Preparation method of controlled-release magnesium hydroxide fertilizer

Abstract

The invention relates to the technical field of inorganic material synthesis, in particular to a preparation method of a controlled-release magnesium hydroxide fertilizer. Comprising the steps of primary magnesium precipitation, hydrothermal treatment, secondary magnesium precipitation, anaerobic heat treatment and pulping. According to the method, firstly, crystal defects are eliminated through hydro-thermal treatment, crystal seeds with complete crystallinity are obtained, then the crystal seeds are added into a magnesium precipitation reaction system, the principle of Gibbs free energy minimization is followed, newly generated magnesium hydroxide tends to precipitate on the surfaces of the crystal seeds, under the interference effect of a crystal form regulator, the newly generated magnesium hydroxide crystal has many defects, and the magnesium precipitation effect is good. And then oxygen-free heat treatment is carried out, so that the defects of the magnesium hydroxide on the surface layer are further increased, and the initial dissolution and release capacity is improved. The controlled-release magnesium fertilizer which is high in early-stage release speed and long-acting in later-stage slow release is synthesized by utilizing a crystal defect engineering tool, and is different from a traditional coating scheme, and a brand new thought is provided for the design of the controlled-release fertilizer.

Description

Technical Field

[0001] The present invention relates to the technical field of inorganic material synthesis, and in particular to a method for preparing a controlled-release magnesium hydroxide fertilizer. Background Art

[0002] Magnesium is a key component in several biological processes such as carbon dioxide fixation in leaf photosynthesis, photophosphorylation, protein and chlorophyll synthesis, phloem loading and assimilate transport, and plays a vital role in ensuring crop productivity. The nutrient release characteristics of magnesium fertilizers directly affect the efficiency of crops in absorbing and utilizing magnesium. If the release rate is too slow, it may lead to a temporary shortage of magnesium, especially for crops with a high magnesium absorption rate; on the contrary, if the release rate is too fast, it is easy to cause magnesium to be lost through leaching when applied under conditions of high soil moisture conductivity and heavy rainfall. In the past, magnesium fertilizers mostly used easily soluble acidic fertilizers such as magnesium sulfate. Although they can increase production, they may also cause soil acidification, resulting in the loss of effective nutrients. Moreover, H in acidic soil + and A1 3+ It inhibits the absorption of magnesium by plants, further inducing magnesium deficiency in plants. Magnesium hydroxide is an alkaline fertilizer. While replenishing magnesium in the soil, it can also reduce the exchangeable aluminum content to a certain extent, thereby alleviating soil acidification, improving the tolerance of crops in acidic soil to aluminum poisoning, and ensuring crop yields.

[0003] However, magnesium hydroxide has poorer solubility than magnesium sulfate and a slower release rate. Although it can achieve the effect of long-term sustained release of magnesium, it is often difficult to meet immediate needs during the peak growth period of crops. Therefore, the ideal magnesium hydroxide fertilizer should have a controlled-release effect, that is, it can quickly release a sufficient amount of magnesium ions in the early stage of application to alleviate the symptoms of magnesium deficiency in crops, and then maintain a stable slow-release supply. This requirement is different from the design concept of traditional controlled-release fertilizers. Existing controlled-release technology uses coating technology to suppress the release rate of nutrients in the early stage, and then accelerates the release after the coating degrades. This controlled-release mode is obviously not suitable for the controlled-release design of magnesium hydroxide fertilizers.

[0004] Therefore, the development of a controlled-release magnesium hydroxide fertilizer that can achieve "rapid release in the early stage and sustained slow release in the later stage" is of great significance to improving the utilization efficiency of magnesium fertilizer and meeting crop needs. Summary of the Invention

[0005] In response to the problems in the background technology, the present invention provides a method for preparing a controlled-release magnesium hydroxide fertilizer based on the theoretical knowledge of crystal dissolution behavior, thereby realizing the "initial rapid release-later continuous slow release" release mode of the magnesium hydroxide fertilizer. The nutrient release curve of the controlled-release fertilizer is consistent with the crop requirements, thereby improving the application efficiency of the magnesium fertilizer.

[0006] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0007] A method for preparing a controlled-release magnesium hydroxide fertilizer, comprising the following steps:

[0008] S1. One-time magnesium precipitation

[0009] Alkali solution is added to the magnesium salt solution to carry out a magnesium precipitation reaction, so that magnesium ions react with hydroxide ions to form magnesium hydroxide precipitate. After the reaction is completed, it is filtered, washed, and dried to obtain magnesium hydroxide powder;

[0010] S2. Hydrothermal treatment

[0011] Mixing the magnesium hydroxide powder obtained in step S1 with water and performing hydrothermal treatment to obtain magnesium hydroxide seed crystals;

[0012] S3, secondary magnesium precipitation

[0013] The magnesium hydroxide seed crystals obtained in step S2 are added to a magnesium salt solution, and an alkali solution and a crystal form regulator are added to perform a secondary magnesium precipitation reaction. After the reaction is completed, the magnesium hydroxide layer is filtered, washed, and dried to obtain a magnesium hydroxide layer;

[0014] S4, oxygen-free heat treatment

[0015] The sample obtained in step S3 is placed in a tube furnace and heat treated in an oxygen-free atmosphere;

[0016] S5. Pulping

[0017] The magnesium hydroxide sample obtained in step S4 is mixed with water and an auxiliary agent to obtain the controlled-release magnesium hydroxide fertilizer.

[0018] In the above technical solution, further, in step S1, the magnesium salt is one or more of magnesium sulfate, magnesium chloride, and magnesium acetate; the alkali solution is one or more of sodium hydroxide solution, ammonia water, and potassium hydroxide solution; and the pH value of the primary magnesium precipitation reaction is controlled at 10-13.

[0019] In the above technical solution, further, in step S2, the temperature of the hydrothermal treatment is 120-140° C., the pressure is 0.2-0.4 MPa, and the time is 2-6 hours; the temperature, pressure and time of the hydrothermal treatment are regulated to promote the growth and complete crystallization of the magnesium hydroxide seed crystals; the solid-liquid ratio of the magnesium hydroxide powder to water is 1 g:10 mL.

[0020] In the above technical solution, further, in step S3, the magnesium salt is one or more of magnesium sulfate, magnesium chloride, and magnesium acetate; the alkali solution is one or more of sodium hydroxide solution, ammonia water, and potassium hydroxide solution; the pH value of the secondary magnesium precipitation reaction is controlled at 10 to 13; the crystal form regulator is one or more of ammonium acetate, ammonium chloride, glycine, and proline; during the secondary magnesium precipitation process, the crystal structure is controlled by adding a crystal form regulator, so that the crystallinity of the magnesium hydroxide layer is incomplete and has defects.

[0021] In the above technical solution, further, in step S3, the mass ratio of the crystal form regulator to the magnesium hydroxide seed crystals is 1:2.

[0022] In the above technical solution, further, in step S4, the oxygen-free atmosphere is a nitrogen atmosphere or a vacuum, the heat treatment temperature is 200-300°C, and the heat treatment time is 2-6 hours; by regulating the heat treatment parameters, the surface defects of the magnesium hydroxide layer are further increased, thereby improving its controlled release performance.

[0023] In the above technical solution, further, in step S5, the mass ratio of the magnesium hydroxide sample, water and the auxiliary agent is 1:(1-2):(0.01-0.1); the auxiliary agent is one or more of sodium polyacrylate, sodium hexametaphosphate, alkyl polyether sulfate, and sodium lignin sulfonate.

[0024] The beneficial effects of the present invention are:

[0025] 1. The dissolution and release of crystals often start from defect sites, first forming dissolution pits, and then the expansion and aggregation of the pits form dissolution steps and diffuse on the crystal surface. Therefore, controlling the defect density of the crystal is the key to allocating its macroscopic dissolution rate. Specifically, the present invention first eliminates crystal defects through hydrothermal treatment to obtain crystal seeds with complete crystallinity, and then adds crystal seeds to the magnesium precipitation reaction system. Following the principle of minimizing Gibbs free energy, the newly generated magnesium hydroxide tends to precipitate on the surface of the crystal seeds. Under the interference of the crystal form regulator, the newly generated magnesium hydroxide crystals have many defects and incomplete crystallinity, thereby obtaining magnesium hydroxide hierarchical crystals with high and low defects coexisting. Then, oxygen-free heat treatment is utilized to further increase the defects of the surface magnesium hydroxide and improve the dissolution and release ability in the initial stage.

[0026] 2. The present invention utilizes crystal defect engineering tools to synthesize a controlled-release magnesium fertilizer with a fast initial release rate and a long-lasting sustained release in the later stage. Compared with traditional coated controlled-release fertilizers, it does not require additional coatings, simplifies the process and is environmentally friendly, providing a new idea for the design of controlled-release fertilizers. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a process flow chart of the preparation method of the present invention;

[0028] Figure 2The results are a comparison of the magnesium hydroxide release effects of the samples prepared in Example 1 and Comparative Examples 1-3 of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with specific embodiments, but the examples do not limit the present invention in any form. Unless otherwise stated, the raw materials and reagents used in the examples of the present invention are conventionally purchased raw materials and reagents.

[0030] Example 1

[0031] A method for preparing a controlled-release magnesium hydroxide fertilizer comprises the following steps:

[0032] S1. One-time magnesium precipitation

[0033] A 40% sodium hydroxide solution was added to a 0.2 mol / L magnesium sulfate solution under stirring, and the pH was adjusted to 12.0. The mixture was stirred for 30 minutes to generate a white magnesium hydroxide precipitate. The precipitate was filtered, washed with water three times, and dried at 80°C for 12 hours to obtain magnesium hydroxide powder.

[0034] S2. Hydrothermal treatment

[0035] The magnesium hydroxide powder obtained in step S1 was added to a high-pressure reactor, and deionized water was added at a solid-liquid ratio of 1:10 (g / mL). After sealing, the temperature was raised to 130°C and the pressure was 0.3 MPa. The reaction was maintained for 4 hours. After the reaction was completed, the reaction was cooled, filtered, and washed, and then dried at 60°C to obtain magnesium hydroxide seed crystals;

[0036] S3, secondary magnesium precipitation

[0037] According to the mass ratio of 1:30, the magnesium hydroxide seed crystals obtained in step S2 were added to 0.15 mol / L magnesium chloride solution, stirred and dispersed for 30 minutes, glycine was added, 40% sodium hydroxide solution was added, and the pH was adjusted to 12.5, wherein the mass ratio of glycine to magnesium hydroxide seed crystals was 1:2, and stirring was continued for 2 hours. The precipitate was filtered, washed until neutral, and dried at 80°C to obtain a magnesium hydroxide layer;

[0038] S4, oxygen-free heat treatment

[0039] The sample obtained in step S3 was placed in a tube furnace, nitrogen was introduced to replace the air for 30 minutes, the temperature was raised to 250°C, the temperature was kept for 3 hours, the temperature was cooled to room temperature, and the sample was taken out;

[0040] S5. Pulping

[0041] The magnesium hydroxide sample obtained in step S4, water, and sodium polyacrylate were mixed in a mass ratio of 1:1.5:0.05 and stirred for 30 minutes to obtain a controlled-release magnesium hydroxide fertilizer.

[0042] Example 2

[0043] The controlled-release magnesium hydroxide fertilizer of this embodiment is prepared by a method similar to that of Example 1, except that the crystal form regulator in step S3 is ammonium chloride, and other conditions are the same as those of Example 1.

[0044] Comparative Example 1

[0045] A preparation method similar to that of Example 1 is adopted, except that the S2 hydrothermal treatment is skipped, and the magnesium hydroxide powder obtained by the primary magnesium precipitation is directly used for secondary magnesium precipitation. Other conditions are the same as those of Example 1, and the method specifically comprises the following steps:

[0046] S1. One-time magnesium precipitation

[0047] A 40% sodium hydroxide solution was added to a 0.2 mol / L magnesium sulfate solution under stirring, and the pH was adjusted to 12.0. The mixture was stirred for 30 minutes to generate a white magnesium hydroxide precipitate. The precipitate was filtered, washed with water three times, and dried at 80°C for 12 hours to obtain magnesium hydroxide powder.

[0048] S2, secondary magnesium precipitation

[0049] The magnesium hydroxide powder obtained in step S1 was added to a 0.15 mol / L magnesium chloride solution at a mass ratio of 1:30, and the mixture was stirred and dispersed for 30 minutes. Glycine was added, and a 40% sodium hydroxide solution was added to adjust the pH to 12.5. The mass ratio of glycine to magnesium hydroxide seed crystals was 1:2. Stirring was continued for 2 hours. The precipitate was filtered, washed until neutral, and dried at 80°C to obtain a magnesium hydroxide layer.

[0050] S3, oxygen-free heat treatment

[0051] The sample obtained in step S2 was placed in a tube furnace, nitrogen was introduced to replace the air for 30 minutes, the temperature was raised to 250°C, the temperature was kept for 3 hours, the sample was cooled to room temperature, and the sample was taken out;

[0052] S4, pulping

[0053] The magnesium hydroxide sample obtained in step S3, water, and sodium polyacrylate were mixed in a mass ratio of 1:1.5:0.05 and stirred for 30 minutes to obtain a magnesium hydroxide fertilizer.

[0054] Comparative Example 2

[0055] A preparation method similar to that of Example 1 is adopted, except that glycine is not added during the secondary magnesium precipitation in step S3. Other conditions are the same as those of Example 1, and specifically include the following steps:

[0056] S1. One-time magnesium precipitation

[0057] A 40% sodium hydroxide solution was added to a 0.2 mol / L magnesium sulfate solution under stirring, and the pH was adjusted to 12.0. The mixture was stirred for 30 minutes to generate a white magnesium hydroxide precipitate. The precipitate was filtered, washed with water three times, and dried at 80°C for 12 hours to obtain magnesium hydroxide powder.

[0058] S2. Hydrothermal treatment

[0059] The magnesium hydroxide powder obtained in step S1 was added to a high-pressure reactor, and deionized water was added at a solid-liquid ratio of 1:10 (g / mL). After sealing, the temperature was raised to 130°C and the pressure was 0.3 MPa. The reaction was maintained for 4 hours. After the reaction was completed, the reaction was cooled, filtered, and washed, and then dried at 60°C to obtain magnesium hydroxide seed crystals;

[0060] S3, secondary magnesium precipitation

[0061] The magnesium hydroxide seed crystals obtained in step S2 were added to a 0.15 mol / L magnesium chloride solution at a mass ratio of 1:30, and the mixture was stirred and dispersed for 30 minutes. A 40% sodium hydroxide solution was added to adjust the pH to 12.5, and stirring was continued for 2 hours. The precipitate was filtered, washed until neutral, and dried at 80°C to obtain a magnesium hydroxide layer;

[0062] S4, oxygen-free heat treatment

[0063] The sample obtained in step S3 was placed in a tube furnace, nitrogen was introduced to replace the air for 30 minutes, the temperature was raised to 250°C, the temperature was kept for 3 hours, the temperature was cooled to room temperature, and the sample was taken out;

[0064] S5. Pulping

[0065] The magnesium hydroxide sample obtained in step S4, water, and sodium polyacrylate were mixed in a mass ratio of 1:1.5:0.05 and stirred for 30 minutes to obtain a magnesium oxide fertilizer.

[0066] Comparative Example 3

[0067] A preparation method similar to that of Example 1 is adopted, except that the S4 heat treatment is skipped and the sample after the secondary magnesium precipitation is directly pulped. Other conditions are the same as those of Example 1, specifically comprising the following steps:

[0068] S1. One-time magnesium precipitation

[0069] A 40% sodium hydroxide solution was added to a 0.2 mol / L magnesium sulfate solution under stirring, and the pH was adjusted to 12.0. The mixture was stirred for 30 minutes to generate a white magnesium hydroxide precipitate. The precipitate was filtered, washed with water three times, and dried at 80°C for 12 hours to obtain magnesium hydroxide powder.

[0070] S2. Hydrothermal treatment

[0071] The magnesium hydroxide powder obtained in step S1 was added to a high-pressure reactor, and deionized water was added at a solid-liquid ratio of 1:10 (g / mL). After sealing, the temperature was raised to 130°C and the pressure was 0.3 MPa. The reaction was maintained for 4 hours. After the reaction was completed, the reaction was cooled, filtered, and washed, and then dried at 60°C to obtain magnesium hydroxide seed crystals;

[0072] S3, secondary magnesium precipitation

[0073] According to the mass ratio of 1:30, the magnesium hydroxide seed crystals obtained in step S2 were added to 0.15 mol / L magnesium chloride solution, stirred and dispersed for 30 minutes, glycine was added, 40% sodium hydroxide solution was added, and the pH was adjusted to 12.5, wherein the mass ratio of glycine to magnesium hydroxide seed crystals was 1:2, and stirring was continued for 2 hours. The precipitate was filtered, washed until neutral, and dried at 80°C to obtain a magnesium hydroxide layer;

[0074] S4, pulping

[0075] The sample obtained in step S3, water, and sodium polyacrylate were mixed in a mass ratio of 1:1.5:0.05 and stirred for 30 minutes to obtain a magnesium hydroxide fertilizer.

[0076] Test Example 1

[0077] The magnesium hydroxide fertilizer samples prepared in Example 1 and Comparative Examples 1-3 were kept at a constant pH of 7 and 25° C. The molar amount of the added acid was measured using an automatic titrator, and the ratio of the remaining magnesium was calculated according to formula (1). The test results are as follows: Figure 2 shown.

[0078]

[0079] As can be seen from the figure, Example 1 released 18% within 24 hours, leaving 82% remaining. After 7 days, 60% remained, and after 28 days, 30% remained. It then entered a slow release phase, with the release curve showing a rapid initial release followed by a slow release, consistent with the high demand characteristics of the crop in the early stages. Comparative Example 1 released 70% within 24 hours, but only 15% remained after 28 days, indicating rapid release in the early stages and insufficient reserves in the later stages. Comparative Examples 2 and 3, with 95% and 90% remaining after 24 hours, respectively, consistently released at a slower rate, indicating insufficient early release capacity.

[0080] In summary, the present invention regulates dissolution behavior through crystal defect engineering, which meets the high demand characteristics of crops in the early stage. Compared to Comparative Example 1, magnesium hydroxide crystal defects can be eliminated by hydrothermal treatment, providing a low-defect substrate for secondary magnesium precipitation to ensure stable release in the later stage. Compared to Comparative Example 2 and Comparative Example 3, the defect density of the secondary grown magnesium hydroxide crystals is increased by a crystal form regulator during the secondary magnesium precipitation process, and the surface defects are further expanded by anaerobic heat treatment to strengthen the initial dissolution ability. This proves that the lack of any step will lead to a decrease in controlled release performance, verifying the necessity of the hierarchical synthesis method.

[0081] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the scope defined in the claims. Other variations or modifications may be made based on the above description. Obvious variations or modifications derived therefrom shall remain within the scope of protection of the present invention.

Claims

1. A method for preparing a controlled-release magnesium hydroxide fertilizer, characterized in that: The method comprises the following steps: S1. One-time magnesium precipitation Alkali solution is added to the magnesium salt solution to carry out a magnesium precipitation reaction, so that magnesium ions react with hydroxide ions to form magnesium hydroxide precipitate. After the reaction is completed, it is filtered, washed, and dried to obtain magnesium hydroxide powder; S2. Hydrothermal treatment Mixing the magnesium hydroxide powder obtained in step S1 with water and performing hydrothermal treatment to obtain magnesium hydroxide seed crystals; S3, secondary magnesium precipitation The magnesium hydroxide seed crystals obtained in step S2 are added to a magnesium salt solution, and an alkali solution and a crystal form regulator are added to perform a secondary magnesium precipitation reaction. After the reaction is completed, the magnesium hydroxide layer is filtered, washed, and dried to obtain a magnesium hydroxide layer; S4, oxygen-free heat treatment The sample obtained in step S3 is placed in a tube furnace and heat treated in an oxygen-free atmosphere; S5. Pulping The magnesium hydroxide sample obtained in step S4 is mixed with water and an auxiliary agent to obtain the controlled-release magnesium hydroxide fertilizer.

2. The preparation method according to claim 1, characterized in that In step S1, the magnesium salt is one or more of magnesium sulfate, magnesium chloride, and magnesium acetate; the alkali solution is one or more of sodium hydroxide solution, ammonia water, and potassium hydroxide solution; and the pH value of the primary magnesium precipitation reaction is controlled at 10-13.

3. The preparation method according to claim 1, characterized in that In step S2, the hydrothermal treatment temperature is 120-140° C., the pressure is 0.2-0.4 MPa, and the time is 2-6 hours; the solid-liquid ratio of magnesium hydroxide powder to water is 1 g:10 mL.

4. The preparation method according to claim 1, characterized in that In step S3, the magnesium salt is one or more of magnesium sulfate, magnesium chloride, and magnesium acetate; the alkali solution is one or more of sodium hydroxide solution, ammonia water, and potassium hydroxide solution; the crystal form regulator is one or more of ammonium acetate, ammonium chloride, glycine, and proline; and the pH value of the secondary magnesium precipitation reaction is controlled at 10 to 13.

5. The preparation method according to claim 1, characterized in that In step S3, the mass ratio of the crystal form regulator to the magnesium hydroxide seed crystals is 1:

2.

6. The preparation method according to claim 1, characterized in that In step S4, the oxygen-free atmosphere is a nitrogen atmosphere or a vacuum atmosphere, the heat treatment temperature is 200-300° C., and the heat treatment time is 2-6 hours.

7. The preparation method according to claim 1, characterized in that In step S5, the mass ratio of the magnesium hydroxide sample, water and the auxiliary agent is 1:(1-2):(0.01-0.1); the auxiliary agent is one or more of sodium polyacrylate, sodium hexametaphosphate, alkyl polyether sulfate, and sodium lignin sulfonate.

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