Attapulgite modified lignin / epoxy resin-based double-layer controlled-release fertilizer and preparation method thereof

Abstract

The invention provides an attapulgite modified lignin / epoxy resin-based double-layer controlled release fertilizer and a preparation method thereof, and belongs to the technical field of fertilizer manufacturing. The preparation method comprises the following steps: mixing and dissolving a liquefying agent and polyol, adding lignin and an acid catalyst for reaction, then adding attapulgite for modification to obtain attapulgite-modified lignin-based polyol, spraying the attapulgite-modified lignin-based polyol, an isocyanate compound and a catalyst onto the surface of a fertilizer for in-situ polymerization reaction to obtain the modified lignin-based polyol. An inner-layer coated fertilizer is obtained; and mixing and dissolving epoxy resin and a wax water-blocking agent to obtain an outer-layer coated solution, spraying the outer-layer coated solution to the outer surface of the inner-layer coated fertilizer, and then spraying an amine compound to obtain the double-layer controlled-release fertilizer. The double-layer slow / controlled-release fertilizer breaks through the dependence of a traditional fertilizer on a non-degradable petroleum-based material, not only realizes the easy degradability of a membrane material and effectively avoids secondary pollution, but also remarkably reduces the production cost, and has extremely high economic benefits.

Description

Technical Field

[0001] The invention belongs to the field of fertilizer manufacturing, and in particular relates to an attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer and a preparation method thereof. Background Art

[0002] Modern agriculture has an increasing demand for fertilizers to support the growing global population's demand for food. However, the application of traditional fertilizers has problems such as low utilization rate and serious nutrient loss, which not only leads to economic waste, but also causes a series of environmental problems, such as eutrophication of water bodies, soil acidification, and greenhouse gas emissions. In order to meet these challenges, slow-release fertilizer technology came into being. It controls the release rate of nutrients so that plants can better absorb and utilize nutrients, thereby improving fertilizer utilization, reducing environmental pollution, and ultimately promoting sustainable agricultural development.

[0003] At present, the coating materials of slow-release and controlled-release fertilizers mainly rely on synthetic polymer materials, such as polyurethane, polyethylene and polypropylene. Although these materials have excellent controlled-release performance, their difficult-to-degrade properties lead to environmental problems such as soil residues and plastic pollution, which is contrary to the concept of sustainable agricultural development. Therefore, finding biodegradable and environmentally friendly coating materials has become a key challenge in the research field of slow-release and controlled-release fertilizers.

[0004] Lignin, as an abundant renewable resource, comes from plant cell walls and is one of the most abundant natural aromatic polymers on earth. It has the advantages of excellent biodegradability, low cost, rich active functional groups and good film-forming properties, making it an ideal candidate for the preparation of slow-release fertilizer coating materials. However, the mechanical properties and water barrier properties of single lignin materials are relatively poor, and it is difficult to meet the performance requirements of slow-release fertilizers for coating materials. In order to overcome these limitations, researchers began to explore the composite modification of lignin with other materials to improve its coating properties, hoping to develop a new type of efficient, environmentally friendly and low-cost slow-release fertilizer, which provides a feasible solution to solve the environmental problems caused by traditional chemical fertilizers and promote the sustainable development of agriculture. Summary of the invention

[0005] The purpose of the present invention is to provide a novel slow-release fertilizer which is efficient, environmentally friendly and low-cost.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer, comprising the following steps:

[0008] S1: mixing and dissolving a liquefier and a polyol to obtain a mixed solution, and then adding lignin and an acid catalyst to the mixed solution to react to obtain a liquefied lignin-based polyol;

[0009] S2: dispersing attapulgite in the liquefied lignin-based polyol described in S1 to obtain attapulgite-modified lignin-based polyol;

[0010] S3: sequentially spraying the attapulgite-modified lignin-based polyol, isocyanate compound and catalyst described in S2 onto the surface of the fertilizer for in-situ polymerization reaction to obtain an inner-layer coated fertilizer;

[0011] S4: mixing and dissolving the epoxy resin and the wax water-blocking agent to obtain an outer coating solution;

[0012] S5: spraying the outer coating solution of S4 onto the outer surface of the inner coating fertilizer of S3, and then spraying the amine compound, and after the reaction is completed, the attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer is obtained.

[0013] Preferably, the mass ratio of the liquefier to the polyol in S1 is 12-14:1-2;

[0014] The liquefier is a mixture of polyethylene glycol and polycaprolactone glycol, with a mass ratio of 8:4-6;

[0015] The dissolving speed is 300-400 rpm, the temperature is 120-180° C., and the time is 40-80 min.

[0016] Preferably, the polyethylene glycol is one or more of polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600 and polyethylene glycol 800;

[0017] The polycaprolactone diol is one or more of polycaprolactone diol 530, polycaprolactone diol 830 and polycaprolactone diol 2000;

[0018] The polyol is one or both of ethylene glycol and glycerol.

[0019] Preferably, the amount of lignin and acid catalyst in S1 is 50-200 g lignin and 0.1-2 g acid catalyst per 100 g mixed solution;

[0020] The lignin is one or more of alkali lignin, sulfate lignin and lignin sulfonate, and the particle size of the lignin is 60 to 120 meshes;

[0021] The acid catalyst is one or more of sulfuric acid and hydrochloric acid;

[0022] The reaction temperature is 120-180° C. and the reaction time is 0.5-1.5 h.

[0023] Preferably, the attapulgite in S2 accounts for 1 to 10 wt% of the total amount of the liquefied lignin-based polyol system;

[0024] The attapulgite is selected from one or more of high-quality attapulgite, opal attapulgite, dolomite attapulgite and montmorillonite attapulgite;

[0025] The temperature when dispersing the attapulgite in the S2 is 120-180° C. and the time is 30-60 min.

[0026] Preferably, the mass ratio of the modified lignin-based polyol to the isocyanate compound in S3 is 1-2:1-1.25;

[0027] The catalyst accounts for 0.1% of the total mass of the modified lignin-based polyol and the isocyanate compound.

[0028] Preferably, the isocyanate compound is selected from one or more of toluene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, methylcyclohexyl diisocyanate or tetramethylxylylene diisocyanate;

[0029] The catalyst is selected from one or more of dibutyltin dilaurate, stannous octoate, zinc naphthenate, zinc isooctanoate, bismuth carboxylate, bismuth isooctanoate, phenylmercuric propionate or phenylmercuric acetate;

[0030] The fertilizer is selected from one or more of urea, ammonium nitrate, ammonium sulfate, monoammonium phosphate, diammonium phosphate and potassium chloride.

[0031] Preferably, in S3, the fertilizer needs to be preheated to 75-85° C. before coating the inner coated fertilizer.

[0032] Preferably, the mass ratio of the epoxy resin to the wax water repellent in S4 is (3-10):1;

[0033] The mass ratio of the outer coating solution to the amine compound is (1-4):1;

[0034] The epoxy resin is selected from one or more of bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, E44 epoxy resin, acrylic resin or brominated epoxy resin;

[0035] The wax water-blocking agent is selected from one or more of paraffin wax, microcrystalline wax, sulfonated paraffin, chlorinated paraffin or polyethylene wax;

[0036] The amine compound is selected from one or more of triethylenetetramine, diethylenetriamine, metaphenylenediamine or polyamide.

[0037] The invention also provides a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0038] Beneficial Effects

[0039] (1) The raw material lignin of the present invention is low-cost, renewable and widely available, while the coating material is easily degradable in the natural environment and is environmentally friendly;

[0040] (2) The present invention uses attapulgite-modified lignin and epoxy resin as double-layer coating materials, which can more finely control the release rate of nutrients, further improve the utilization efficiency of fertilizers, and extend the effective action time of fertilizers, which can make up for the shortcomings of single-layer coated fertilizers that have a short slow release time and are difficult to control;

[0041] (3) The present invention utilizes attapulgite to increase the roughness of the membrane material, improve the surface energy, and improve the hydrophobicity of the membrane material; attapulgite is a natural nano-clay mineral with a unique rod-like structure, a large specific surface area, excellent adsorption and ion exchange properties. The composite of attapulgite and lignin can effectively improve the mechanical strength, water barrier properties and nutrient adsorption capacity of the lignin-based coating material, thereby improving the performance of the slow-release fertilizer.

[0042] (4) The preparation process of the present invention is simple, easy to operate, and low in cost. The nutrient requirements of different crops during their growth period can be met by adjusting the coating thickness. The prepared double-layer slow-release fertilizer has a good controlled-release effect, with an initial nutrient release rate of ≤2% and a cumulative release period of ≥28d.

[0043] The present invention uses lignin, a renewable resource, to develop a new type of environmentally friendly controlled-release fertilizer, breaking through the reliance of traditional fertilizers on non-degradable petroleum-based materials. It not only achieves the easy degradation of the film material and effectively avoids secondary pollution, but also significantly reduces production costs and has extremely high economic benefits. This fertilizer with excellent controlled-release performance, low cost and environmentally friendly characteristics conforms to the trend of sustainable development and provides a strong guarantee for the sustainable development of future agriculture. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is the cumulative nitrogen release of the double-layer slow-release fertilizer prepared in Examples 1 to 5. DETAILED DESCRIPTION

[0045] The methods are conventional methods unless otherwise specified. The materials are available from public commercial sources unless otherwise specified.

[0046] The high-quality attapulgite in the present invention has a attapulgite content of more than 85%, and the main impurities are micron-sized fragmentary quartz, feldspar, and a small amount of authigenic opal and dolomite.

[0047] The lignin used in the embodiments and comparative examples of the present invention comes from Shandong Xinglong Paper (Group) Co., Ltd., urea is purchased from Shaanxi Shanhua Coal Chemical Industry Group Co., Ltd., and polybenzyl polyisocyanate is purchased from Guangzhou Baichuan Chemical Industry Co., Ltd.

[0048] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0049] Example 1

[0050] S1: Polyethylene glycol 400, polycaprolactone diol 530 and propylene glycol were mixed in a mass ratio of 8:4:1 and placed in a three-necked flask with a stirrer and a condenser, heated to 160°C in an oil bath, and stirred at 350 rpm for 40 min to obtain a mixed solution; then, 70g of alkali lignin passed through a 100-mesh sieve and 2g of concentrated sulfuric acid with a mass fraction of 98% were added to 100g of the mixed solution, mixed evenly, and continued to react at 160°C and normal pressure (101.325 kPa) for 1h to obtain a liquefied lignin-based polyol;

[0051] S2: adding 3 wt% of high-quality attapulgite to the liquefied lignin-based polyol in S1, and continuing the reaction at 160° C. for 50 min until the mixture is uniformly mixed to obtain attapulgite-modified lignin-based polyol;

[0052] S3: Weigh 1000 g of large granular urea with an average particle size of 3 to 5 mm, place it in a drum coating machine and preheat it at 80° C. for 15 minutes to obtain a preheated fertilizer;

[0053] Weigh 9.62 g of the attapulgite-modified lignin-based polyol described in S2, 12.03 g of hexamethylene diisocyanate, and stannous octoate (0.1% of the total mass of the attapulgite-modified lignin-based polyol and the hexamethylene diisocyanate), spray them onto the preheated fertilizer surface in sequence, and perform in-situ polymerization at 80° C. for 30 min. After the reaction is completed, cool naturally to obtain an inner-layer coated fertilizer;

[0054] S4: E44 epoxy resin and paraffin are mixed in a mass ratio of 9:1 and heated to 90°C to completely dissolve to obtain an outer coating solution;

[0055] S5: Weigh 7.42g of the outer coating solution and spray it evenly on the surface of the inner coating fertilizer described in S3, and react it with 1.86g of triethylenetetramine at 80°C for 20min to obtain a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0056] Example 2

[0057] S1: Polyethylene glycol 400, polycaprolactone diol 830 and propylene glycol were mixed in a mass ratio of 8:6:1 and placed in a three-necked flask with a stirrer and a condenser, heated to 180°C in an oil bath, and stirred at 350 rpm for 50 min to obtain a mixed solution; then, 70g of alkali lignin passed through a 100-mesh sieve and 2g of concentrated sulfuric acid with a mass fraction of 98% were added to 100g of the mixed solution, mixed evenly, and continued to react at 180°C and normal pressure (101.325 kPa) for 1h to obtain a liquefied lignin-based polyol;

[0058] S2: adding 5 wt% of high-quality attapulgite to the liquefied lignin-based polyol described in S1, and continuing the reaction at 160° C. for 50 min until the mixture is uniformly mixed to obtain attapulgite-modified lignin-based polyol;

[0059] S3: Weigh 1000 g of large granular urea with an average particle size of 3 to 5 mm, place it in a drum coating machine and preheat it at 80° C. for 15 minutes to obtain a preheated fertilizer;

[0060] Weigh 14.81 g of the attapulgite-modified lignin-based polyol described in S2, 18.52 g of hexamethylene diisocyanate, and stannous octoate (accounting for 0.1% of the total mass of the attapulgite-modified lignin-based polyol and the hexamethylene diisocyanate), spray them onto the preheated fertilizer surface in sequence, and perform in-situ polymerization at 80° C. for 30 min. After the reaction is completed, cool naturally to obtain an inner-layer coated fertilizer;

[0061] S4: E44 epoxy resin and paraffin are mixed in a mass ratio of 9:1 and heated to 90°C to completely dissolve to obtain an outer coating solution;

[0062] S5: Weigh 6.67g of the outer coating solution and spray it evenly on the surface of the inner coating fertilizer in step S3, and react it with 1.67g of triethylenetetramine at 80°C for 20 minutes to obtain a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0063] Example 3

[0064] S1: Polyethylene glycol 200, polycaprolactone diol 530 and propylene glycol were mixed in a mass ratio of 8:4:2 and placed in a three-necked flask with a stirrer and a condenser, heated to 160°C in an oil bath, and stirred at 350 rpm for 60 min to obtain a mixed solution; then, 65g of alkali lignin passed through a 100-mesh sieve and 1.5g of concentrated sulfuric acid with a mass fraction of 98% were added to 100g of the mixed solution, mixed evenly, and continued to react at 160°C and normal pressure (101.325 kPa) for 1h to obtain a liquefied lignin-based polyol;

[0065] S2: adding 5 wt% of high-quality attapulgite to the liquefied lignin-based polyol described in S1, and continuing the reaction at 160° C. for 50 min until the mixture is uniformly mixed to obtain attapulgite-modified lignin-based polyol;

[0066] S3: Weigh 1000 g of large granular urea with an average particle size of 3 to 5 mm, place it in a drum coating machine and preheat it at 80° C. for 15 minutes to obtain a preheated fertilizer;

[0067] Weigh 14.81 g of the attapulgite-modified lignin-based polyol described in S2, 18.52 g of hexamethylene diisocyanate, and stannous octoate (accounting for 0.1% of the total mass of the attapulgite-modified lignin-based polyol and the hexamethylene diisocyanate), spray them onto the preheated fertilizer surface in sequence, and perform in-situ polymerization at 80° C. for 30 min. After the reaction is completed, cool naturally to obtain an inner-layer coated fertilizer;

[0068] S4: E44 epoxy resin and paraffin are mixed in a mass ratio of 9:1 and heated to 90°C to completely dissolve to obtain an outer coating solution;

[0069] S5: Weigh 6.67g of the outer coating solution and spray it evenly on the surface of the inner coating fertilizer in step S3, and react it with 1.67g of triethylenetetramine at 80°C for 20 minutes to obtain a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0070] Example 4

[0071] S1: Polyethylene glycol 600, polycaprolactone diol 830 and propylene glycol were mixed in a mass ratio of 8:4:1 and placed in a three-necked flask with a stirrer and a condenser, heated to 160°C in an oil bath, and stirred at 350 rpm for 70 min to obtain a mixed solution; then, 65g of alkali lignin passed through a 100-mesh sieve and 2g of concentrated sulfuric acid with a mass fraction of 98% were added to 100g of the mixed solution, mixed evenly, and continued to react at 160°C and normal pressure (101.325 kPa) for 1h to obtain a liquefied lignin-based polyol;

[0072] S2: adding 3 wt% of high-quality attapulgite to the liquefied lignin-based polyol in S1, and continuing the reaction at 160° C. for 50 min until the mixture is uniformly mixed to obtain attapulgite-modified lignin-based polyol;

[0073] S3: Weigh 1000 g of large granular urea with an average particle size of 3 to 5 mm, place it in a drum coating machine and preheat it at 80° C. for 15 minutes to obtain a preheated fertilizer;

[0074] Weigh 18.71 g of the attapulgite-modified lignin-based polyol described in S2, 23.39 g of hexamethylene diisocyanate, and stannous octoate (accounting for 0.1% of the total mass of the attapulgite-modified lignin-based polyol and the hexamethylene diisocyanate), spray them onto the preheated fertilizer surface in sequence, and perform in-situ polymerization at 80° C. for 30 min. After the reaction is completed, cool naturally to obtain an inner-layer coated fertilizer;

[0075] S4: E44 epoxy resin and paraffin are mixed in a mass ratio of 9:1 and heated to 90°C to completely dissolve to obtain an outer coating solution;

[0076] S5: Weigh 8.42g of the outer coating solution and spray it evenly on the surface of the inner coating fertilizer described in S3, and react it with 2.11g of triethylenetetramine at 80°C for 20min to obtain a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0077] Example 5

[0078] S1: Polyethylene glycol 400, polycaprolactone diol 2000 and propylene glycol were mixed in a mass ratio of 8:6:2 and placed in a three-necked flask with a stirrer and a condenser, heated to 180°C in an oil bath, and stirred at 350 rpm for 80 min to obtain a mixed solution; then, 70g of alkali lignin passed through a 100-mesh sieve and 1.5g of concentrated sulfuric acid with a mass fraction of 98% were added to 100g of the mixed solution, mixed evenly, and reacted at 180°C and normal pressure (101.325 kPa) for 1h to obtain a liquefied lignin-based polyol;

[0079] S2: adding 5 wt% of high-quality attapulgite to the liquefied lignin-based polyol described in S1, and continuing the reaction at 160° C. for 50 min until the mixture is uniformly mixed to obtain attapulgite-modified lignin-based polyol;

[0080] S3: Weigh 1000 g of large granular urea with an average particle size of 3 to 5 mm, place it in a drum coating machine and preheat it at 80° C. for 15 minutes to obtain a preheated fertilizer;

[0081] Weigh 18.71 g of the attapulgite-modified lignin-based polyol described in S2, 23.39 g of hexamethylene diisocyanate, and stannous octoate (accounting for 0.1% of the total mass of the attapulgite-modified lignin-based polyol and the hexamethylene diisocyanate), spray them onto the preheated fertilizer surface in sequence, and perform in-situ polymerization at 80° C. for 30 min. After the reaction is completed, cool naturally to obtain an inner-layer coated fertilizer;

[0082] S4: E44 epoxy resin and paraffin are mixed in a mass ratio of 9:1 and heated to 90°C to completely dissolve to obtain an outer coating solution;

[0083] S5: Weigh 8.42g of the outer coating solution and spray it evenly on the surface of the inner coating fertilizer in step S3, and react it with 2.11g of triethylenetetramine at 80°C for 20min to obtain a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer.

[0084] Test example

[0085] The obtained double-layer slow-release fertilizer was tested for controlled-release performance according to the national standard GB / T 23348-2009. The steps are as follows: weigh 10g of double-layer slow-release fertilizer and place it in a 100-mesh nylon mesh bag. After sealing, put the mesh bag containing the double-layer slow-release fertilizer into a 300mL plastic bottle, add 250mL of deionized water, cover and seal, and place it in a biochemical incubator at 25°C for culture. Take samples at regular intervals (sampling time is 24h, 3d, 5d, 7d, 10d, 13d, 16d, 19d, 22d, 25d, 28d, 35d, 42d, 49d, 56d, 63d...). When sampling, turn the plastic bottle upside down three times to ensure that the concentration of the liquid in the bottle is consistent. Take 50mL of the extract and cool it down for determination of total nitrogen. The coating thickness and controlled release performance of different fertilizers are shown in Table 1. The coating thickness is expressed as the percentage of coating material to the core fertilizer. The date corresponding to 80% cumulative nutrient release is defined as the cumulative nutrient release period of the fertilizer. Figure 1 shown.

[0086] Table 1 Thickness of different fertilizer coatings and controlled release performance

[0087] Envelope thickness Initial dissolution rate of nitrogen Controlled release period Example 1 3％ 1.55％ 42d Example 2 4％ 1.01％ 49d Example 3 4％ 0.98％ 56d Example 4 5％ 0.79％ 56d Example 5 5％ 0.75％ 63d

[0088] Depend on Figure 1 As shown in Table 1, the prepared double-layer slow-release fertilizer has a good controlled-release effect, with an initial nutrient dissolution rate of ≤2% and a cumulative release period of ≥28 days.

[0089] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer, characterized in that: The following steps are involved: S1: mixing and dissolving a liquefier and a polyol to obtain a mixed solution, and then adding lignin and an acid catalyst to the mixed solution to react to obtain a liquefied lignin-based polyol; S2: dispersing attapulgite in the liquefied lignin-based polyol described in S1 to obtain attapulgite-modified lignin-based polyol; S3: sequentially spraying the attapulgite-modified lignin-based polyol, isocyanate compound and catalyst described in S2 onto the surface of the fertilizer for in-situ polymerization reaction to obtain an inner-layer coated fertilizer; S4: mixing and dissolving the epoxy resin and the wax water-blocking agent to obtain an outer coating solution; S5: spraying the outer coating solution of S4 onto the outer surface of the inner coating fertilizer of S3, and then spraying the amine compound, and after the reaction is completed, the attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer is obtained.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the liquefier to the polyol in S1 is 12-14:1-2; The liquefier is a mixture of polyethylene glycol and polycaprolactone glycol, with a mass ratio of 8:4-6; The dissolving speed is 300-400 rpm, the temperature is 120-180° C., and the time is 40-80 min.

3. The preparation method according to claim 2, characterized in that: The polyethylene glycol is one or more of polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600 and polyethylene glycol 800; The polycaprolactone diol is one or more of polycaprolactone diol 530, polycaprolactone diol 830 and polycaprolactone diol 2000; The polyol is one or both of ethylene glycol and glycerol.

4. The preparation method according to claim 3, characterized in that: The amount of lignin and acid catalyst in S1 is 50-200g lignin and 0.1-2g acid catalyst per 100g mixed solution; The lignin is one or more of alkali lignin, sulfate lignin and lignin sulfonate, and the particle size of the lignin is 60 to 120 meshes; The acid catalyst is one or more of sulfuric acid and hydrochloric acid; The reaction temperature is 120-180° C. and the reaction time is 0.5-1.5 h.

5. The preparation method according to claim 4, characterized in that: The attapulgite in S2 accounts for 1 to 10 wt% of the total amount of the liquefied lignin-based polyol system; The attapulgite is selected from one or more of high-quality attapulgite, opal attapulgite, dolomite attapulgite and montmorillonite attapulgite; The temperature when dispersing the attapulgite in the S2 is 120-180° C. and the time is 30-60 min.

6. The preparation method according to claim 5, characterized in that: The mass ratio of the modified lignin-based polyol to the isocyanate compound in S3 is 1-2:1-1.25; The catalyst accounts for 0.1% of the total mass of the modified lignin-based polyol and the isocyanate compound.

7. The preparation method according to claim 6, characterized in that: The isocyanate compound is selected from one or more of toluene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, methylcyclohexyl diisocyanate or tetramethylxylylene diisocyanate; The catalyst is selected from one or more of dibutyltin dilaurate, stannous octoate, zinc naphthenate, zinc isooctanoate, bismuth carboxylate, bismuth isooctanoate, phenylmercuric propionate or phenylmercuric acetate; The fertilizer is selected from one or more of urea, ammonium nitrate, ammonium sulfate, monoammonium phosphate, diammonium phosphate and potassium chloride.

8. The preparation method according to claim 7, characterized in that: In S3, the fertilizer needs to be preheated to 75-85° C. before coating the inner coated fertilizer.

9. The preparation method according to claim 8, characterized in that: The mass ratio of the epoxy resin to the wax water-blocking agent in S4 is (3-10):1; The mass ratio of the outer coating solution to the amine compound is (1-4):1; The epoxy resin is selected from one or more of bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, E44 epoxy resin, acrylic resin or brominated epoxy resin; The wax water-blocking agent is selected from one or more of paraffin wax, microcrystalline wax, sulfonated paraffin, chlorinated paraffin or polyethylene wax; The amine compound is selected from one or more of triethylenetetramine, diethylenetriamine, metaphenylenediamine or polyamide.

10. A attapulgite-modified lignin / epoxy resin-based double-layer slow-release fertilizer prepared by the preparation method according to any one of claims 1 to 9.

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