Modified kaolin fertilizer coating liquid as well as preparation method and application thereof

Abstract

The invention discloses a modified kaolin fertilizer coating liquid as well as a preparation method and application thereof, and belongs to the technical field of fertilizer production. The preparation method comprises the following steps: modifying kaolin by using a chemical expansion method, modifying the kaolin by using dimethyl sulfoxide and hexadecyl trimethyl ammonium bromide, and adding the modified kaolin into a polyvinyl alcohol / starch composite film solution to obtain a modified kaolin fertilizer coating solution; the fertilizer coating liquid is used as a coating material of a biochar-based compound fertilizer and is applied to preparation of a coated slow-release fertilizer by using a coating machine, and the prepared coated slow-release fertilizer has a better fertilizer nutrient slow-release effect and a longer fertilizer release period and has the advantages of low fertilizer nutrient loss, high crop utilization rate, environmental friendliness and the like; wide market application prospects are realized.

Description

Technical Field

[0001] The invention relates to a modified kaolin fertilizer coating liquid and a preparation method and application thereof, belonging to the technical field of fertilizer production. Background Art

[0002] Coated slow-release fertilizer is a new type of fertilizer that uses coating materials to regulate the release rate of nutrients. It has unique slow-release characteristics and can release nutrients smoothly over a long period of time. It can not only greatly improve the utilization efficiency of fertilizers and avoid excessive use of fertilizers and the resulting loss of fertility, but also effectively reduce the pollution of fertilizers to the environment. It has broad application prospects. The key to the slow-release effect of coated slow-release fertilizers lies in the performance of the coating materials. However, traditional coating materials mostly use polymers or natural polymers, which have limited slow-release effects and high costs. Therefore, the development of coating materials that are both cost-effective and have excellent performance has become the focus of current research.

[0003] In recent years, the introduction of inorganic materials to improve the performance of coated slow-release fertilizers has attracted widespread attention from researchers. Among them, kaolin, as a natural clay mineral, has a large specific surface area and good ion exchange capacity. It can effectively regulate the release rate of fertilizers and prolong the nutrient release time. It is an important raw material for coated slow-release fertilizers. Chinese patent CN100999427A discloses a coated controlled-release vegetable-specific compound fertilizer and a preparation method thereof. The coated controlled-release vegetable-specific compound fertilizer combines kaolin with polyvinyl alcohol / starch film to achieve the purpose of extending the fertilizer slow-release cycle and improving the utilization rate of fertilizers by crops. However, the preparation of the compound fertilizer involves the use of multiple raw materials, which is relatively costly, and the binding of kaolin to the membrane material is poor, which limits the application of the technical solution. Another Chinese patent CN101759498A discloses a degradable slow-release fertilizer, which uses modified urea-formaldehyde resin and kaolin as the main coating materials, has a simple preparation process, and has a significant slow-release effect on nitrogen. However, due to the insufficient modification effect of kaolin in the slow-release fertilizer, the slow-release fertilizer is still prone to reorganization, and the fertilizer particles are only formed by physical agglomeration, with poor mechanical properties. The membrane expansion after water absorption easily causes the particles to rupture, which limits its use effect.

[0004] In summary, due to the strong agglomeration of kaolin itself, the kaolin used in the existing kaolin-containing slow-release fertilizers is difficult to disperse evenly, thus affecting the actual performance of the fertilizer. Therefore, the prior art still lacks a kaolin fertilizer coating liquid and coated slow-release fertilizer with high kaolin dispersibility, excellent fertilizer nutrient slow-release effect, and long fertilizer release period. Summary of the invention

[0005] In view of the existing problems, the present invention proposes a modified kaolin fertilizer coating liquid and a preparation method and application thereof; the present invention uses a chemical expansion method to modify kaolin, and after kaolin is modified with dimethyl sulfoxide and hexadecyltrimethylammonium bromide, it is added to a polyvinyl alcohol / starch composite film liquid to obtain a modified kaolin fertilizer coating liquid; the fertilizer coating liquid is used as a coating material for biochar-based composite fertilizer and a coating machine is used to apply it to the preparation of coated slow-release fertilizer, the fertilizer nutrient slow-release effect and fertilizer release cycle of the prepared coated slow-release fertilizer are significantly improved, and it has the advantages of less nutrient loss, high crop utilization rate, environmental friendliness, etc., and has broad market application prospects.

[0006] The technical solution of the present invention is as follows:

[0007] A modified kaolin fertilizer coating liquid comprises modified kaolin, polyvinyl alcohol, corn starch, glycerol, potassium persulfate and water. By mass percentage, the polyvinyl alcohol comprises 2% to 4%, the corn starch comprises 3% to 5%, the glycerol comprises 1%, the potassium persulfate comprises 0.04% to 0.08% and the water comprises 89% to 95%. The addition amount of the modified kaolin is 1.0% to 1.4% of the total mass of the polyvinyl alcohol and the corn starch.

[0008] Furthermore, the modified kaolin is kaolin modified by dimethyl sulfoxide and hexadecyltrimethylammonium bromide.

[0009] Furthermore, the fertilizer coating liquid is coated on the fertilizer surface to form a film in a coating machine by spraying.

[0010] Furthermore, the kaolin is one or more of calcined kaolin, washed kaolin and metakaolin.

[0011] The present invention also provides a method for preparing the modified kaolin fertilizer coating liquid, comprising the following steps:

[0012] S1, mixing kaolin powder with a dimethyl sulfoxide solution and stirring, washing the powder with methanol; then mixing the powder with a hexadecyltrimethylammonium bromide solution and ultrasonicating, washing and drying the powder to obtain modified kaolin;

[0013] S2. Dissolve starch and polyvinyl alcohol in water and mix them respectively, add modified kaolin, glycerol and potassium persulfate solution to the mixed solution, and heat and stir to obtain the modified kaolin fertilizer coating solution.

[0014] Furthermore, in step S1, the volume ratio of dimethyl sulfoxide to water in the dimethyl sulfoxide solution is 7-9:1, and the mass ratio of cetyltrimethylammonium bromide to water in the cetyltrimethylammonium bromide solution is 1-3:4-6.

[0015] Furthermore, in step S1, the stirring speed is 300-500 rpm, the stirring time is 4-6 hours, and the ultrasonic time is 1-2 days.

[0016] Furthermore, the concentration of the potassium persulfate solution in step S2 is 0.1-1 mol / L.

[0017] Furthermore, in step S2, the temperature of heating and stirring is 85-95° C., the stirring time is 2-3 h, and the stirring speed is 300-500 rpm.

[0018] The present invention applies the modified kaolin fertilizer coating liquid to the preparation of coated slow-release fertilizer.

[0019] The present invention also provides a method for preparing a coated slow-release fertilizer, comprising the following steps:

[0020] (1) mixing compound fertilizer and biochar, crushing and granulating, and drying to obtain a biochar-based compound fertilizer core;

[0021] (2) using a coating machine to spray the modified kaolin fertilizer coating liquid onto the surface of the biochar-based compound fertilizer core, and then drying to form a film to obtain the coated slow-release fertilizer;

[0022] The biochar is derived from one or more of wheat straw, rice straw and corn straw; the compound fertilizer can be one of the general compound fertilizers with a total nutrient content of ≥45%, preferably N-P2O5-K2O 15-15-15 or N-P2O5-K2O 14-16-15.

[0023] Furthermore, the drying temperature in step (1) is 40-60°C; the temperature of the coating machine in step (2) is 40-60°C, and the rotation speed is 20-80 rpm.

[0024] Different from the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention provides a modified kaolin fertilizer coating liquid, which uses kaolin to modify polyvinyl alcohol / starch composite film liquid; the addition of kaolin significantly reduces the swelling of the generated polyvinyl alcohol / starch composite film, can effectively control water penetration, and the biodegradability of the composite film is also significantly enhanced; the slow-release fertilizer prepared using the modified kaolin fertilizer coating liquid not only has excellent soil water retention effect, but also can be quickly degraded, reducing environmental burden, and has broad agricultural application prospects. It shows good soil water retention in 30 days, with a water retention rate of 24.50% and a microbial degradability of 34.11%, which is a significant improvement compared with uncoated compound fertilizers.

[0026] 2. Different from the existing technology, the present invention uses a chemical expansion method to modify kaolin. After the kaolin is modified with dimethyl sulfoxide and hexadecyltrimethylammonium bromide, it has a more significant stratification effect, can be evenly and stably dispersed in the fertilizer coating liquid, and has good physical and chemical properties; the modified kaolin can also effectively regulate the structure of the polyvinyl alcohol / starch composite film through a cross-linking reaction, further improve the slow-release performance and mechanical properties of the film, thereby improving the nutrient utilization rate of the fertilizer, reducing nutrient loss, reducing the amount and frequency of fertilization, and regulating its nutrient release cycle to better match the growth needs of crops, thereby improving the absorption and utilization efficiency of nutrients by crops.

[0027] 3. Compared with the preparation process of traditional general-purpose fertilizers and coated fertilizers, the preparation method of the modified kaolin coated slow-release fertilizer provided by the present invention has the advantages of simple process, low cost, and significant slow-release effect; the results of the soil column leaching experiment show that the cumulative nitrogen, phosphorus, and potassium release rates of the modified kaolin coated slow-release fertilizer prepared by the present invention within 30 days are 19.11%, 48.35%, and 72.35%, respectively, and it has good slow-release performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the nutrient release curve of nitrogen, phosphorus and potassium of the modified kaolin coated biochar-based slow-release fertilizer in Example 4 in the soil column leaching test.

[0029] Figure 2 The swelling, degradability and water retention of the modified kaolin-coated biochar-based slow-release fertilizer of Example 4.

[0030] Figure 3 Key agronomic parameters of wheat under different fertilization treatments. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and preferred embodiments. The given embodiments are only for illustrating the present invention, rather than for limiting the scope of the present invention.

[0032] Unless otherwise specified, the materials, reagents, etc. used in the following examples can be obtained from commercial sources; the methods in the following examples are conventional methods unless otherwise specified.

[0033] Example 1

[0034] The present embodiment provides a modified kaolin fertilizer coating liquid, the fertilizer coating liquid comprising modified kaolin, polyvinyl alcohol, corn starch, glycerol, potassium persulfate and water, wherein, by mass percentage, polyvinyl alcohol 2% to 3%, corn starch 3% to 3.5%, glycerol 1%, potassium persulfate 0.04% to 0.05%, water 93% to 95%, and the addition amount of the modified kaolin is 1.0% of the total mass of the polyvinyl alcohol and corn starch;

[0035] Among them, the modified kaolin is kaolin modified by dimethyl sulfoxide and hexadecyltrimethylammonium bromide. The kaolin can be a mixture of one or more of calcined kaolin, washed kaolin, and metakaolin, and modified kaolin with similar properties can be obtained. In this embodiment, calcined kaolin is used for kaolin; the fertilizer coating liquid is coated on the fertilizer surface in a coating machine by spraying.

[0036] The preparation method of the modified kaolin fertilizer coating liquid comprises the following steps:

[0037] S1. Mix kaolin powder and dimethyl sulfoxide solution (V 二甲基亚砜 :V 水 =8:1) and stirred at 300 rpm for 6 h, and then the powder was washed with methanol; then the powder was mixed with a hexadecyltrimethylammonium bromide solution (prepared by 20 g of hexadecyltrimethylammonium bromide and 50 mL of water) and ultrasonicated for 1 d, and the powder was washed and dried to obtain modified kaolin;

[0038] S2. Dissolve 6 g of starch in 80-90° C. water by stirring, and dissolve 4 g of polyvinyl alcohol in 90-100° C. water by stirring, then mix the two solutions, add 1.0% of the total mass of polyvinyl alcohol and corn starch, 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution to the mixture, and heat and stir at 85° C. at 300 rpm for 2 h to obtain the modified kaolin fertilizer coating solution.

[0039] Example 2

[0040] The present embodiment provides a modified kaolin fertilizer coating liquid, which comprises modified kaolin, polyvinyl alcohol, corn starch, glycerol, potassium persulfate and water. In terms of mass percentage, the polyvinyl alcohol is 3% to 3.5%, the corn starch is 3.5% to 4%, the glycerol is 1%, the potassium persulfate is 0.05% to 0.06%, and the water is 91% to 93%. The amount of modified kaolin added is 1.2% of the total mass of the polyvinyl alcohol and the corn starch.

[0041] The preparation of modified kaolin is as described in Example 1. In this example, the kaolin is water-washed kaolin.

[0042] The preparation method of the modified kaolin fertilizer coating liquid comprises the following steps:

[0043] S1. Mix kaolin powder and dimethyl sulfoxide solution (V 二甲基亚砜 :V 水=9:1) and stirred at 400 rpm for 5 h, and then the powder was washed with methanol; then the powder was mixed with a hexadecyltrimethylammonium bromide solution (prepared from 10 g of hexadecyltrimethylammonium bromide and 60 mL of water) and ultrasonicated for 1.5 d, and the powder was washed and dried to obtain modified kaolin;

[0044] S2. Dissolve 6 g of starch in 80-90° C. water by stirring, and dissolve 4 g of polyvinyl alcohol in 90-100° C. water by stirring, then mix the two solutions, add 1.2% of the total mass of polyvinyl alcohol and corn starch, 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution to the mixture, and heat and stir at 90° C. at 400 rpm for 3 h to obtain the modified kaolin fertilizer coating solution.

[0045] Example 3

[0046] The present embodiment provides a modified kaolin fertilizer coating liquid, which comprises modified kaolin, polyvinyl alcohol, corn starch, glycerol, potassium persulfate and water. In terms of mass percentage, the polyvinyl alcohol is 3.5% to 4%, the corn starch is 4% to 5%, the glycerol is 1%, the potassium persulfate is 0.06% to 0.08%, and the water is 89% to 91%. The amount of modified kaolin added is 1.4% of the total mass of the polyvinyl alcohol and the corn starch.

[0047] The preparation of modified kaolin is as described in Example 1, and the kaolin in this example is metakaolin.

[0048] The preparation method of the modified kaolin fertilizer coating liquid comprises the following steps:

[0049] S1. Mix kaolin powder and dimethyl sulfoxide solution (V 二甲基亚砜 :V 水 =7:1) and stirred at 500 rpm for 4 h, and then the powder was washed with methanol; then the powder was mixed with a hexadecyltrimethylammonium bromide solution (prepared from 30 g of hexadecyltrimethylammonium bromide and 40 mL of water) and ultrasonicated for 2 d, and the powder was washed and dried to obtain modified kaolin;

[0050] S2. Dissolve 6 g of starch in 80-90° C. water by stirring, and dissolve 4 g of polyvinyl alcohol in 90-100° C. water by stirring, then mix the two solutions, add 1.4% of the total mass of polyvinyl alcohol and corn starch, 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution to the mixture, and heat and stir at 95° C. at 500 rpm for 2.5 h to obtain the modified kaolin fertilizer coating solution.

[0051] Example 4

[0052] In this example, the modified kaolin fertilizer coating liquid described in Example 1 is applied to the preparation of coated slow-release fertilizer, which includes the following steps:

[0053] S1. Compound fertilizer and biochar were mixed in a mass ratio of 2:5, crushed, and granulated using a granulator, and dried at 40° C. to obtain a biochar-based compound fertilizer core;

[0054] Wherein, the biochar is derived from one or more of wheat straw, rice straw, and corn straw. In this embodiment, the biochar is wheat straw; the general compound fertilizer can be one of the compound fertilizers with a total nutrient content of ≥45%, such as N-P2O5-K2O 15-15-15 and N-P2O5-K2O 14-16-15, and the obtained coated slow-release fertilizer has similar effects; in this embodiment, the biochar is wheat straw, and the general compound fertilizer is N-P2O5-K2O15-15-15;

[0055] S2. Use a coating machine to spray the modified kaolin fertilizer coating liquid onto the surface of the biochar-based compound fertilizer core, and then dry it into a film to obtain a coated slow-release fertilizer, which is a modified kaolin coated biochar-based slow-release fertilizer; the temperature of the coating machine is 40° C. and the rotation speed is 20 rpm.

[0056] Example 5

[0057] In this example, the modified kaolin fertilizer coating liquid described in Example 2 is applied to the preparation of coated slow-release fertilizer, which includes the following steps:

[0058] S1. Compound fertilizer and biochar are mixed in a mass ratio of 2:5, crushed, and granulated using a granulator, and dried at 50° C. to obtain a biochar-based compound fertilizer core; in this embodiment, the biochar is corn stalks, and the general compound fertilizer is N-P2O5-K2O14-16-15;

[0059] S2. Use a coating machine to spray the modified kaolin fertilizer coating liquid on the surface of the biochar-based compound fertilizer core, and then dry it into a film to obtain a coated slow-release fertilizer, which is a modified kaolin coated biochar-based slow-release fertilizer; the temperature of the coating machine is 50° C. and the rotation speed is 40 rpm.

[0060] Example 6

[0061] In this example, the modified kaolin fertilizer coating liquid described in Example 3 is applied to the preparation of coated slow-release fertilizer, which includes the following steps:

[0062] S1. Compound fertilizer and biochar are mixed in a mass ratio of 2:5, crushed, and granulated using a granulator, and dried at 60° C. to obtain a biochar-based compound fertilizer core; in this embodiment, the biochar is rice straw, and the general compound fertilizer is N-P2O5-K2O 15-15-15;

[0063] S2. Use a coating machine to spray the modified kaolin fertilizer coating liquid on the surface of the biochar-based compound fertilizer core, and then dry it into a film to obtain a coated slow-release fertilizer, which is a modified kaolin coated biochar-based slow-release fertilizer; the temperature of the coating machine is 60° C. and the rotation speed is 80 rpm.

[0064] Comparative Example 1

[0065] This comparative example provides a method for preparing a coated biochar-based slow-release fertilizer, the steps of which are as follows:

[0066] S1, 6 g of starch was stirred and dissolved in 80-90° C. water, 4 g of polyvinyl alcohol was stirred and dissolved in 90-100° C. water, the two solutions were then mixed, 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution were added to the mixture, and the mixture was heated and stirred at 85° C. for 2 h to obtain a polyvinyl alcohol / starch composite film liquid;

[0067] S2. Mix a general-purpose compound fertilizer and biochar in a mass ratio of 2:5 and then crush them, wherein the general-purpose compound fertilizer may be N-P2O5-K2O 15-15-15, and the biochar may be wheat straw, to obtain a biochar-compound fertilizer powder; granulate and dry the biochar-compound fertilizer powder to obtain a biochar-based compound fertilizer core, and use a coating machine to coat the fertilizer core with the prepared film liquid to obtain the coated biochar-based slow-release fertilizer; wherein the temperature of the coating machine is 40° C. and the rotation speed is 20 rpm.

[0068] Comparative Example 2

[0069] This comparative example provides a method for preparing a biochar-based compound fertilizer, and the steps are as follows:

[0070] The general compound fertilizer and biochar are mixed in a mass ratio of 2:5 and then crushed, the general compound fertilizer is N-P2O5-K2O15-15-15, and the biochar is wheat straw, to obtain biochar-compound fertilizer powder; the biochar-compound fertilizer powder is granulated and dried to obtain biochar-based compound fertilizer.

[0071] Comparative Example 3

[0072] This comparative example provides a universal compound fertilizer, which is N-P2O5-K2O15-15-15.

[0073] Performance Testing

[0074] (1) Field test setup

[0075] In a non-grain farmland in Lin'an District, Zhejiang Province, a 5m long and 4m wide experimental area was set up to test the effect of modified kaolin coated carbon-based slow-release fertilizer on wheat growth. The soil in non-grain farmland is poor and has low nutrient content, and has good feedback effects on different fertilization treatments.

[0076] (2) Performance evaluation of different fertilizers

[0077] The modified kaolin-coated biochar-based slow-release fertilizer described in Examples 4 to 6, the coated biochar-based slow-release fertilizer of Comparative Example 1, the biochar-based compound fertilizer of Comparative Example 2, and the universal chemical fertilizer of Comparative Example 3 were applied to the plots, respectively, with 3 parallels for each treatment. The amount of fertilizer applied was determined in accordance with the "Guiding Opinions on Scientific Fertilization of Wheat and Rape in Zhejiang Province in the Autumn and Winter of 2022", and the nitrogen conversion was performed; the fertilization time was the greening period of winter wheat. During the filling and maturity periods of wheat, wheat plants were collected in each treated plot, air-dried and threshed, and the number and weight of grains per ear and the thousand-grain weight of wheat were recorded. The final yield of wheat treated with different fertilization treatments was estimated based on these data. Tables 1 and 2 show the effects of different addition ratios of modified kaolin on agronomic parameters of wheat at different stages in Examples 4 to 6. Modified kaolin-coated biochar-based slow-release fertilizers with different modified kaolin addition ratios have higher wheat yields.

[0078] Table 1 Effects of different addition ratios of modified kaolin on agronomic parameters of wheat during grain filling period

[0079]

[0080] Table 2 Effects of different addition ratios of modified kaolin on agronomic parameters of wheat at maturity

[0081]

[0082] The results of the soil column leaching experiment of the modified kaolin coated biochar-based slow-release fertilizer of Example 4 are as follows: Figure 1 As shown, the results showed that the cumulative nitrogen, phosphorus and potassium release rates of the prepared coated fertilizer within 30 days were 19.11%, 48.35% and 72.35%, respectively, which were significantly lower than those of the control group fertilizer, indicating that its slow-release performance was significantly better than that of conventional fertilizers.

[0083] The swelling, degradability and water retention of the modified kaolin coated biochar-based slow-release fertilizer of Example 4 are as follows: Figure 2As shown, the modified kaolin-coated biochar-based slow-release fertilizer showed good soil water retention within a 30-day period, with a water retention rate of 24.50%, which is higher than that of general-purpose fertilizers and biochar-based compound fertilizers. The modified kaolin significantly reduced the swelling of the polyvinyl alcohol / starch composite film (86.11%) and effectively controlled water penetration. At the same time, the microbial degradability of the coated fertilizer in the soil was 34.11%, which was significantly higher than that of the composite film without modified kaolin. These results show that the slow-release fertilizer of the present invention not only has excellent soil water retention effect, but also can be quickly degraded to reduce environmental burden.

[0084] Comparison of key agronomic parameters of the modified kaolin coated biochar-based slow-release fertilizer of Example 4 and the fertilizers of Comparative Examples 1 to 3 Figure 3 The results show that the wheat yield of the modified kaolin coated carbon-based slow-release fertilizer of Example 4 and the coated carbon-based fertilizer without modified kaolin is significantly higher than that of the uncoated fertilizer, and the wheat yield is increased by 97% to 132%. The wheat yield of the modified kaolin coated carbon-based slow-release fertilizer is higher than that of the coated carbon-based fertilizer without modified kaolin, indicating that the addition of modified kaolin can effectively improve the nutrient slow-release effect of the slow-release fertilizer and improve the crop utilization rate.

[0085] In addition, from Figure 3 It can be seen that the number of grains per ear and the weight of wheat per ear in the treatment with modified kaolin coated biochar-based slow-release fertilizer are higher than those in the treatments with other chemical fertilizers, indicating that modified kaolin coated biochar-based slow-release fertilizer can better support the key reproductive stages of wheat, especially the flowering and filling stages, which are often limited by soil nutrient shortages. This shows that modified kaolin coated biochar-based slow-release fertilizer can supply nutrients more continuously, allowing plants to distribute nutrients more efficiently, thereby promoting the formation of more and fuller grains.

[0086] In summary, the modified kaolin coated carbon-based slow-release fertilizer of the present invention has significant leaching and slow-release effects, promotes crop growth and increases yield, reduces the amount and frequency of fertilizer application, improves the nutrient utilization efficiency of crops, and the nutrient release cycle is more in line with the crop growth cycle. It has the advantages of being cheap, environmentally friendly, and highly efficient, and has broad application prospects.

Claims

1. A modified kaolin fertilizer coating solution, characterized in that: The fertilizer coating liquid comprises modified kaolin, polyvinyl alcohol, corn starch, glycerol, potassium persulfate and water. In terms of mass percentage, the polyvinyl alcohol comprises 2% to 4%, the corn starch comprises 3% to 5%, the glycerol comprises 1%, the potassium persulfate comprises 0.04% to 0.08%, and the water comprises 89% to 95%. The amount of modified kaolin added is 1.0% to 1.4% of the total mass of the polyvinyl alcohol and the corn starch. Wherein, the modified kaolin is kaolin modified by dimethyl sulfoxide and hexadecyltrimethylammonium bromide.

2. The modified kaolin fertilizer coating liquid according to claim 1, characterized in that: The kaolin is one or more of calcined kaolin, washed kaolin and metakaolin.

3. A method for preparing the modified kaolin fertilizer coating solution as claimed in claim 1 or 2, characterized in that: The steps include: S1, mixing kaolin powder with a dimethyl sulfoxide solution and stirring, washing the powder with methanol; then mixing the powder with a hexadecyltrimethylammonium bromide solution and ultrasonicating, washing and drying the powder to obtain modified kaolin; S2. Dissolve starch and polyvinyl alcohol in water and mix them respectively, add modified kaolin, glycerol and potassium persulfate solution to the mixed solution, and heat and stir to obtain the modified kaolin fertilizer coating solution.

4. The method for preparing modified kaolin fertilizer coating liquid according to claim 3, characterized in that: In step S1, the volume ratio of dimethyl sulfoxide to water in the dimethyl sulfoxide solution is 7-9:1, and the mass ratio of cetyltrimethylammonium bromide to water in the cetyltrimethylammonium bromide solution is 1-3:4-6.

5. The method for preparing modified kaolin fertilizer coating liquid according to claim 3, characterized in that: In step S1, the stirring speed is 300-500 rpm, the stirring time is 4-6 hours, and the ultrasonic time is 1-2 days.

6. The method for preparing modified kaolin fertilizer coating solution according to claim 3, characterized in that: The concentration of the potassium persulfate solution in step S2 is 0.1-1 mol / L.

7. The method for preparing modified kaolin fertilizer coating liquid according to claim 3, characterized in that: The heating and stirring temperature in step S2 is 85-95° C., the stirring time is 2-3 h, and the stirring speed is 300-500 rpm.

8. Using the modified kaolin fertilizer coating liquid as claimed in any one of claims 1 or 2 in the preparation of coated slow-release fertilizer.

9. A method for preparing a coated slow-release fertilizer, characterized in that: The steps include: (1) mixing compound fertilizer and biochar, crushing and granulating, and drying to obtain a biochar-based compound fertilizer core; (2) using a coating machine to spray the modified kaolin fertilizer coating liquid as claimed in claim 1 or 2 onto the surface of the biochar-based compound fertilizer core, and then drying to form a film to obtain the coated slow-release fertilizer; Wherein, the biochar is one or more of wheat straw, rice straw, and corn straw, and the compound fertilizer is a general compound fertilizer with a total nutrient content of ≥45%.

10. The method for preparing the coated slow-release fertilizer according to claim 9, characterized in that: The drying temperature in step (1) is 40-60° C.; the temperature of the coating machine in step (2) is 40-60° C. and the rotation speed is 20-80 rpm.

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