A moisture-absorbing and swelling composite material, its preparation method and application

Abstract

The present invention relates to the field of soil improvement, and particularly to a hygroscopic expansion composite material, a preparation method thereof and an application; the composite material comprises the following raw material components in parts by weight: 65-70 parts of chemical fertilizer, 7-10 parts of sodium alginate, 20-25 parts of calcium chloride solution, and 1-2 parts of sodium polyacrylate powder; the high-molecular material sodium alginate is used to improve the problem of soil compaction in saline-alkali land. After being applied to the soil, sodium alginate swells when encountering water, and can maintain the soil bulk density and structure, solving the problem that traditional chemical fertilizers have almost no expansibility when encountering water, dissolve in a short time, and the ions in the fertilizer act on soil particles, resulting in the destruction of aggregates and soil compaction when the soil loses water; the stability and toughness of the gel coating formed by crosslinking calcium chloride solution and sodium polyacrylate are used for modification, improving the physical properties such as water absorption, expansibility and toughness of the composite material, enhancing the strength of the composite material, and effectively coping with the harsh environment of saline-alkali land.

Description

Technical Field

[0001] The present invention relates to the technical field of soil improvement, and particularly relates to a moisture-absorbing and swelling composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Polymer hydrogel materials have excellent water absorption and water retention properties and are widely used in the fields of hygiene, medicine, etc. With the continuous development of modern agricultural technologies, hydrogel materials show good prospects in soil improvement. Mixing improvement materials with hydrogels can modify the hydrogel materials, which can be used as a new agricultural technology for soil improvement. However, for spherical hydrogel materials, the effective substances contained therein are less, and the hydrogel spheres themselves are fragile, with a short action time, and will hydrolyze relatively quickly especially under the soil conditions of saline-alkali land with high salt content and high pH.

[0003] Saline-alkali land has a high salt content, which easily leads to soil compaction, poor permeability, and difficulty in effectively infiltrating and retaining water, thereby affecting the normal growth of crops. Applying traditional chemical fertilizers will aggravate the compaction of saline-alkali land, cause deterioration of the soil structure, and lead to setbacks in the improvement of saline-alkali land.

[0004] How to apply hydrogel materials to the comprehensive improvement of saline-alkali soil has important strategic significance for ensuring national food security. Summary of the Invention

[0005] In view of the deficiencies in the above-mentioned prior art, the present invention provides a moisture-absorbing and swelling composite material, a preparation method thereof, and an application thereof, which can improve the moisture condition and permeability of soil and improve the fertilizer utilization rate.

[0006] The present invention provides a moisture-absorbing and swelling composite material, comprising the following raw material components in parts by weight:

[0007] Chemical fertilizer 65 - 70 parts, sodium alginate 7 - 10 parts, calcium chloride solution 20 - 25 parts, sodium polyacrylate powder 1 - 2 parts.

[0008] In some embodiments of the present application, the mass percentage concentration of the calcium chloride solution is 0.1% - 0.5%.

[0009] As another aspect of the present application, there is also provided a preparation method of the above-mentioned moisture-absorbing and swelling composite material, which specifically comprises the following steps:

[0010] S1. Granulation:

[0011] Mix and grind the chemical fertilizer and sodium alginate in proportion to obtain a compound mixture. Put the compound mixture into a granulator, and spray the calcium chloride solution to control the humidity of the compound mixture within a certain range. Start the granulator for granulation to obtain compound granules;

[0012] S2. Drying:

[0013] Dry the compound granules at 55 - 60 °C for 12 - 24 h;

[0014] S3. Forming a viscous adsorption gel:

[0015] Spray the calcium chloride solution onto the dried compound granules while stirring, so that the calcium chloride solution is evenly coated on the surface of the compound granules to form a viscous adsorption gel;

[0016] S4. Coating:

[0017] Put sodium polyacrylate powder into the compound granules forming the viscous adsorption gel in step S3 and stir, so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel, and obtain coated compound granules;

[0018] S5. Secondary drying and shaping:

[0019] Dry the coated compound granules at 55 - 60 °C for 12 - 24 h to dry the coating gel and make it shaped into a solid coating;

[0020] S6. Repeat steps S3 - S5 until the thickness of the coating on the surface of the coated compound granules reaches the requirement to obtain the composite material.

[0021] In some embodiments of the present application, in step S1, when the fineness of the compound material is 60 - 80 mesh, stop grinding and put the compound material into a granulator.

[0022] In some embodiments of the present application, in step S1, the humidity of the compound material is controlled at 15 - 20%, and too low or too high humidity will affect the granulation effect.

[0023] In some embodiments of the present application, in step S3, the viscous adsorption gel is formed by the reaction of the calcium chloride solution with sodium alginate on the surface of the compound granules, and its reaction formula is as follows:

[0024] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0025] In some embodiments of the present application, in step S3, the spraying amount of the calcium chloride solution does not exceed 5% - 7% of the weight of the compound granules.

[0026] In some embodiments of the present application, in step S4, the fineness of the sodium polyacrylate powder is between 80 and 100 meshes.

[0027] In some embodiments of the present application, in step S4, the feeding speed of the sodium polyacrylate is 5%-10% / min of the total feeding mass of the sodium polyacrylate powder, and the stirring speed is 10-15 r / min.

[0028] As another aspect of the present application, the water-absorbing and swelling composite material prepared by the above preparation method is applied to saline-alkali soil improvement. The surface coating forms a semi-solid gel state by solid-state water absorption and swelling. The nutrient elements in the chemical fertilizer are slowly released into the soil through the gel, and a microenvironment with a higher water content is formed in the soil. After the soil loses water, the coating also loses water and returns to the solid state, and no longer releases the nutrient elements in the chemical fertilizer; after the soil is re-watered again, the coating absorbs water and swells into a gel state again, and slowly releases the nutrient elements again. The above process is repeated multiple times until all the nutrient elements in the chemical fertilizer are released, and the coating gradually decomposes during the process of water absorption and water loss.

[0029] Based on the above technical solutions, the present invention uses the moisture-absorbing and swelling polymer material sodium alginate to improve the problem of soil compaction. After sodium alginate is applied to the soil, it swells when it encounters water, and can maintain the soil bulk density and structure, solving the problem that traditional chemical fertilizers have almost no expansibility when they encounter water, dissolve in a short time, the ions in the fertilizer interact with soil particles, and the aggregates are damaged when the soil loses water, resulting in soil compaction; using chemical fertilizers in saline-alkali soil overcomes the technical prejudice that traditional chemical fertilizers cannot be used in saline-alkali soil and will aggravate soil compaction;

[0030] Aiming at the problem that the gel strength of sodium alginate in the compound fertilizer is not high and it is easy to break and hydrolyze in high-salt and high-pH environments, by spraying calcium chloride solution, the gel coating with stronger stability and toughness formed by cross-linking calcium chloride solution and sodium polyacrylate is used for modification to improve the water absorption, expansibility and toughness and other physical properties of the composite material, and improve the strength of the composite material. By coating multiple times, a composite material with a specific coating thickness can be formulated, which can effectively cope with the harsh environment of saline-alkali soil;

[0031] After the composite material absorbs water and swells, its state is stable, and its volume expands by more than 5 times. Adding 10% of the composite material to the surface compacted soil can increase the soil water holding capacity by 2-5% and reduce the soil bulk density by 20-30% when the soil moisture is sufficient, which can significantly improve the problem of soil compaction in saline-alkali soil; at the same time, as the coating slowly decomposes, the nutrients and calcium ions in the chemical fertilizer are slowly released and exchanged with the sodium ions in the soil, which is beneficial to the desalination of saline-alkali soil. Description of the Drawings

[0032] The accompanying drawings described herein are used to provide a further understanding of the present invention and form a part of this application. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0033] Figure 1 is the swelling rate of the composite materials prepared in Examples 1-5 of the present invention;

[0034] Figure 2 is a comparison chart of the water absorption and swelling of the composite material particles of Comparative Example 1, Comparative Example 2, and Example 6;

[0035] Figure 3 is a schematic diagram of the change in the diameter of the composite material particles of Comparative Example 1, Comparative Example 2, and Example 6 after water absorption. Detailed implementation manners

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] Example 1

[0038] S1. Granulation:

[0039] Mix 65 parts of commercially available ordinary chemical fertilizer and 7 parts of sodium alginate, grind them until the particle fineness is 60-80 mesh to obtain a compound mixture. Put the compound mixture into a disk granulator and spray 20 parts of a calcium chloride solution with a mass percentage concentration of 0.5%. Use a soil moisture sensor to measure the humidity of the compound mixture, and control the humidity of the compound mixture powder at about 15-20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3-4 mm, the disk granulator stops to obtain compound granules;

[0040] S2. Drying:

[0041] Start the hot air drying function of the disk granulator, set the drying temperature at 55 °C, and dry for 12 h;

[0042] S3. Form a viscous adsorption gel:

[0043] Load 5 parts of a calcium chloride solution with a mass percentage concentration of 0.5% into a spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator for stirring, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with the sodium alginate on the surface as follows:

[0044] 2C 17 H35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0045] Form a viscous adsorption gel. In this step, the spraying amount of calcium chloride solution does not exceed 6% of the mass of the compound fertilizer particles.

[0046] S4. Coating:

[0047] Start the stirring function of the disk granulator, add 1 part of sodium polyacrylate powder with a fineness of 80 - 100 meshes to the material feeding device. The addition amount is about 1.1% of the total mass of the compound fertilizer particles obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (5%×1 part) / min, and stir at a speed of 10 r / min to make the sodium polyacrylate powder evenly adsorbed on the surface of the compound fertilizer particles, forming a coating gel to obtain coated compound fertilizer particles.

[0048] S5. Secondary drying and shaping:

[0049] Start the hot air drying function of the disk granulator, and dry the above-mentioned coated compound fertilizer particles at 55 °C for 12 h to dry the coating gel and make it shaped into a solid coating.

[0050] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating on the surface of the coated compound fertilizer particles reaches 1 - 2 mm to obtain the composite material.

[0051] Example 2

[0052] S1. Granulation:

[0053] Mix 65 parts of commercially available ordinary chemical fertilizer and 7 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 meshes to obtain a compound fertilizer mixture. Put the compound fertilizer mixture into a disk granulator, and spray 20 parts of calcium chloride solution with a mass percentage concentration of 0.1%. Use a soil moisture sensor to measure the humidity of the compound fertilizer mixture, and control the humidity of the compound fertilizer powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3 - 4 mm, stop the disk granulator to obtain compound fertilizer particles.

[0054] S2. Drying:

[0055] Start the hot air drying function of the disk granulator, and set the drying temperature at 55 °C for 12 h.

[0056] S3. Form a viscous adsorption gel:

[0057] Put 5 parts of calcium chloride solution with a mass percentage concentration of 0.1% into the spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with sodium alginate on the surface as follows:

[0058] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0059] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 5% of the mass of the compound granules;

[0060] S4. Coating:

[0061] Start the stirring function of the disk granulator, add 1 part of sodium polyacrylate powder with a fineness of 80-100 mesh to the material feeding device. The addition amount is about 1.1% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (5%×1 part) / min and stir at a speed of 10 r / min, so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel, and obtain coated compound granules;

[0062] S5. Secondary drying and shaping:

[0063] Start the hot air drying function of the disk granulator, dry the above-mentioned coated compound granules at 55°C for 12 h, dry the coating gel to make it shaped into a solid coating;

[0064] S6. Repeat steps S3-S5 for 2-3 times until the thickness of the coating on the surface of the coated compound granules reaches 1-2 mm to obtain the composite material.

[0065] Example 3

[0066] S1. Granulation:

[0067] Mix 65 parts of commercially available ordinary chemical fertilizer and 7 parts of sodium alginate, grind them until the particle fineness is 60-80 mesh to obtain a compound material. Put the compound material into a disk granulator and spray 20 parts of calcium chloride solution with a mass percentage concentration of 0.3%. Use a soil moisture sensor to measure the humidity of the compound material powder, and control the humidity of the compound material powder at about 15-20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3-4 mm, the disk granulator stops to obtain compound granules;

[0068] S2. Drying:

[0069] Start the hot air drying function of the disk granulator, set the drying temperature at 55 °C, and dry for 12 h;

[0070] S3. Form a viscous adsorption gel:

[0071] Put 5 parts of calcium chloride solution with a mass percentage concentration of 0.3% into the spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with sodium alginate on the surface as follows:

[0072] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0073] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 6% of the mass of the compound granules;

[0074] S4. Coating:

[0075] Start the stirring function of the disk granulator, add 1 part of sodium polyacrylate powder with a fineness of 80-100 meshes to the material feeding device, and the addition amount is about 1.1% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (5%×1 part) / min and stir at a speed of 10 r / min, so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel, and obtain coated compound granules;

[0076] S5. Secondary drying and shaping:

[0077] Start the hot air drying function of the disk granulator, dry the above-mentioned coated compound granules at 55 °C for 12 h to dry the coating gel and make it shaped into a solid coating;

[0078] S6. Repeat steps S3-S5 for 2-3 times until the thickness of the coating on the surface of the coated compound granules reaches 1-2 mm to obtain the composite material.

[0079] Example 4

[0080] S1. Granulation:

[0081] Mix 65 parts of commercially available ordinary chemical fertilizer and 7 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 mesh to obtain a compound mixture. Put the compound mixture into a disc granulator and spray 20 parts of calcium chloride solution with a mass percentage concentration of 1%. Use a soil moisture sensor to measure the humidity of the compound mixture powder, and control the humidity of the compound mixture powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disc granulator for granulation. When the granulation diameter reaches 3 - 4 mm, stop the disc granulator to obtain compound granules;

[0082] S2. Drying:

[0083] Start the hot air drying function of the disc granulator, set the drying temperature at 55 °C, and dry for 12 h;

[0084] S3. Form a viscous adsorption gel:

[0085] Load 5 parts of calcium chloride solution with a mass percentage concentration of 1% into the spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disc granulator for stirring, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with the sodium alginate on the surface as follows:

[0086] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0087] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 6% of the mass of the compound granules;

[0088] S4. Coating:

[0089] Start the stirring function of the disc granulator, add 1 part of sodium polyacrylate powder with a fineness of 80 - 100 mesh to the material feeding device. The addition amount is about 1.1% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disc granulator at a feeding speed of (5%×1 part) / min and stir at a speed of 10 r / min, so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel and obtain coated compound granules;

[0090] S5. Secondary drying and shaping:

[0091] Start the hot air drying function of the disc granulator, dry the above-mentioned coated compound granules at 55 °C for 12 h to dry the coating gel and make it shaped into a solid coating;

[0092] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating on the surface of the coated compound granules reaches 1 - 2 mm to obtain the composite material.

[0093] Example 5

[0094] S1. Granulation:

[0095] Mix 65 parts of commercially available ordinary chemical fertilizer and 7 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 mesh to obtain the compound mixture. Put the compound mixture into a disk granulator and spray 20 parts of a calcium chloride solution with a mass percentage concentration of 3%. Use a soil moisture sensor to measure the humidity of the compound mixture and control the humidity of the compound mixture powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3 - 4 mm, stop the disk granulator to obtain compound granules.

[0096] S2. Drying:

[0097] Start the hot air drying function of the disk granulator, set the drying temperature at 55 °C, and dry for 12 h.

[0098] S3. Form a viscous adsorption gel:

[0099] Load 5 parts of a calcium chloride solution with a mass percentage concentration of 3% into a spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with the sodium alginate on the surface as follows:

[0100] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0101] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 6% of the mass of the compound granules.

[0102] S4. Coating:

[0103] Start the stirring function of the disk granulator, add 1 part of sodium polyacrylate powder with a fineness of 80 - 100 mesh to the material feeding device. The addition amount is about 1.1% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (5%×1 part) / min and stir at a speed of 10 r / min so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel and obtain coated compound granules.

[0104] S5. Secondary drying and shaping:

[0105] Start the hot air drying function of the disk granulator, and dry the above-coated compound fertilizer granules at 55 °C for 12 hours to dry the coated gel and make it shaped into a solid coating film.

[0106] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating film on the surface of the coated compound fertilizer granules reaches 1 - 2 mm to obtain the composite material.

[0107] Example 6

[0108] S1. Granulation:

[0109] Mix 67 parts of commercially available ordinary chemical fertilizer and 8 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 mesh to obtain a compound mixture. Put the compound mixture into a disk granulator, and spray 17 parts of calcium chloride solution with a mass percentage concentration of 0.5%. Use a soil moisture sensor to measure the humidity of the compound mixture, and control the humidity of the compound mixture powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3 - 4 mm, stop the disk granulator to obtain compound fertilizer granules.

[0110] S2. Drying:

[0111] Start the hot air drying function of the disk granulator, and set the drying temperature at 60 °C for 24 hours.

[0112] S3. Forming a viscous adsorption gel:

[0113] Load 6 parts of calcium chloride solution with a mass percentage concentration of 0.5% into the spraying device, spray it on the dried compound fertilizer granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound fertilizer granules and reacts with the sodium alginate on the surface as follows:

[0114] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0115] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 7% of the mass of the compound fertilizer granules.

[0116] S4. Coating:

[0117] Start the stirring function of the disk granulator, add 1.4 parts of sodium polyacrylate powder with a fineness of 80 - 100 mesh to the material feeding device, and the addition amount is about 1.5% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (10% × 1.4 parts) / min, and stir at a speed of 15 r / min to make the sodium polyacrylate powder evenly adsorb on the surface of the compound granules, forming a coating gel to obtain coated compound granules;

[0118] S5. Secondary drying and shaping:

[0119] Start the hot air drying function of the disk granulator, dry the above-mentioned coated compound granules at 60 °C for 24 h to dry the coating gel and make it shaped into a solid coating;

[0120] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating on the surface of the coated compound granules reaches 1 - 2 mm to obtain the composite material.

[0121] Example 7

[0122] S1. Granulation:

[0123] Mix 68 parts of commercially available ordinary chemical fertilizer and 9 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 mesh to obtain a compound material. Put the compound material into a disk granulator, and spray 16 parts of calcium chloride solution with a mass percentage concentration of 0.4%. Use a soil moisture sensor to measure the humidity of the compound material powder, and control the humidity of the compound material powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3 - 4 mm, the disk granulator stops to obtain compound granules;

[0124] S2. Drying:

[0125] Start the hot air drying function of the disk granulator, set the drying temperature at 58 °C and dry for 15 h;

[0126] S3. Forming a viscous adsorption gel:

[0127] Load 4 parts of calcium chloride solution with a mass percentage concentration of 0.4% into the spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with the sodium alginate on the surface as follows:

[0128] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H35 Ca(OO)2

[0129] Form a viscous adsorption gel. In this step, the spraying amount of calcium chloride solution does not exceed 5% of the mass of the compound fertilizer granules.

[0130] S4. Coating:

[0131] Start the stirring function of the disk granulator. Add 1.4 parts of sodium polyacrylate powder with a fineness of 80 - 100 meshes to the material feeding device. The addition amount is about 1.5% of the total mass of the compound fertilizer granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (6% × 1.4 parts) / min and stir at a speed of 15 r / min to make the sodium polyacrylate powder evenly adsorbed on the surface of the compound fertilizer granules, forming a coating gel to obtain coated compound fertilizer granules.

[0132] S5. Secondary drying and shaping:

[0133] Start the hot air drying function of the disk granulator. Dry the above-mentioned coated compound fertilizer granules at 58 °C for 15 h to dry the coating gel and make it shaped into a solid coating.

[0134] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating on the surface of the coated compound fertilizer granules reaches 1 - 2 mm to obtain the composite material.

[0135] Example 8

[0136] S1. Granulation:

[0137] Mix 70 parts of commercially available ordinary chemical fertilizer and 10 parts of sodium alginate, grind them until the particle fineness reaches 60 - 80 meshes to obtain a compound fertilizer mixture. Put the compound fertilizer mixture into a disk granulator and spray 18 parts of calcium chloride solution with a mass percentage concentration of 0.2%. Use a soil moisture sensor to measure the humidity of the compound fertilizer mixture and control the humidity of the compound fertilizer powder at about 15 - 20%. Too low or too high humidity will affect the granulation effect. Start the disk granulator for granulation. When the granulation diameter reaches 3 - 4 mm, stop the disk granulator to obtain compound fertilizer granules.

[0138] S2. Drying:

[0139] Start the hot air drying function of the disk granulator and set the drying temperature at 59 °C for 20 h.

[0140] S3. Form a viscous adsorption gel:

[0141] Put 6 parts of a calcium chloride solution with a mass percentage concentration of 0.2% into a spraying device, spray it on the dried compound granules, and at the same time start the stirring function of the disk granulator to stir, so that the calcium chloride solution is evenly coated on the surface of the compound granules and reacts with sodium alginate on the surface as follows:

[0142] 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca

[0143] Form a viscous adsorption gel. In this step, the spraying amount of the calcium chloride solution does not exceed 7% of the mass of the compound granules;

[0144] S4. Coating:

[0145] Start the stirring function of the disk granulator, add 1.5 parts of sodium polyacrylate powder with a fineness of 80 - 100 meshes to the material feeding device, and the addition amount is about 1.5% of the total mass of the compound granules obtained in step S3. Slowly add the sodium polyacrylate powder to the disk granulator at a feeding speed of (8% × 1.5 parts) / min and stir at a speed of 15 r / min, so that the sodium polyacrylate powder is evenly adsorbed on the surface of the compound granules to form a coating gel, and obtain coated compound granules;

[0146] S5. Secondary drying and shaping:

[0147] Start the hot air drying function of the disk granulator, dry the above-mentioned coated compound granules at 59 °C for 20 h, dry the coating gel to make it shaped into a solid coating;

[0148] S6. Repeat steps S3 - S5 for 2 - 3 times until the thickness of the coating on the surface of the coated compound granules reaches 1 - 2 mm to obtain the composite material.

[0149] The concentration of the calcium chloride solution has an important influence on the water absorption and swelling rate of the composite material. By implementing Examples 1 - 5, calcium chloride solutions with five concentrations of 0.1%, 0.3%, 0.5%, 1.0% and 3.0% in mass percentage concentration are prepared. Take the obtained composite material particles respectively and put them into beakers containing 100 ml of distilled water and let them stand for 1 h to fully absorb water. A gel is formed on the surface of the particles. Then take out the composite material particles and immediately remove the water on the surface with filter paper and weigh them. Conduct 5 groups of experiments independently at each concentration, calculate the water absorption and swelling rate respectively, and take the average value.

[0150] As Figure 1As shown in the figure, as the concentration of the calcium chloride solution increases from 0.1% to 0.5%, the water absorption and swelling rate of the composite material gradually increases. However, when the concentration of the calcium chloride solution exceeds 0.5%, the water absorption and swelling rate of the composite material no longer increases significantly. Therefore, it is determined that the concentration of the calcium chloride spraying solution is 0.1% - 0.5%, and the most suitable concentration is 0.5%.

[0151] Comparative Example 1:

[0152] Commercially available ordinary fertilizer granules without adding sodium alginate, calcium chloride solution and sodium polyacrylate.

[0153] Comparative Example 2:

[0154] Commercially available ordinary fertilizer is only applied with sodium alginate and calcium chloride, and granulation is carried out according to Steps S1 and S2 of Example 6.

[0155] Take 10 compound granules of Comparative Example 1, Comparative Example 2 and Example 6 in a petri dish, respectively measure the diameter of the granules, and then spray 10 ml of water into the petri dish. After 1 h, measure the diameter and shape of the fertilizer after water absorption to evaluate the expansibility, water absorbency and stability of the composite material;

[0156] After 1 h of water absorption, observe the morphology of each material, as Figures 2 - 3 shown; specifically, Figure 2 in A, the ordinary compound granules of Comparative Example 1 have dissolved with water and have no obvious solid characteristics; Figure 2 in B, the compound granules of the ordinary fertilizer with sodium alginate added in Comparative Example 2 maintain a relatively regular spherical shape, but individual granules have the phenomenon of excessive swelling resulting in morphological collapse. The average diameter increases from 3.90 mm before water absorption to 6.52 mm after water absorption, and the volume expands 4.7 times, as Figure 3 shown;

[0157] Figure 2 in C, the granules of Example 6 maintain a relatively perfect spherical shape after water absorption and have good stability, indicating that the coating plays a very good role in binding and supporting the skeleton and can maintain the soil bulk density and structure; the average diameter of the granules increases from 4.28 mm before water absorption to 7.34 mm after water absorption, and the volume expands 5.0 times, as Figure 3 shown.

[0158] In other embodiments, the fertilizer can also be replaced with other water-soluble soil additives, such as microbial inoculants, to promote soil nutrient release, improve soil aggregate structure and enhance plant stress resistance; it can also be chitosan oligosaccharide to enhance soil water permeability and water retention capacity, which is beneficial to the root growth of plants. Chitosan oligosaccharide contains various trace elements and amino acid and other nutrients, which can increase soil fertility.

[0159] Based on the above technical solution, in this application, the hydroscopic and swelling polymer material sodium alginate is used to improve the problem of soil compaction. After being applied to the soil, sodium alginate swells when it encounters water, which can maintain the soil bulk density and structure, and solves the problem that traditional chemical fertilizers have almost no expansibility when encountering water, dissolve in a short time, the ions in the fertilizer act on the soil particles, and the aggregates are destroyed when the soil loses water, resulting in soil compaction; applying chemical fertilizers to saline-alkali land overcomes the technical prejudice that traditional chemical fertilizers cannot be used in saline-alkali land and will aggravate soil compaction.

[0160] Aiming at the problem that the gel strength of sodium alginate in the compound fertilizer is not high and it is easy to break and hydrolyze in high-salt and high-pH environments, by spraying calcium chloride solution, the compound fertilizer is modified with a gel coating with stronger stability and toughness formed by cross-linking calcium chloride solution and sodium polyacrylate, which can improve the water absorption, expansibility and toughness and other physical properties of the composite material, and improve the strength of the composite material. By coating multiple times, a composite material with a specific coating thickness can be formulated, which can effectively cope with the harsh environment of saline-alkali land.

[0161] After the composite material absorbs water and swells, its state is stable, and its volume expands by more than 5 times. Adding 10% of the composite material to the surface compacted soil can increase the soil water holding capacity by 2-5% and reduce the soil bulk density by 20-30% when the soil moisture is sufficient, which can significantly improve the problem of soil compaction in saline-alkali soil; at the same time, as the coating slowly decomposes, the nutrients and calcium ions in the chemical fertilizer are slowly released and exchange with the sodium ions in the soil, which is beneficial to the desalination of saline-alkali soil.

[0162] Finally, it should be noted that: the embodiments in this specification are described in a progressive manner, and the key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other.

[0163] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or some technical features can be equivalently replaced; without departing from the spirit of the technical solutions of the present invention, they should all be covered within the scope of the technical solutions claimed by the present invention.

Claims

1. A method for preparing a hygroscopic and expandable composite material, characterized in that: The raw material components include the following parts by weight: 65-70 parts of fertilizer, 7-10 parts of sodium alginate, 20-25 parts of calcium chloride solution, 1-2 parts of sodium polyacrylate powder; The preparation method of the hygroscopic and expandable composite material comprises the following steps: S1. Granulation: The fertilizer and sodium alginate are mixed and ground in proportion to obtain a composite material, the composite material is put into a granulator, and a calcium chloride solution is sprayed to control the humidity of the composite material within a certain range, and the granulator is started to granulate to obtain composite particles; S2. Drying: Drying the composite particles at 55-60°C for 12-24 hours; S3, forming a viscous adsorption gel: Spraying the calcium chloride solution onto the dried composite particles while stirring, so that the calcium chloride solution is evenly coated on the surface of the composite particles to form composite particles with a viscous adsorption gel on the surface; S4, Encapsulation: Adding sodium polyacrylate powder to the composite particles that form the viscous adsorption gel in step S3, and stirring the mixture, so that the sodium polyacrylate powder is uniformly adsorbed on the surface of the composite particles to form a coating gel, thereby obtaining coated composite particles; S5, Secondary drying and shaping: Drying the coated composite particles at 55-60° C. for 12-24 hours to dry the coating gel to form a solid coating; S6. Repeat steps S3-S5 until the thickness of the coating on the surface of the coated composite particles reaches the required value, thereby obtaining the composite material.

2. The method for preparing the hygroscopic and expandable composite material according to claim 1, characterized in that: The mass percentage concentration of the calcium chloride solution is 0.1%-0.5%.

3. The method for preparing the hygroscopic and expandable composite material according to claim 1, characterized in that: In step S1, when the fineness of the composite material is 60-80 mesh, the grinding is stopped and the composite material is put into a granulator.

4. The method for preparing the hygroscopic and expandable composite material according to claim 3, characterized in that: In step S1, the humidity of the composite material is controlled at 15-20%.

5. The method for preparing the hygroscopic and expandable composite material according to claim 1, characterized in that: In step S3, the viscous adsorption gel is formed by the reaction of the calcium chloride solution and the sodium alginate on the surface of the composite particles, and the reaction formula is as follows: 2C 17 H 35 COO - +Ca 2+ ==(C 17 H 35 COO)2Ca。 6. The method for preparing the hygroscopic and expandable composite material according to claim 5, characterized in that: In step S3, the spraying amount of the calcium chloride solution does not exceed 5%-7% of the weight of the composite particles.

7. The method for preparing the hygroscopic and expandable composite material according to claim 3, characterized in that: In step S4, the fineness of the sodium polyacrylate powder is between 80-100 meshes.

8. The method for preparing the hygroscopic and expandable composite material according to claim 7, characterized in that: In step S4, the feeding speed of the sodium polyacrylate powder is 5%-10% / min of the total feeding mass of the sodium polyacrylate powder, and the stirring speed is 10-15 r / min.

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