fertilizer can be slowly and timely released according to the fertilizer requirement of the crops to meet the growth needs of different crops. Part of the water-soluble nutrient elements in the coated fertilizer layer can be released from the pores formed by dissolution of the water-soluble part of the coating layer to be absorbed by the crops. The water-insoluble part of the coating layer is slowly decomposed under the action of soil microorganisms, to release all the remaining undissolved fertilizers in the fertilizer layer to be absorbed by the crops. As the coating layer decomposes layer by layer, the coated fertilizer layer is released layer by layer until the entire fertilizer granules are completely dispersed in the soil. Since the coating layer itself is also a fertilizer, the fertilizer does not produce any residue while satisfying the growth of the crop.

Reference labels: <NUM> - fertilizer core; <NUM> - fertilizer layer; <NUM> - coating layer; <NUM> - powdering tank; <NUM> - powdering machine; <NUM> - first slurry tank; <NUM> - second slurry tank; <NUM> - first reactor; <NUM> - second reactor; <NUM> - liquid slurry pump; <NUM> - rotor drum granulator; <NUM> - drying machine; <NUM> - sieving machine.

In order to clarify the objects, technical solution and advantages of embodiments of the present invention, the technical solution in embodiments of the present invention will be described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by manufacturers are followed. Any reagent or instrument used which does not have a manufacturer specified is a commercially available conventional product.

The method for preparing multi-layer coating structure slow/controlled-release fertilizers in embodiments of the present invention are specifically described below.

The present invention provides a method for preparing multi-layer coating structure slow/controlled-release fertilizer, the multi-layer coating structure slow/controlled-release fertilizer comprising a layer-by-layer coated fertilizer layer <NUM> and a coating layer <NUM>, wherein the fertilizer layer <NUM> is separated from the coating layer <NUM>, there are at least two layers of fertilizer layer <NUM> and at least two layers of coating layer <NUM>, and the innermost layer of the fertilizer layers <NUM> is a fertilizer core <NUM>. The fertilizer layer <NUM> includes at least two of nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer, and the coating layer <NUM> is polyurea formaldehyde, the raw materials of which are urea and formaldehyde, the molar ratio of the urea and the formaldehyde is <NUM>-<NUM>:<NUM>, and the polyurea formaldehyde includes at least a polyurea formaldehyde having two to five carbon atoms. More specifically, the polyurea formaldehyde may have two to three carbon atoms, including dimethylol urea and monomethylenediurea that are soluble in cold water. In another embodiment, the polyurea formaldehyde may have two to five carbon atoms, including polyurea formaldehyde that is soluble in cold water, dimethylenetriurea and trimethylenetetraurea that are insoluble in cold water but soluble in hot water, and urea with a chain length having more than four carbons that is insoluble in hot water.

The polyurea formaldehyde includes at least a polyurea formaldehyde having two to five carbon atoms. It should be noted that when the fertilizer layer <NUM> contains two fertilizers such as nitrogen fertilizer, phosphorus fertilizer or potassium fertilizer, a single component can be a layer of the fertilizer layer <NUM>. The mixture of the two fertilizers can be a layer of the fertilizer layer <NUM>; or the two fertilizers can each form a layer, and collectively form a fertilizer layer <NUM>. When the fertilizer layer <NUM> includes nitrogen fertilizer, phosphate fertilizer and potassium fertilizer, the mixture of the three fertilizers can form a layer of fertilizer layer <NUM>. Or when the mixture of two fertilizers forms one layer and the other fertilizer forms another layer, the two layers can jointly form the fertilizer layer <NUM>. In another embodiment, when each of the three fertilizers forms a layer separately, the three layers can collectively form a fertilizer layer <NUM> altogether. The NPK fertilizer may be a quick-acting urea, ammonium sulfate, ammonium hydrogen phosphate or potassium sulfate, or a mixture of slow-released urea formaldehyde, isobutylidene diurea, oxamide, etc. It should be noted that the NPK fertilizer itself may contain other medium and trace elements, or some specially added medium and trace elements.

The multi-layer coating structure slow/controlled-release fertilizer forms a spherical-shaped unit including a fertilizer layer <NUM> and a coating layer <NUM> with layer-by-layer coating, and the fertilizer layer <NUM> is disposed separately from the coating layer <NUM>. The ratio of nutrients such as nitrogen, phosphorus and potassium and other medium and trace elements of the fertilizer layer <NUM> can be predetermined according to different soil structures, different climatic conditions and different crop growth needs. For the polyurea formaldehyde as a coating layer <NUM>, the fertilizer in the coated fertilizer <NUM> can be slowly and timely released according to the fertilizer requirement of the crops to meet the growth needs for different crops. Part of the water-soluble nutrient elements in the coated fertilizer layer <NUM> can be released from the pores formed by dissolution of the water-soluble part of the coating layer <NUM> and can be absorbed by crops. The water-insoluble part of the coating layer <NUM> can then be slowly decomposed under the action of soil microorganisms, to release all the remaining undissolved fertilizers in the fertilizer layer <NUM> for the crops to absorb. As the coating layer <NUM> decomposes layer by layer, the coated fertilizer layer <NUM> is released until the entire fertilizer granules are completely dispersed in the soil. Since the polyurea formaldehyde itself of the coating layer <NUM> is also a fertilizer, the fertilizer does not produce any residue while satisfying the needs for the growth of crops.

In an embodiment of the present invention, one-time application of the multi-layer coating structure slow/controlled-release fertilizer can meet the needs of all nutrients in the entire growth cycle of a crop. The nutrient release can be customized according to the crop type, so that the nutrient release time and intensity of the fertilizer can substantially synchronize with the fertilizer needs of the crop. When the crop needs a large amount of nutrients, it can be supplied effectively and quickly. On the other hand, when the crop needs a small amount of nutrients, it can be supplied slowly, and the medium and trace elements can also be designed to be placed on different fertilizer layers <NUM>. This slow releasing mechanism improves fertilizer utilization rate and reduces the loss and/or immobilization of fertilizer, to achieve the purpose of applying adequate base fertilizer at one time, without the need for top dressing.

Polyurea formaldehyde is mainly prepared by the following steps: mixing urea and formaldehyde at a molar ratio of <NUM>- <NUM>:<NUM>, adjusting the pH of the reaction system to <NUM>-<NUM> and the temperature to <NUM>-<NUM>, and then adjusting the pH to <NUM>~<NUM>. In one embodiment of the invention, for the method for preparing polyurea formaldehyde as a coating layer <NUM>, the urea needs not to be dissolved in the water, but dissolved and reacted in the formaldehyde solution to obtain the mixed solution of methylol urea and urea, which has lower percentage of water, for example, <NUM>%-<NUM>%. After adjusting the solution to an acid solution, the polymerization reaction will not occur in the aqueous phase, but after sprayed onto the surface of the fertilizer core <NUM> or the fertilizer layer <NUM>, the film is formed by polymerization and solidification after water evaporation, and cross-linking and entanglement occur between the molecules. The film structure is compact and not disintegrated in the soil, thereby achieving the purpose of a controlled and slow release of various nutrients in the fertilizer core <NUM> and the fertilizer layer <NUM>.

In some embodiments of the present invention, the molar ratio of urea to formaldehyde is <NUM>-<NUM>:<NUM> or <NUM>-<NUM>:<NUM> or <NUM>-<NUM>:<NUM>.

The compositions and contents of fertilizer core <NUM> and fertilizer layer <NUM>, and the carbon chain length and coating thickness of coating layer <NUM> can be calculated and determined according to the fertilizer requirements of different crops in the growth cycle thereof under different soil and climatic conditions. It is noted that the fertilizer requirement is determined according to the crop fertilizer efficiency tests carried out in scientific research institutions in the country and around the world, or the soil tests and fertilizer efficiency tests carried out by the manufacturers.

In conventional polyurea formaldehyde preparation method, the particulate urea is dissolved in water at first, formaldehyde solution is then added, and when the mixed solution is heated to a predetermined temperature, the pH value is adjusted to initiate a methylolation reaction. After the reaction is completed, the pH value and temperature are adjusted again to initiate a polymerization reaction. The polyurea formaldehyde prepared by the conventional method is a suspension and the polymerization reaction is completed in the water. After the polymerization reaction, the water content is as high as <NUM>%, and it is required to remove partial water by discharging, cooling, and centrifuging or plate-and-frame filtration. At this time, the filter cakes still contain <NUM>% to <NUM>% of water, which must be removed by high-temperature heating provided by a drying machine <NUM>, and finally a powdery and blocky mixture of polyurea formaldehyde is obtained. Due to high percentage of water in the polyurea formaldehyde prepared by the conventional method, it is impossible to achieve spraying to form a film on its own. Although it is capable of wrapping as a film after being mixed with other binders, cross-linking and entanglement between molecules will not occur, so the film may not be dense enough in the structure, and it may be easily dispersed and disintegrated in the soil, and the granules of fertilizer core <NUM> cannot be released slowly.

In some embodiments, the thickness of each coating layer <NUM> prepared by the method of the present invention and its composition determine the release time of the coated fertilizer. In still some embodiments, the multi-layer coating structure slow/controlled-release fertilizer comprises at least two layers of the coating layer <NUM>, with the same thickness. In some embodiments of the present invention, the at least two layers of the coating layer <NUM> have different thicknesses. In other embodiments, the polyurea formaldehyde of at least two layers of coating layer <NUM> has components of the same carbon chain lengths. In other embodiments, the polyurea formaldehyde of at least two layers of coating layer <NUM> has components of different carbon chain lengths.

Generally speaking, polyurea formaldehyde is a slow-release nitrogen fertilizer containing four forms of nitrogen in urea formaldehyde, and includes a part of unreacted excess urea, monomethylol urea, dimethylol urea and monomethylenediurea that are soluble in cold water, dimethylenetriurea and trimethylenetetraurea that are insoluble in cold water but soluble in hot water, and the urea having a carbon chain length exceeding four carbon atoms that is insoluble in hot water. The main components of polyurea formaldehyde are a series of condensates of urea and formaldehyde such as dimethylenetriurea, trimethylenetetraurea, etc. The condensate is slightly soluble in water and can control the release of nitrogen and prevent nitrogen loss, and it can be slowly decomposed by microorganisms in the soil. The polyurea formaldehyde, as the coating layer <NUM>, can control the release amount of the coated fertilizer, so that it can be fully and timely absorbed and utilized by crops.

In some embodiments, the polyurea formaldehyde is a slow release fertilizer containing the above four forms of nitrogen. In some embodiments, the polyurea formaldehyde is a slow release fertilizer containing the first three forms of nitrogen. In some embodiments, the polyurea formaldehyde is a slow release fertilizer containing the latter three forms of nitrogen. In some embodiments, the polyurea formaldehyde is a slow release fertilizer containing two forms of nitrogen, that is, the nitrogen that is insoluble in cold water but soluble in hot water, and the nitrogen that is insoluble in hot water.

In the present invention, the term "fertilizer layer <NUM>" includes at least two of nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer. In some embodiments, the fertilizer layer <NUM> comprises: (a) at least one macronutrient fertilizer from nitrogen fertilizer, phosphorus fertilizer or potassium fertilizer; and optionally (b) at least one trace element fertilizer. In some embodiments, the fertilizer layer <NUM> may include: (a) macronutrient fertilizers nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer; and optionally (b) at least one trace element fertilizer. In some embodiments, the trace element fertilizer is selected from boron fertilizer, zinc fertilizer, molybdenum fertilizer, iron fertilizer, manganese fertilizer, copper fertilizer, etc. or combinations thereof. In some embodiments, the fertilizer layer <NUM> is a fertilizer other than polyurea formaldehyde on its own.

In addition, it should be noted that the polyurea formaldehyde as the coating layer <NUM> itself is a nitrogen fertilizer. The nitrogen fertilizer in the coated fertilizer layer <NUM> may be one or more slow-acting or quick-acting fertilizers of polyurea formaldehyde, urea, isobutylidene diurea, crotonylidenediurea, oxamide, ammonium sulfate, and ammonium nitrate.

It should be noted that the coated fertilizer layer <NUM> can be mixed with polyurea formaldehyde and other nutrient elements such as Ca, Mg, etc. Alternatively, other elements such as Ca and Mg may be mixed in the phosphorus fertilizer and the potassium fertilizer, so that it can provide other nutrients required for the crop when the polyurea formaldehyde as the coating layer <NUM> is slowly decomposed.

In some embodiments, the multi-layer coating structure slow/controlled-release fertilizer comprises at least two layers of coating layer <NUM>, or at least three layers of coating layer <NUM>, or at least four layers of coating layer <NUM>. In some embodiments, the outermost layer of the multi-layer coating structure slow/controlled-release fertilizer is a polyurea formaldehyde coating layer <NUM>, as shown in <FIG>. In some embodiments, the outermost layer of the multi-layer coating structure slow/controlled-release fertilizer is the fertilizer layer <NUM> that contains polyurea formaldehyde, as shown in <FIG>. In some embodiments, the outermost layer of the multi-layer coating structure slow/controlled-release fertilizer is a fertilizer layer <NUM> that does not contain polyurea formaldehyde.

The fertilizer core <NUM> may be a common chemical fertilizer purchased from the market, or may be a fertilizer component prepared in proportion, or may be granular fertilizer containing polyurea formaldehyde. For example, the fertilizer component may include at least one of the following: urea, monoamine phosphate, diamine phosphate, potassium dihydrogen phosphate, potassium chloride, potassium sulfate, potassium nitrate, ammonium sulfate and ammonium chloride, etc..

In some embodiments, the release amount of each fertilizer element of the multi-layer coating structure slow/controlled-release fertilizer is substantially the same as the fertilizer requirement of the corn crop (herein referred to as "corn-specific fertilizer").

In some embodiments, the corn-specific fertilizer includes the following from the inside to outside:.

In some embodiments, the release amount of each fertilizer element of the multi-layer coating structure slow/controlled-release fertilizer is substantially the same as the fertilizer requirement of the wheat crop (herein referred to as "wheat-specific fertilizer"). In some embodiments, the wheat-specific fertilizer includes the following from the inside to outside:.

In some embodiments, the release amount of each fertilizer element of the multi-layer coating structure slow/controlled-release fertilizer is substantially the same as the fertilizer requirement of the rice crop (herein referred to as "rice-specific fertilizer"). In some embodiments, the rice-specific fertilizer includes the following from the inside to outside:.

In embodiments of this invention, the multi-layer coating structure slow/controlled-release fertilizer has a quick-acting, intermediate-acting and long-acting NPK nutrient distribution and it can achieve a reasonable combination of various elements for slow/controlled release. The quick-acting fertilizers such as urea, ammonium sulfate, monoammonium phosphate, diammonium phosphate and potassium chloride in the present invention can be quickly released for crop absorption, and the calcium magnesium phosphate fertilizer can provide the citric-acid-soluble phosphorus, calcium, magnesium and silicon. The polyurea formaldehyde in the fertilizer can provide intermediate-acting and long-acting nitrogen nutrients after being decomposed by soil microorganisms to meet the nutrient requirements of the entire slow growth cycle of the crop. In addition, phosphorus, potassium and other medium and trace elements can be mixed with the urea formaldehyde to achieve slow controlled release.

Moreover, for the coating layer <NUM> in the embodiments of the present invention, the methylol urea has an adsorption function, which promotes the combination of fertilizer nutrients and soil granules to adsorb on the roots of the crop and form a gel-like chelate, thereby reducing nutrient loss, and achieving full nutrient absorption by crops and improving the utilization rate of the fertilizer. Compared with other controlled release fertilizers, the product is less affected by climate (temperature) and soil conditions during nutrient release, and the fertilizer efficiency is more stable and lasting.

The present invention concerns a method of preparing the above multi-layer coating structure slow/controlled-release fertilizer as defined in the set of claims. The method includes the steps of: applying fertilizer corresponding to the fertilizer layer <NUM> and the coating layer <NUM> to the periphery of the fertilizer core <NUM> in a predetermined order to form layer-by-layer coated fertilizer granules. Application is performed by spraying, rolling agglomerates or a combination thereof.

Specifically, in some embodiments of the present invention, the main steps of the spraying method may include: first separately preparing liquid coating material of different carbon chain lengths in a plurality of reactors, preparing the components of each fertilizer layer <NUM> according to a ratio in a plurality of slurry tanks or powdering tanks <NUM>. The components to be made as fertilizer core <NUM> are first fed to a rotor drum granulator <NUM>, then the liquid coating material of different carbon chain lengths in a plurality of successive reactors and components of fertilizer layer <NUM> in the slurry tanks and/ or powdering tanks <NUM> are sprayed in the predetermined sequence into the rotor drum granulator <NUM>, so that the fertilizer granules continue to enlarge toward the predetermined thickness of fertilizer layer <NUM> and the coating layer <NUM> until the target value is reached.

It should be noted that, due to different spraying sequences and spraying times, different thicknesses of fertilizer layer <NUM> and the coating layer <NUM> can be formed, to form controlled-release fertilizers with different fertilizer effects. In addition, the spraying may be carried out intermittently or continuously according to the preparation needs of the fertilizer, and the granulating and drying may also be carried out in different granulators and drying machines <NUM>, respectively.

In addition, a layer of fertilizer layer <NUM> may be formed by spraying one time or multiple times. For the fertilizer layer <NUM> formed by multiple layers of spraying, the fertilizers being sprayed multiple times may be different or the same. A layer of coating layer <NUM> may be formed by spraying one time or multiple times, and the components of coating layer <NUM> and polyurea formaldehyde slow-release fertilizer after being sprayed multiple times may be the same or different.

The preparation of the multi-layer coating structure slow/controlled-release fertilizer in the present invention needs a reactor, a slurry tank and/or a powdering tank <NUM>, a granulator, a drying machine <NUM> and supporting facilities. The preparation features are to control the proportion of each layer of fertilizer, coating amount, the coating position in the entire fertilizer granules, the thickness and carbon chain length of each layer of coating layer <NUM> according to the design procedure. The entire process can be carried out intermittently or continuously, to achieve mass production.

Furthermore, the fertilizer granules are sprayed with hot air into a drying machine <NUM> for drying, and then sieved, cooled, packaged, and stored. Also, the polyurea formaldehyde of the coating layer <NUM> is obtained by in-situ polymerization, while evaporating and removing water on the surface of the fertilizer layer <NUM>, of liquid coating material including methylol urea under acidic conditions.

Specifically, urea and formaldehyde are formed into a methylol urea at a pH of <NUM>-<NUM> and a temperature of <NUM> to <NUM>, and then the pH is adjusted to <NUM> to <NUM> to form a raw polyurea formaldehyde coating liquid. The liquid coating material is then sprayed on the surface of the fertilizer core <NUM> to form a coating layer <NUM>, and then a second layer of fertilizer is sprayed on the surface of the coating layer <NUM> to form a fertilizer layer <NUM>, and then the liquid coating material is sprayed to form a second layer of coating layer <NUM>, and this process is repeated sequentially. In addition, as needed, the liquid polyurea formaldehyde coating material can also be mixed and sprayed together with a fertilizer as a component of the fertilizer layer <NUM>.

In some embodiments, in order to accelerate the rate of water evaporation, the fertilizer core <NUM> can be preheated. For example, the fertilizer core <NUM> is preheated to <NUM> -<NUM>, or <NUM> -<NUM> ° C, or <NUM> -<NUM>, or <NUM> -<NUM>.

Since the carbon chains contained in the polyurea formaldehyde are mostly of length <NUM> to <NUM>, the percentage of water content is only <NUM>%-<NUM>%, and the concentration is high. When the liquid coating material is sprayed onto the surface of the fertilizer layer <NUM>, the moisture is evaporated, and polymerization and solidification take place so as to coat the surface of the fertilizer layer <NUM>.

The carbon chain length of the polyurea formaldehyde that produces the coating layer <NUM> can then be changed by adjusting the molar ratio of urea and formaldehyde, the pH and temperature of the reaction system.

Since polymerization and solidification happen during evaporation, the molecules having different chain lengths are cross-linked to each other during the formation of the coating layer <NUM>. In the soil, some of the water-soluble components of the coating layer <NUM> are first dissolved to form pores, so that some of the water-soluble components in the fertilizer layer <NUM> are dissolved to provide nutrients for crops. The coating layer <NUM> is then slowly decomposed, so that the coated fertilizer components are released slowly to provide more nutrients to crops. When the operating conditions are different, the carbon chain length distributions of the coating layer <NUM> are different and the slow/controlled-release strengths are different. Under the action of soil microorganisms, the coating layer <NUM> is dissolved and decomposed from outside to inside layer by layer, and the coated fertilizer layer <NUM> is slowly released for crop absorption.

In some embodiments of the present invention, the water content of liquid coating material is within the range of <NUM>% to <NUM>% by weight.

The preparation method of the controlled release fertilizer according to the embodiment of the invention has the characteristics of short process route and low energy consumption, so a large-scale industrial production can be carried out. The features and performance of the present invention are further described in detail below in combination with the embodiments.

Example <NUM>: The method for preparing Corn-specific fertilizer in Henan Region According to the fertilizer requirements of corn in the fluvo-aquic soil on the alluvial plain of the Yellow River and the soil test results in the test field searched in relevant literatures, we design the fertilizer core <NUM>, each fertilizer layer <NUM> and coating layer <NUM> according to previous plot tests on the release and utilization of various nutrients in the multi-layer coating structure slow/controlled-release fertilizer. And according to the design, we produce the corn-specific fertilizer with the content of <NUM>-<NUM>-<NUM>, and carry out field tests.

Major raw materials of the fertilizer include: urea, <NUM>% of formaldehyde solution, powdered monoammonium phosphate with a content of <NUM>-<NUM>-<NUM>, potassium chloride with a content of <NUM>-<NUM>-<NUM>, zinc sulfate and dolomite powder.

The ratio of various elements for preparation of <NUM> corn-specific fertilizer is calculated as following: Polyurea formaldehyde <NUM>, urea <NUM>, monoammonium phosphate <NUM>, potassium chloride <NUM>, zinc sulfate <NUM> and dolomite powder <NUM> are required. The contents of nitrogen, phosphorus and potassium of core granules are <NUM>%, <NUM>% and <NUM>% of the total NPK of the fertilizer granules respectively. The first fertilizer layer <NUM> contains <NUM>%, <NUM>% and <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively, and the second fertilizer layer <NUM> contains <NUM>%, <NUM>% and <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The nitrogen content of the first coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules, and the nitrogen content of the second coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules. The sustained release nitrogen content of polyurea formaldehyde accounts for <NUM>% of the total nitrogen of the fertilizer granules.

Fertilizer core <NUM>: the fertilizer core <NUM> is prepared according to the mass ratio of urea, monoammonium phosphate and potassium chloride at <NUM>:<NUM>:<NUM>. The fertilizer granules having a diameter of about <NUM> are prepared in advance from urea, monoammonium phosphate and potassium chloride in the above ratio by using high-tower granulation equipment.

Reactor: put the urea and formaldehyde in a reactor at a molar ratio of <NUM>:<NUM>, heating while stirring; at the same time add sodium carbonate solution, adjust the pH value to <NUM>-<NUM>, raise the temperature to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes. Then add dilute sulfuric acid solution, adjust the pH to <NUM>~<NUM>, and control the reaction time for about <NUM> minutes.

First slurry tank <NUM>: pour <NUM> of urea, <NUM> of monoammonium phosphate and <NUM> of potassium chloride to a tank which has water already, continuously stir and heat to <NUM> for standby.

Powdering tank <NUM>: pour <NUM> of urea, <NUM> of monoammonium phosphate, <NUM> of potassium chloride, <NUM> of zinc sulfate and <NUM> of dolomite powder that have been treated into the tank to mix evenly.

First feed a calculated amount of the granules of fertilizer core <NUM> to a rotor drum granulator <NUM>, preheat the fertilizer granules to around <NUM> with hot air at <NUM>. Spray the coating liquid in the reactor to the surface of the granules of fertilizer core <NUM> with a liquid slurry pump <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a first coating layer <NUM>. Then, start the liquid slurry pump <NUM> of the first slurry tank <NUM>, spray the slurry containing urea, monoammonium phosphate, and potassium chloride to rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the first coating layer <NUM> by adhesion. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a first fertilizer layer <NUM>. Start the liquid slurry pump <NUM> of the reactor to spray the coating liquid to the surface of the first fertilizer layer <NUM> to initiate polymerization and solidification on the surface of granules. Spray for <NUM>, simultaneously remove the water by hot air evaporation to form a second coating layer <NUM>. Finally start the powdering machine <NUM> of the powdering tank <NUM>, feed the urea, monoammonium phosphate, potassium chloride, zinc sulfate and dolomite powder which have been processed and mixed to the granulator evenly, and meanwhile start the liquid slurry pump <NUM> of the reactor to spray the polyurea formaldehyde into the granulator. The urea formaldehyde and powder are adhered to the surface of the second coating layer <NUM>, until the addition of materials is complete. At the same time, remove the water by hot air evaporation to form a second fertilizer layer <NUM>, and then carry out the operations of sieving, cooling and packaging.

Example <NUM>: The method for preparing wheat-specific fertilizer in Henan Region According to the fertilizer requirements of wheat in the fluvo-aquic soil on the alluvial plain of the Yellow River and the soil test results in the test field searched in relevant literatures, we design fertilizer core <NUM>, each fertilizer layer <NUM> and coating layer <NUM> on the basis of previous plot tests on the release and utilization of various nutrients in the multi-layer coating structure slow/controlled-release fertilizer in the present invention. And according to the design, we produce the wheat-specific fertilizer with content of <NUM>-<NUM>-<NUM>, and carry out field tests.

Major raw materials of the fertilizer include urea, <NUM>% of formaldehyde solution, powdered monoammonium phosphate with a content of <NUM>-<NUM>-<NUM>, potassium chloride with a content of <NUM>-<NUM>-<NUM>, zinc sulfate, borax and dolomite powder.

The ratio of various elements for preparation of <NUM> wheat-specific fertilizer is calculated as following. Polyurea formaldehyde <NUM>, urea <NUM>, monoammonium phosphate <NUM>, potassium chloride <NUM>, zinc sulfate <NUM>, borax <NUM> and dolomite powder <NUM> are needed. The contents of nitrogen, phosphorus and potassium of core granules are <NUM>%, <NUM>% and <NUM>% of the total NPK of the fertilizer granules respectively. The first layer of fertilizer layer <NUM> contains <NUM>%, <NUM>%, <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The second fertilizer layer <NUM> contains <NUM>%, <NUM>%, <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The percentage of nitrogen of the first coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules, the percentage of nitrogen of the second coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules, and the percentage of nitrogen of the third coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules. The outermost layer is the third coating layer <NUM> of the polyurea formaldehyde. The sustained release nitrogen content of polyurea formaldehyde accounts for <NUM>% of the total nitrogen of the fertilizer granules.

First reactor <NUM>: put the urea and formaldehyde in the first reactor <NUM> at a molar ratio of <NUM>:<NUM>, heating while stirring; at the same time, add sodium carbonate solution, adjust the pH value to <NUM>-<NUM>, raise the temperature to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes. Then add dilute sulfuric acid solution, adjust the pH value to <NUM>~<NUM>, and control the reaction time for about <NUM> minutes.

Second reactor <NUM>: put the urea and formaldehyde in the second reactor <NUM> at a molar ratio of <NUM>:<NUM>, heating while stirring; at the same time, add sodium carbonate solution, adjust the pH value to <NUM>-<NUM>, raise the temperature to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes. Add dilute sulfuric acid solution, adjust the pH value to <NUM>~<NUM>, and control the reaction time for about <NUM> minutes.

First slurry tank <NUM>: pour <NUM> of urea, <NUM> of monoammonium phosphate, <NUM> of potassium chloride to a tank which has water already, and continuously stir and heat to <NUM> for standby.

Second slurry tank <NUM>: pour <NUM> of urea, <NUM> of monoammonium phosphate, <NUM> of potassium chloride to a tank which has water already, and continuously stir and heat to <NUM> for standby.

Powdering tank <NUM>: pour <NUM> of zinc sulfate, <NUM> of borax, <NUM> of dolomite powder that have been treated into the tank to mix evenly.

First feed a calculated amount of the granules of fertilizer core <NUM> to a rotor drum granulator <NUM>, preheat the fertilizer granules to around <NUM> with hot air at <NUM>. Spray the coating liquid in the first reactor <NUM> to the surface of the granules of fertilizer core <NUM> with a liquid slurry pump <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a first coating layer <NUM>. Then, start the liquid slurry pump <NUM> of the first slurry tank <NUM>, spray the slurry containing urea, monoammonium phosphate, and potassium chloride to rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the first coating layer <NUM> by adhesion. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a first fertilizer layer <NUM>. Start the liquid slurry pump <NUM> of the second reactor <NUM> to spray the coating liquid to the surface of the first fertilizer layer <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a second coating layer <NUM>. Then start the liquid slurry pump <NUM> of the second slurry tank <NUM>, spray the slurry containing urea, monoammonium phosphate, and potassium chloride to the rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the second coating layer <NUM> by adhesion. Spray for <NUM> minutes, start the powdering machine <NUM> of the powdering tank <NUM>, feed the zinc sulfate, borax, dolomite powder which have been processed and mixed into the rotor drum granulator <NUM> evenly, simultaneously remove the water by hot air evaporation to form a second fertilizer layer <NUM>. Finally start the liquid slurry pump <NUM> of the first reactor <NUM>, spray the remaining coating liquid onto the surface of the second fertilizer layer <NUM>, and simultaneously remove the water by hot air evaporation to form a third coating layer <NUM>. Then, the fertilizer granules are dried in a drying machine <NUM>, sieved by a sieving machine <NUM>, and then proceed for cooling and packaging.

According to the fertilizer requirements of rice in the fluvo-aquic soil on the alluvial plain of the Yellow River and the soil test results in the test field searched in relevant literatures, we design fertilizer core <NUM>, each fertilizer layer <NUM> and coating layer <NUM> on the basis of previous plot tests on the release and utilization of various nutrients in the multi-layer coating structure slow/controlled release fertilizer. And according to the design, we produce the wheat-specific fertilizer with the content of <NUM>-<NUM>-<NUM>, and carry out field tests.

Major raw materials of fertilizer: urea, <NUM>% of formaldehyde solution, powdered diammonium phosphate with a content of <NUM>-<NUM>-<NUM>, <NUM>-<NUM>-<NUM> calcium magnesium phosphate fertilizer, potassium chloride with a content of <NUM>-<NUM>-<NUM>.

The ratio of various elements for preparation of <NUM> rice-specific fertilizer is calculated as following. Polyurea formaldehyde <NUM>, urea <NUM>, diammonium phosphate <NUM>, calcium magnesium phosphate fertilizer <NUM>, potassium chloride <NUM> are needed. The contents of nitrogen, phosphorus and potassium of core granules are <NUM>%, <NUM>%, <NUM>% of the total NPK of the fertilizer granules respectively. The first layer of fertilizer layer <NUM> contains <NUM>%, <NUM>%, <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The second fertilizer layer <NUM> contains <NUM>%, <NUM>%, <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The third fertilizer layer <NUM> contains <NUM>%, <NUM>%, <NUM>% of the total nitrogen, phosphorus and potassium of the fertilizer granules respectively. The percentage of nitrogen of the first coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules, the percentage of nitrogen of the second coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules, and the percentage of nitrogen of the third coating layer <NUM> is <NUM>% of the total nitrogen of the fertilizer granules.

The inner layer of the outermost layer is mixed urea, diammonium phosphate and potassium chloride, and the outer layer is mixed calcium magnesium phosphate fertilizer and polyurea formaldehyde slow release nitrogen fertilizer. The slow release nitrogen content of the polyurea formaldehyde accounts for <NUM>% of the total nitrogen of the fertilizer granules.

Fertilizer core <NUM>: the fertilizer core <NUM> is prepared according to the mass ratio of urea, diammonium phosphate and potassium chloride at <NUM>:<NUM>:<NUM>. The fertilizer granules having a diameter of about <NUM> are prepared in advance from urea, diammonium phosphate and potassium chloride in the above ratio by using high-tower granulation equipment.

First reactor <NUM>: put the urea and formaldehyde in the reactor at a molar ratio of <NUM>:<NUM>, heating while stirring; at the same time, add sodium hydroxide solution, adjust the pH value to <NUM>-<NUM>, raise the temperature to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes. Add dilute sulfuric acid solution, adjust the pH value to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes.

Second reactor <NUM>: put the urea and formaldehyde in the reactor at a molar ratio of <NUM>:<NUM>, heating while stirring; at the same time, add sodium hydroxide solution, adjust the pH value to <NUM>-<NUM>, raise the temperature to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes. Add <NUM>% sulfuric acid solution, adjust the pH value to <NUM>-<NUM>, and control the reaction time for about <NUM> minutes.

First slurry tank <NUM>: pour <NUM> of urea, <NUM> of diammonium phosphate, <NUM> of potassium chloride to a tank which has water already, and continuously stir and heat to <NUM> for standby.

Second slurry tank <NUM>: Pour <NUM> of urea, <NUM> of diammonium phosphate, <NUM> of potassium chloride to a tank which has water already, and continuously stir and heat to <NUM> for standby.

Third slurry tank: pour <NUM> of urea, <NUM> of diammonium phosphate, <NUM> of potassium chloride to a tank which has water already, and continuously stir and heat to <NUM> for standby.

Powdering tank <NUM>: pour calcium magnesium phosphate fertilizer into the tank for use.

First feed a calculated amount of the granules of fertilizer core <NUM> to a rotor drum granulator <NUM>, preheat the fertilizer granules to around <NUM> with hot air at <NUM>. Spray the coating liquid in the first reactor <NUM> to the surface of the granules of fertilizer core <NUM> with a liquid slurry pump <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a first coating layer <NUM>. Then, start the liquid slurry pump <NUM> of the first slurry tank <NUM>, spray the slurry containing urea, monoammonium phosphate, and potassium chloride to rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the first coating layer <NUM> by adhesion. Spray for <NUM>, simultaneously remove the water by hot air evaporation to form a first fertilizer layer <NUM>. Start the liquid slurry pump <NUM> of the second reactor <NUM> to spray the coating liquid to the surface of the first fertilizer layer <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a second coating layer <NUM>. Then start the liquid slurry pump <NUM> of the second slurry tank <NUM>, spray the slurry containing urea, diammonium phosphate, and potassium chloride to rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the second coating layer <NUM> by adhesion. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a second fertilizer layer <NUM>. Start the liquid slurry pump <NUM> on the second reactor <NUM> to spray the coating liquid to the surface of the second fertilizer layer <NUM>, to initiate polymerization and solidification on the surface of granules. Spray for <NUM> minutes, simultaneously remove the water by hot air evaporation to form a third coating layer <NUM>. Then start the liquid slurry pump <NUM> of the third slurry tank, spray the slurry containing urea, diammonium phosphate, and potassium chloride to rolling granules in the rotor drum granulator <NUM>, to uniformly cover the surface of the third coating layer <NUM> by adhesion until the coating is finished. At the same time, remove the water by hot air evaporation to form the inner layer of fertilizer of a third fertilizer layer <NUM>. Restart the first reactor <NUM> and simultaneously start the powdering machine <NUM> of the powdering tank <NUM>, feed the calcium magnesium phosphate fertilizer to the granulator evenly, and the urea formaldehyde and powder are attached to the surface of fertilizer granules.

Meanwhile, remove the water by hot air evaporation to form an outer layer of fertilizer of a third fertilizer layer <NUM>, and then carry out the operations of sieving, cooling and packaging.

The fertilizer in Comparative Example <NUM> is a conventional compound fertilizer with the same contents as the fertilizer in Example <NUM>.

The multi-layer coating structure slow/controlled-release fertilizers of Example <NUM>-<NUM> in the present invention and the conventional fertilizers as Comparative Examples <NUM>-<NUM> are applied to the experimental fields of corn, wheat and rice in different plots. One-time base fertilizer is used, without topdressing. The experimental fields are located in Henan region, and their soils belong to fluvo-aquic soil. Before the implementation, soil assay is performed on the test plots. According to the assay results, we design the test program, record agronomic matters of crops during the growth process and perform statistical analysis on crop yields after harvesting.

It is noted that the amount applied to the fields of conventional quick-acting compound fertilizers and multi-layer coating structure slow/controlled-release fertilizers in the present invention is the same. They are applied in one time as base fertilizers, without topdressing. The content of NPK is the same, for corn: <NUM>-<NUM>-<NUM>, for wheat: <NUM>-<NUM>-<NUM> and for rice: <NUM>-<NUM>-<NUM>.

Analysis of results: the above results of field experiments show that comparing Examples <NUM>-<NUM> and the Comparative Examples <NUM>-<NUM>, when the growth cycle and the amount of fertilization are the same, the multi-layer coating structure slow/controlled-release fertilizers in embodiments of the present invention herein can provide the nutrients needed by the crops in a timely manner, and the crops show healthy growth trends in each growth cycle, and the crop yields increase significantly, indicating that the fertilizer utilization rate has been significantly improved so it has a good promotion value. Although the conventional compound fertilizers can quickly provide the nutrients needed by the crops in the early stage of crop growth, it shows a significant lack of fertilizer in the middle and late stages of crop growth. Since the crops may not obtain nutrients in time, the crops grow slowly, and the seeds and grains are not full, for example corn has a long bald tip of corn cobs, small cobs, and light hundred-grain weight; and the wheat and rice have more inefficient tillers, few grains per spike, and light thousand-grain weight, etc., eventually leading to a decline in yields. The multi-layer coating structure slow/controlled-release fertilizers in the present invention can be designed as a layer-by-layer coated slow release structure according to the crop growth needs, to provide nutrients in the crop growth cycles in time, so that the crops grow healthily, with more and full grains, and high yields. The micronutrient fertilizer can also be designed in a suitable fertilizer layer <NUM> for the crop absorption at a certain growth stage.

Claim 1:
A method for preparing a multi-layer coating structure slow/controlled-release fertilizer, said multi-layer coating structure slow/controlled release fertilizer comprising a layer-by-layer coated fertilizer layer and a coating layer, wherein the fertilizer layer is separated from the coating layer, the fertilizer layer has at least two layers, the coating layer has at least two layers, and the innermost layer of the fertilizer layers is a fertilizer core; the fertilizer layer includes at least two of nitrogen fertilizer, phosphorus fertilizer and potassium fertilizer, the coating layer is polyurea formaldehyde, wherein the raw materials of the polyurea formaldehyde are urea and formaldehyde, the molar ratio of the urea to the formaldehyde is <NUM>-<NUM>:<NUM>, and the polyurea formaldehyde includes at least a polyurea formaldehyde having two to five carbon atoms,
wherein the method comprises steps of: applying the fertilizer layer and the fertilizer corresponding to the coating layer to the periphery of the fertilizer core according to a preset order, to form layer-by-layer coated fertilizer granules and
wherein polyurea formaldehyde is prepared by the following steps: mixing urea and formaldehyde at a molar ratio of <NUM>- <NUM>:<NUM>, adjusting the pH of the reaction system to <NUM>-<NUM> and the temperature to <NUM>-<NUM>, and then adjusting the pH to <NUM>~<NUM>.