Slow-release fertilizer and preparation method thereof

Abstract

The invention relates to a slow-release fertilizer and a preparation method thereof, and the preparation method comprises the following steps: providing a material for preparing a slow-release nitrogen fertilizer, and carrying out microwave drying on the material to obtain the slow-release fertilizer. Through microwave drying, the slow release performance of the fertilizer is remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of chemical fertilizers, and in particular to a slow-release fertilizer and a preparation method thereof. Background Art

[0002] According to the Food and Agriculture Organization of the United Nations (FAO), the global population is expected to exceed 9 billion by 2050, and agricultural production must increase by about 60% to feed the growing global population. The production of crops is inseparable from the use of fertilizers. Urea is one of the typical fertilizers and the nitrogen fertilizer with the highest nitrogen content. It is usually used to grow rice, wheat, corn, etc. However, urea dissolves quickly when it comes into contact with water, and some of its nutrients are washed away before being absorbed by the crop roots, resulting in serious urea loss. In addition, unabsorbed urea particles continue to form ammonia, polluting waterways and ultimately causing greenhouse gases to be released into the atmosphere. Slow-release fertilizers are fertilizers that can slowly release nutrient components. Traditional slow-release fertilizers consist of urea coated with water-insoluble sulfur or polymers, but they are expensive and have not been proven to actually increase crop yields. They are not suitable for widespread use worldwide, especially in developing countries. Therefore, it is necessary to develop an efficient slow-release fertilizer to achieve higher crop yields at a low cost without causing adverse effects on the environment. Summary of the invention

[0003] According to the first aspect, in one embodiment, a method for preparing a slow-release fertilizer is provided, comprising:

[0004] Materials for preparing slow-release nitrogen fertilizer are provided, and the materials are subjected to microwave drying to obtain the slow-release fertilizer.

[0005] According to the second aspect, in one embodiment, a slow-release fertilizer prepared by the preparation method described in any one of the first aspects is provided.

[0006] In one embodiment, the present invention significantly improves the slow-release performance of the fertilizer through microwave drying. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 This is the effect of drying time, material amount and microwave power on the dehydration rate and dehydration amount of slow-release fertilizer samples;

[0008] Figure 2 This is the microscopic nanostructure of the sustained-release material under transmission electron microscopy (urea / calcium hydroxide feed ratio is 10 / 1);

[0009] Figure 3 This is the time-dependent change of the UV spectrum of common urea released in water;

[0010] Figure 4The ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time when the feed ratio of urea / calcium hydroxide is 1 / 1;

[0011] Figure 5 When the feed ratio of urea to calcium hydroxide is 3 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time;

[0012] Figure 6 The ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time when the feed ratio is urea / calcium hydroxide = 5 / 1;

[0013] Figure 7 When the feed ratio of urea to calcium hydroxide is 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time;

[0014] Figure 8 The ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time when the feed ratio of urea / calcium hydroxide is 8 / 1;

[0015] Fig. 9 The ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer changes with time when the feed ratio of urea / calcium hydroxide is 10 / 1;

[0016] Fig.10 This is a comparison chart of the ultraviolet absorbance of urea released in water from ordinary urea and nano slow-release fertilizers prepared with different urea / calcium hydroxide feed ratios over time;

[0017] Fig.11 The ultraviolet spectrum of urea released in water from the nano slow-release fertilizer prepared by a centrifugal spray dryer changes with time when the feed ratio is urea / calcium hydroxide = 6 / 1;

[0018] Fig.12 When the feed ratio of urea / calcium hydroxide is 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer crude product after drying at 65°C for 6 hours in a blast dryer changes with time;

[0019] Fig.13 When the feed ratio of urea / calcium hydroxide is 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer sample after drying at 65°C for 12 hours in a forced air dryer changes with time;

[0020] Fig.14 When the feed ratio is urea / calcium hydroxide = 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer sample after drying at 65°C for 6 hours and then drying with microwave for 30 minutes is plotted as a function of time;

[0021] Fig.15The figure is a comparison of the change in ultraviolet absorbance of urea released in water from nano slow-release fertilizers prepared by different drying methods when the feed ratio is urea / calcium hydroxide = 6 / 1. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below by specific embodiments in conjunction with the accompanying drawings. In the following embodiments, many detailed descriptions are intended to enable the present application to be better understood. However, those skilled in the art can effortlessly recognize that some of the features can be omitted in different situations, or can be replaced by other materials and methods. In some cases, some operations related to the present application are not shown or described in the specification, in order to avoid the core part of the present application being overwhelmed by too much description, and for those skilled in the art, it is not necessary to describe these related operations in detail, and they can fully understand the related operations according to the description in the specification and the general technical knowledge in the art.

[0023] In addition, the features, operations or characteristics described in the specification can be combined in any appropriate manner to form various implementations. At the same time, the steps or actions in the method description can also be interchanged or adjusted in a manner that is obvious to those skilled in the art. Therefore, the various sequences in the specification and the drawings are only for the purpose of clearly describing a certain embodiment and are not meant to be a required sequence, unless otherwise specified that a certain sequence must be followed.

[0024] The serial numbers assigned to the components in this article, such as "first", "second", etc., are only used to distinguish the objects described and do not have any order or technical meaning.

[0025] According to the first aspect, in one embodiment, a method for preparing a slow-release fertilizer is provided, comprising:

[0026] Materials for preparing slow-release nitrogen fertilizer are provided, and the materials are subjected to microwave drying to obtain the slow-release fertilizer.

[0027] In one embodiment, the material is pre-dried first and then microwave dried. After pre-drying to remove most of the water, microwave drying is performed, which is beneficial to improving the drying efficiency.

[0028] In one embodiment, the material is pre-dried to a moisture content of ≤30 wt% and then microwave dried.

[0029] In one embodiment, after microwave drying, the water content of the material is ≤1 wt%.

[0030] In one embodiment, the pre-drying includes air-blast drying and / or belt drying.

[0031] In one embodiment, the pre-drying temperature is 50-80°C, including but not limited to 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, and 80°C.

[0032] In one embodiment, the pre-drying time is 6 to 12 hours, including but not limited to 6, 7, 8, 9, 10, 11, and 12 hours.

[0033] In one embodiment, the power of the microwave drying is 500-1000 W, including but not limited to 500 W, 550 W, 600 W, 650 W, 700 W, 750 W, 800 W, 850 W, 900 W, 950 W, 1000 W and the like.

[0034] In one embodiment, the microwave power is 700-900W.

[0035] In one embodiment, the microwave power is 700-800W.

[0036] In one embodiment, the microwave drying time is 10 to 60 minutes, including but not limited to 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, etc.

[0037] In one embodiment, the microwave drying time is 15 to 55 minutes.

[0038] In one embodiment, the microwave drying time is 20 to 55 minutes.

[0039] In one embodiment, the amount of material used for drying is 10-100g, including but not limited to 10g, 15g, 20g, 25g, 30g, 35g, 40g, 45g, 50g, 55g, 60g, 65g, 70g, 75g, 80g, 85g, 90g, 95g, 100g and the like.

[0040] In one embodiment, during microwave drying, the moisture content of the material satisfies the following formula ①:

[0041] Y1=0.72-0.15χ1-0.088χ2+0.16χ3+0.02χ1χ2-0.068χ1χ3-0.065χ2χ3-0.15χ1 2 -0.087χ2 2 -0.14χ3 2 ;

[0042] Y1 is the moisture content, χ1 is the drying temperature, χ2 is the drying time, and χ3 is the material amount.

[0043] In one embodiment, during microwave drying, the water loss rate of the material satisfies the following formula ②:

[0044] Y2=0.33+0.20χ1-0.44χ2+0.14χ3-0.19χ1χ2+0.11χ1χ3+0.38χ2χ3+0.23χ1 2 +0.36χ2 2 -0.037χ3 2 ;

[0045] Y2 is the water loss rate, χ1 is the drying temperature, χ2 is the drying time, and χ3 is the material amount.

[0046] In one embodiment, in the formula ① and / or formula ②, the material amount χ3 is mass.

[0047] In one embodiment, the fertilizer includes but is not limited to nitrogen fertilizer, nitrogen-phosphorus-potassium fertilizer or organic compound fertilizer.

[0048] In one embodiment, the material includes urea and calcium hydroxide.

[0049] In one embodiment, the molar ratio of urea to calcium hydroxide is (1-10): 1, including but not limited to 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, and 10:1.

[0050] In one embodiment, the molar ratio of urea to calcium hydroxide is (3-10):1.

[0051] In one embodiment, the molar ratio of urea to calcium hydroxide is (3-8):1.

[0052] In one embodiment, the molar ratio of urea to calcium hydroxide is (5-8):1.

[0053] In one embodiment, the molar ratio of urea to calcium hydroxide is 6:1.

[0054] In one embodiment, the material further comprises phosphoric acid. In some embodiments, the amount of phosphoric acid added is determined based on calcium hydroxide, for example, generally about 0.6 times the molar amount of calcium hydroxide.

[0055] In one embodiment, phosphoric acid can be prepared into an aqueous solution and then added to a mixed solution of urea and calcium hydroxide. The concentration of the phosphoric acid aqueous solution is not limited, for example, it can be 1.2 mol / L. In actual production, the concentration of the phosphoric acid aqueous solution can be increased to save water.

[0056] In one embodiment, the material further includes a solvent.

[0057] In one embodiment, the solvent includes but is not limited to water.

[0058] In one embodiment, in actual production, the material can be placed on a conveyor belt or other equipment for continuous and large-scale rapid drying.

[0059] According to the second aspect, in one embodiment, a slow-release fertilizer prepared by the preparation method described in any one of the first aspects is provided.

[0060] In one embodiment, the present invention uses urea, calcium hydroxide and phosphoric acid as main reaction raw materials, adopts one-step in-situ synthesis method and rapid drying technology, and prepares a highly efficient slow-release nano hybrid, namely urea-hydroxyapatite nano hybrid (Urea-HA nano hybrid). The specific implementation scheme is as follows: (1) calcium hydroxide is dispersed in water, solid urea is added to the calcium hydroxide suspension, and stirred for 30 to 45 minutes to dissolve it; the temperature is raised to 50°C, phosphoric acid solution is added dropwise to the mixed solution of urea and calcium hydroxide, and the stirring reaction is continued for 2 to 3 hours to obtain a slurry; (2) the above reaction slurry is first dried at 65°C in an ordinary blast drying oven for 6 to 12 hours to obtain a slow-release fertilizer crude product with a water content of less than 30%, and then the slow-release fertilizer crude product with a water content of less than 30% is dried in a microwave oven for 30 to 60 minutes to obtain a final slow-release fertilizer product with a water content of less than 1%. The above reaction materials are mixed in the following molar ratio: urea: calcium hydroxide: phosphoric acid = (1-10): 1: 0.6. In the actual feeding process, the molar ratio of urea and calcium hydroxide can be adjusted in the range of 1 / 1-10 / 1 to obtain a slow-release fertilizer with a nitrogen content of 17.5%-42%. The lower the nitrogen content, the longer the slow-release time of the slow-release fertilizer.

[0061] In one embodiment, the present invention uses microwave drying to improve the slow-release efficiency for the following reasons: 1. Microwaves can easily penetrate the material and quickly dry the material from the inside out; 2. Water is a polar substance, and the molecules in the microwave field will polarize to produce dipoles, thereby quickly absorbing microwaves and being evenly heated; the above two points may keep the material small in particle size during the drying process, improve the binding efficiency of urea, and thus improve the slow-release efficiency (the time required to release 80% of urea). The slow-release efficiency is 30 to 40 times that of ordinary nitrogen fertilizers, while the slow-release efficiency of existing ordinary centrifugal spray drying is only 8 to 20 times that of ordinary nitrogen fertilizers.

[0062] In one embodiment, the present invention uses ultraviolet spectroscopy to analyze the release behavior of urea in nano slow-release fertilizer in water. The experimental results show that compared with ordinary urea, the urea release time in the slow-release fertilizer prepared by the above method is 8 to 40 times that of ordinary urea.

[0063] In one embodiment, the present invention utilizes nanomaterials that are common and easily available in nature and have high biocompatibility to adsorb urea, and no other materials are required to coat the fertilizer. The fertilizer has a long release period and a high absorption rate, which reduces the number of fertilization times, does not pollute crops, has good safety, and reduces farmers' production costs.

[0064] Example

[0065] Urea, calcium hydroxide and phosphoric acid are used as reaction raw materials, and a one-step in-situ synthesis method and a rapid drying technology are adopted to prepare a highly efficient slow-release nano hybrid, namely, urea-hydroxyapatite nano hybrid (Urea-HA nano hybrid). The specific implementation scheme is as follows: (1) calcium hydroxide is dispersed in water, and solid urea is added to the calcium hydroxide suspension, and stirred for 30 to 45 minutes to dissolve it; the temperature is raised to 50° C., and a phosphoric acid aqueous solution (with a concentration of 1.2 mol / L) is added dropwise to the mixed solution of urea and calcium hydroxide (the added molar amount of phosphoric acid is about 0.6 times the molar amount of calcium hydroxide), and the stirring reaction is continued for 2 to 3 hours to obtain a slurry; (2) the above reaction slurry is first dried in an ordinary blast drying oven at 65° C. for 6 to 12 hours to obtain a slow-release fertilizer crude product with a water content of less than 30%, and then the slow-release fertilizer crude product with a water content of less than 30% is placed in a microwave oven and dried for 30 to 60 minutes to obtain a final slow-release fertilizer product with a water content of less than 1%.

[0066] Follow-up Figures 4 to 10 In the examples, the samples were first dried in a conventional air drying oven at 65°C for 6 hours and then microwave dried.

[0067] Figures 3 to 15 In the graph, the vertical axis “Absorbance” refers to the absorbance value.

[0068] Figures 3 to 14 In the figure, the horizontal axis “Wavelength” refers to the wavelength.

[0069] Fig.15 In the figure, the horizontal axis “Time” refers to time.

[0070] Microwave drying power measurement

[0071] The drying time, material amount and microwave power, which have a greater impact on the drying effect of slow-release fertilizer samples, were selected as the main factors, and the influence of each factor on the dehydration rate and dehydration amount of slow-release fertilizer samples was investigated. Figure 1 This is a graph showing the effects of drying time, material amount and microwave power on the dehydration rate and dehydration amount of slow-release fertilizer samples. Figure 1 The material amount in (a) is 50 g and the microwave power is 900 W. Figure 1 The microwave power in (b) is 900 W and the time is 15 min. Figure 1The amount of material in (c) is 50 g and the time is 15 min.

[0072] Depend on Figure 1 It can be seen that under a certain microwave power, the longer the drying time, the more water evaporates and the lower the water content of the sample; as the amount of material increases, the sample dehydration rate gradually decreases. By performing multivariate regression fitting on the experimental results of microwave drying, it is found that the moisture content and water loss rate conform to the linear and quadratic equation models respectively, and the fitting equations are shown in equations ① and ② respectively.

[0073] Formula ① is as follows:

[0074] Y1=0.72-0.15χ1-0.088χ2+0.16χ3+0.02χ1χ2-0.068χ1χ3-0.065χ2χ3-0.15χ1 2 -0.087χ2 2 -0.14χ3 2

[0075] Y1 is the moisture content, χ1 is the drying temperature (the microwave dryer can control the internal heating temperature of the material by controlling the power consumption of the machine), χ2 is the drying time, and χ3 is the material amount.

[0076] Formula ② is as follows:

[0077] Y2=0.33+0.20χ1-0.44χ2+0.14χ3-0.19χ1χ2+0.11χ1χ3+0.38χ2χ3+0.23χ1 2 +0.36χ2 2 -0.037χ3 2

[0078] Y2 is the water loss rate, χ1 is the drying temperature, χ2 is the drying time, and χ3 is the material amount. The water loss rate refers to the mass of water lost by the material per unit time.

[0079] The above regression model was used to optimize the process parameters and obtain the optimal process parameters for microwave deep drying of nano slow-release fertilizer. The results are shown in the following table:

[0080] Table 1 Optimal process parameters of microwave deep drying slow-release fertilizer sample (urea / calcium hydroxide feed molar ratio is 10 / 1)

[0081]

[0082] It can be seen from the above table that the predicted value of the response surface is close to the verified value with a small deviation, which shows that the prediction model is appropriate, the optimization of process conditions is feasible, and further industrial research can be carried out.

[0083] The microstructure of nano slow-release fertilizer under transmission electron microscope Figure 2 As shown, the microwave power is 750 W, the drying temperature is 90° C., the drying time is 10 min, and the material amount is 50 g. Figure 2 The micro-nanostructure image of the sustained-release material under transmission electron microscopy (the molar ratio of urea / calcium hydroxide is 10 / 1), (a) hydroxyapatite forms a rod-like structure at the microscopic level; (b) urea microparticles are evenly coated on the nanostructure skeleton.

[0084] The release of urea in slow-release fertilizer in water was determined by ultraviolet spectroscopy. The experimental results are as follows: Figures 3 to 10 As shown:

[0085] Figure 3 This is a graph showing the change in the ultraviolet spectrum of ordinary urea released in water over time.

[0086] Figures 4 to 10 In the experiment, the microwave power was 750 W, the drying temperature was 90 °C, the material amount was 26 g, and the drying time was 10 min.

[0087] Figure 4 This is a graph showing the change in the ultraviolet spectrum of urea released in water from the prepared nano slow-release fertilizer over time when the feed ratio of urea to calcium hydroxide is 1 / 1.

[0088] Figure 5 The graph shows the change of the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer over time when the feed ratio of urea / calcium hydroxide is 3 / 1.

[0089] Figure 6 The graph shows the change of the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer over time when the feed ratio of urea / calcium hydroxide is 5 / 1.

[0090] Figure 7 The graph shows the change of the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer over time when the feed ratio of urea / calcium hydroxide is 6 / 1.

[0091] Figure 8 The graph shows the change of the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer over time when the feed ratio of urea / calcium hydroxide is 8 / 1.

[0092] Fig. 9 The graph shows the change of the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer over time when the feed ratio of urea / calcium hydroxide is 10 / 1.

[0093] Fig.10 This is a comparison chart of the changes in ultraviolet absorbance of urea released in water from ordinary urea and nano-slow-release fertilizers prepared with different urea / calcium hydroxide feed ratios.

[0094] according to Fig.10It can be seen that:

[0095] When the urea / calcium hydroxide feed ratio is 1 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 17.5%. According to the time when the urea release in the fertilizer is 80% of the total amount, the slow-release time of the slow-release fertilizer is about 40 times that of ordinary urea.

[0096] When the urea / calcium hydroxide feed ratio is 3 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 29.8%, and the slow-release time of the slow-release fertilizer is about 30 times that of ordinary urea;

[0097] When the urea / calcium hydroxide feed ratio is 5 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 34.9%, and the slow-release time of the slow-release fertilizer is about 20 times that of ordinary urea;

[0098] When the urea / calcium hydroxide feed ratio is 6 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 36.4%, and the slow-release time of the slow-release fertilizer is about 12.5 times that of ordinary urea;

[0099] When the urea / calcium hydroxide feed ratio is 8 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 38.5%, and the slow-release time of the slow-release fertilizer is about 9 times that of ordinary urea;

[0100] When the urea / calcium hydroxide feed ratio is 10 / 1, the nitrogen content of the slow-release fertilizer prepared at this time is 42%, and the slow-release time of the slow-release fertilizer is about 8 times that of ordinary urea.

[0101] Comparative experiment on slow-release performance of slow-release fertilizers prepared by different drying methods:

[0102] When the urea / calcium hydroxide feed ratio was selected as 6 / 1, nano slow-release fertilizer samples were prepared using three different drying methods, namely centrifugal spray dryer, ordinary blast dryer and microwave dryer. The slow-release properties of each sample were measured by ultraviolet spectroscopy, and the results are as follows:

[0103] Fig.11 The ultraviolet spectrum of urea released in water from the nano slow-release fertilizer prepared by a centrifugal spray dryer changes with time when the feed ratio is urea / calcium hydroxide = 6 / 1.

[0104] Fig.12 When the feed ratio of urea / calcium hydroxide is 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer crude product after drying at 65°C in a forced air dryer for 6 hours changes with time.

[0105] Fig.13 When the feed ratio of urea / calcium hydroxide is 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer sample after drying at 65°C in a forced air dryer for 12 hours changes with time.

[0106] Fig.14When the feed ratio is urea / calcium hydroxide = 6 / 1, the ultraviolet spectrum of urea released in water in the prepared nano slow-release fertilizer sample is plotted as a function of time after being dried in a blast dryer at 65°C for 6 hours and then dried in a microwave for 30 minutes.

[0107] Fig.15 The figure is a comparison of the change in ultraviolet absorbance of urea released in water from nano slow-release fertilizers prepared by different drying methods when the feed ratio is urea / calcium hydroxide = 6 / 1.

[0108] Figures 11 to 15 In the experiment, the amount of dried material was 500 g.

[0109] pass Fig.15 It can be seen that when the feed ratio of urea / calcium hydroxide = 6 / 1, the release rate of urea in the slow-release fertilizer prepared by a centrifugal spray dryer is about 8 times that of ordinary urea, the release rate of urea in the slow-release fertilizer after being dried at 65°C in a blower dryer for 6 hours is about 4 times that of ordinary urea, the release rate of urea in the slow-release fertilizer after being dried at 65°C in a blower dryer for 12 hours is about 10 times that of ordinary urea, and the release rate of urea in the slow-release fertilizer after being dried at 65°C in a blower dryer for 6 hours and then dried in a microwave dryer for 30 minutes is about 12.5 times that of ordinary urea.

[0110] The centrifugal spray dryer can dry the material in the shortest time, but when the temperature is lower than 150℃, the volume heat transfer coefficient is low, the equipment used is large, and the thermal efficiency is not high. The thermal efficiency of the general downstream tower spray dryer is only 30% to 50%. The ordinary blast dryer uses the box blast cycle to drive the hot air flow to achieve drying. The moisture is transferred from the inner layer of the material to the surface of the material in the form of steam, and then from the surface to the hot air. This process requires overcoming the air film resistance. In the conventional drying process, the direction of steam diffusion is opposite to the temperature gradient, resulting in diffusion obstruction and reduced heat transfer efficiency. In the microwave drying process, all the moisture in the material, that is, the inner and surface moisture, is selectively heated by microwaves at the same time. The moisture is instantly vaporized after being heated, and the inner layer of the material and the surface air film and air produce a huge pressure difference, forming a huge driving force, so that the moisture is quickly transferred from the inside of the material to the air. In the microwave drying process, the temperature gradient and heat transfer direction inside the material are the same as the direction of water vapor transfer, so it exhibits the characteristics of heating from the inside to the surface.

[0111] Considering the advantages and disadvantages of the above different drying methods and the equipment cost, in a preferred embodiment, the slow-release fertilizer is prepared by using a blast dryer for rough drying and microwave heating for deep drying.

[0112] The above specific examples are used to illustrate the present invention, which is only used to help understand the present invention and is not intended to limit the present invention. For those skilled in the art, according to the concept of the present invention, some simple deductions, modifications or substitutions can be made.

Claims

1. A method for preparing a slow-release fertilizer, characterized in that: include: Materials for preparing slow-release nitrogen fertilizer are provided, and the mixture is subjected to microwave drying to obtain the slow-release fertilizer.

2. The preparation method according to claim 1, characterized in that The material is first pre-dried and then microwave dried.

3. The preparation method according to claim 1, characterized in that: The material is pre-dried until the water content is ≤30wt% and then microwave dried.

4. The preparation method according to claim 1, characterized in that: After microwave drying, the water content of the material is ≤1wt%.

5. The preparation method according to claim 2, characterized in that: The pre-drying includes air-blast drying and / or belt drying.

6. The preparation method according to claim 2, characterized in that: The pre-drying temperature is 50-80°C; Alternatively, the pre-drying time is 6 to 12 hours.

7. The preparation method according to claim 1, characterized in that: The power of the microwave drying is 500-1000W; Alternatively, the microwave drying time is 10 to 60 minutes.

8. The preparation method according to claim 1, characterized in that: During microwave drying, the moisture content of the material satisfies the following formula ①: Y1=0.72-0.15χ1-0.088χ2+0.16χ3+0.02χ1χ2-0.068χ1χ3-0.065χ2χ3-0.15χ1 2 -0.087x2 2 -0.14x3 2 ; Y1 is the moisture content, χ1 is the drying temperature, χ2 is the drying time, and χ3 is the material amount; Or, during microwave drying, the water loss rate of the material satisfies the following formula ②: Y2=0.33+0.20χ1-0.44χ2+0.14χ3-0.19χ1χ2+0.11χ1χ3+0.38χ2χ3+0.23χ1 2 +0.36x2 2 -0.037x3 2 ; Y2 is the water loss rate, χ1 is the drying temperature, χ2 is the drying time, and χ3 is the material amount.

9. The preparation method according to claim 1, characterized in that: The fertilizer includes nitrogen fertilizer, nitrogen-phosphorus-potassium fertilizer or organic compound fertilizer; Or, the materials include urea and calcium hydroxide; Or, the molar ratio of urea to calcium hydroxide is (1-10):1; Or, the material further includes phosphoric acid; Or, the material further comprises a solvent; Alternatively, the solvent comprises water.

10. The slow-release fertilizer obtained by the preparation method according to any one of claims 1 to 9.

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