Formulation and preparation method of an acidic non-aqueous medium element fertilizer

Abstract

The application discloses a formula and a preparation method of an acidic non-water-soluble secondary and microelement fertilizer, and relates to the technical field of fertilizers.The method comprises the following steps: mixing and crushing a flotation phosphorus tailing with a grinding activator; spraying a first acid liquor into a closed container to perform low-temperature pre-acidification; adding a hole-expanding agent, heating, and then adding a second acid liquor in batches to perform medium-temperature deep activation, and the reaction end point is controlled through pH; then, multi-stage aging treatment is performed; finally, drying and crushing are performed, and the acidic non-water-soluble secondary and microelement fertilizer is obtained.The application realizes the resource utilization of the flotation phosphorus tailing through a synergistic process of mechanical activation, low-temperature pre-acidification, medium-temperature deep activation and multi-stage aging, and prepares the acidic non-water-soluble secondary and microelement fertilizer.The pH of the fertilizer is controlled to be 4.5-5.5, the fertilizer is suitable for alkaline soil improvement and acid-loving crop fertilization, the fertilizer has a porous structure, has good dispersibility in water, and can meet the use requirements of drip irrigation.

Description

Technical Field

[0001] This invention relates to the field of fertilizer technology, and more specifically, to a formulation and preparation method for an acidic, water-insoluble, medium-element fertilizer. Background Technology

[0002] Calcium, magnesium, and sulfur are essential nutrients for crop growth and play a crucial role in regulating soil pH balance and promoting nutrient absorption. Currently, most medium-element fertilizers are water-soluble, such as calcium magnesium nitrate and calcium ammonium nitrate. While these products can quickly replenish nutrients, the nitrate ions they contain may exacerbate soil acidification during application. For already acidified soils, the effectiveness of these fertilizers is limited. Meanwhile, non-water-soluble medium-element fertilizers are often prepared from alkaline raw materials such as light calcium carbonate, light magnesium hydroxide, and magnesium oxide, suitable for improving acidic soils. However, existing products are not well-suited for alkaline soils or the fertilization needs of acid-loving crops (such as blueberries, potatoes, and tea trees).

[0003] Chinese patent CN114031443A discloses a medium-element non-water-soluble solid fertilizer and its preparation method, using high-purity raw materials such as light calcium carbonate, light magnesium hydroxide, and fumed silica. The product is alkaline and suitable for raising the pH value of acidic soils. This technical solution focuses on solving the problem of fertilizer sedimentation in water, but the product's alkaline nature limits its applicability. Chinese patent CN118771941A discloses a non-water-soluble alkaline magnesium fertilizer with soil-improving properties, using modified nano-suspended magnesium oxide as the main raw material. It is also suitable for improving acidic soils, but its application effect in alkaline soil environments requires further verification.

[0004] Existing non-water-soluble medium-element fertilizers are mostly based on alkaline materials, resulting in generally high pH values ​​that fail to meet the dual requirements of improving alkaline soils and fertilizing acid-loving crops. Furthermore, existing technologies often use high-purity chemical raw materials, leading to high production costs, and lack effective ways to utilize the large amount of tailings generated during phosphate rock beneficiation. Therefore, a formula and preparation method for an acidic, water-insoluble, medium-element fertilizer are proposed to address the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a formula and preparation method for an acidic non-water-soluble medium-element fertilizer. Using flotation phosphate tailings as raw material, a non-water-soluble medium-element fertilizer with acidic characteristics, porous structure, and suitable for drip irrigation is prepared through a specific chemical modification process. This fertilizer is used for alkaline soil improvement and fertilization of acid-loving crops.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention first provides a formulation for an acidic, non-water-soluble medium-element fertilizer. This fertilizer is composed of a modified product obtained from flotation phosphate tailings through grinding activation, low-temperature pre-acidification, medium-temperature deep activation, multi-stage aging, drying, and pulverization. The fertilizer has a pH of 4.5–5.5, an acidic characteristic that makes it suitable for pH adjustment in alkaline soils and for meeting the nutrient requirements of acid-loving crops, filling the technological gap in existing non-water-soluble medium-element fertilizers that are primarily alkaline products. The bulk density of the fertilizer is 0.58 g / cm³. 3 ~0.63g / cm 3 A sedimentation rate of ≤13% in water indicates that the product has a loose and porous structure, good suspension and dispersion in water, and can meet the requirements of drip irrigation without easily clogging the equipment.

[0007] This invention also provides a method for preparing an acidic, water-insoluble, medium-element fertilizer, comprising the following steps: S1. Mix the flotation phosphate tailings with the grinding aid and activator, then pulverize to 80-120 mesh to obtain activated phosphate tailings powder.

[0008] Flotation tailings are solid waste generated during phosphate ore beneficiation, mainly composed of calcium carbonate and magnesium carbonate. Due to the dense crystal structure of natural minerals, the powder obtained by direct crushing has low reactivity. In this step, a grinding aid activator is added during the crushing process. The polar groups in the grinding aid activator molecules adsorb onto the surface of the newly formed particles, reducing surface energy and preventing fine particle agglomeration. Furthermore, it causes lattice distortion and dislocations in the mineral crystals, increasing surface active sites and providing a larger reaction interface for subsequent acidification reactions. Crushing to 80-120 mesh ensures sufficient specific surface area while avoiding excessive dust and processing difficulties caused by overly fine particles.

[0009] S2. Place the activated phosphorus tailings powder in a sealed container, spray the first acid solution at 15℃~25℃, control the liquid-solid mass ratio to be 0.3:1~0.5:1, and react for 2 hours to 4 hours. During the reaction, maintain a slightly positive pressure environment of 0.1MPa~0.2MPa in the sealed container to obtain surface acidified material.

[0010] This step employs low-temperature pre-acidification, ensuring that the acid only partially reacts with the carbonates on the particle surface, forming a core-shell structure of "acid on the outside, base on the inside." The low temperature of 15℃–25℃ helps slow the reaction rate, preventing excessive surface reaction and the formation of a dense calcium phosphate passivation layer. Adding the acid via spray ensures uniform distribution on the particle surface, preventing excessively high local acid concentrations. A liquid-to-solid ratio of 0.3:1–0.5:1 ensures sufficient wetting of the particle surface without causing excessive moisture and clumping. Maintaining a slight positive pressure environment of 0.1MPa–0.2MPa during the reaction traps the generated carbon dioxide gas inside the material, creating conditions for the subsequent formation of a porous structure. A reaction time of 2–4 hours ensures the surface acidification layer reaches an appropriate thickness, guaranteeing sufficient acidic substance formation without excessive diffusion into the interior.

[0011] S3. Add a pore-expanding agent to the surface acidified material, mix evenly, and heat to 55℃~65℃. Then add the second acid solution in 4 to 6 portions, stirring for 15 to 20 minutes after each addition. Detect the pH value of the 1:10 aqueous suspension of the material. Add the next portion when the pH value is higher than 5.5, until all the second acid solution is added. Control the pH value of the material at the reaction endpoint to be 4.5~5.5 to obtain a porous activated material.

[0012] This step employs a medium-temperature deep activation treatment, utilizing the decomposition of a pore-expanding agent to generate gas and controlled acid addition in stages to construct interconnected pores within the particles and precisely control the acid endpoint. The pore-expanding agent decomposes at 55℃–65℃, producing ammonia and carbon dioxide gases, which escape and form microporous channels within the particles. Heating to this temperature range ensures rapid decomposition of the pore-expanding agent without causing thermal decomposition of the already generated acidic substances. The second acid solution is added in stages, with stirring for 15–20 minutes after each addition, allowing sufficient time for the acid to react with the carbonates within the particles. The reaction process is monitored in real-time using pH detection; the next batch is added only when the pH value exceeds 5.5, ensuring that each batch of acid is fully consumed before replenishing, preventing localized acid accumulation that could lead to uncontrolled reaction. The final pH value of the material is controlled at 4.5–5.5, giving the product stable acidic characteristics.

[0013] S4. Spread the porous activated material into a 3cm to 5cm thick layer and place it in a temperature and humidity controlled environment for multi-stage aging treatment to obtain aged material.

[0014] The aging process is a dissolution-recrystallization process. By controlling the temperature and humidity gradient, the reaction products form a stable acidic mineral phase. Spreading the material into a 3cm-5cm thick layer ensures air permeability and uniform temperature and humidity transfer. The multi-stage aging treatment uses gradient temperature and humidity conditions to allow the acid salt to successively undergo three stages: hydration recrystallization, crystal growth and densification, and final drying and solidification, forming a stable acidic mineral structure and preventing alkali return during storage.

[0015] S5. The aged material is dried at 60℃~65℃ until the moisture content is ≤5% to obtain the dried material.

[0016] The drying temperature is controlled at 60℃~65℃, below the decomposition temperature of acid salts, to ensure the stability of the acidic mineral phase during the drying process. Moisture content is controlled at ≤5% to meet the standard requirements for solid fertilizers, while also preventing clumping during storage.

[0017] S6. The dried material is pulverized to 150-200 mesh to obtain an acidic, non-water-soluble medium-element fertilizer.

[0018] The particles are crushed to 150-200 mesh to meet the requirements of drip irrigation. Particles within this fineness range have good suspension and dispersibility in water, are not easy to settle and clog drip irrigation equipment, and at the same time ensure sufficient reactivity after being applied to the soil.

[0019] Furthermore, the grinding aid activator mentioned in S1 is triethanolamine, and the amount added is 0.8% to 1.2% of the mass of the flotation phosphate tailings. Triethanolamine molecules contain hydroxyl and amino groups, which can be adsorbed on the surface of carbonate particles. By reducing the surface energy, it prevents agglomeration. At the same time, it forms a complex with surface calcium ions, which relaxes the surface lattice and improves the activity of subsequent acidification reactions.

[0020] Furthermore, the first acid solution in S2 is a composite acid solution, composed of phosphoric acid, citric acid, tartaric acid, and water by mass percentage. Phosphoric acid accounts for 50%–70%, citric acid for 15%–25%, and tartaric acid for 5%–15%, with the sum of the three not exceeding 90%, resulting in a total acid concentration of 12%–18%. Phosphoric acid reacts with carbonates to form acidic phosphates, providing the main acidic component. Citric acid and tartaric acid are natural organic acids that can form soluble complexes with calcium ions, preventing the formation of an insoluble calcium phosphate passivation layer and ensuring uniform reaction. The fact that the sum of the three does not exceed 90% ensures sufficient water as a reaction medium.

[0021] Furthermore, the injection described in S2 is carried out via a spray method, and the carrier gas used for the spray is a mixed gas containing 10% to 20% carbon dioxide by volume. The carbon dioxide carrier gas dissolves in water to form carbonic acid, which works synergistically with phosphoric acid to enhance the etching of the particle surface; at the same time, it increases the partial pressure of carbon dioxide in the environment, inhibits the formation reaction of carbonate, and helps maintain an acidic environment; carbon dioxide forms microbubbles in the material, providing more gas nuclei for subsequent pore formation.

[0022] Further, the pore-expanding agent mentioned in S3 is ammonium bicarbonate or urea, added at a rate of 1% to 3% of the mass of the flotation phosphate tailings. Ammonium bicarbonate and urea decompose at 55°C to 65°C to produce ammonia and carbon dioxide, which escape to form interconnected pores. The second acid solution is a 25% to 30% phosphoric acid solution, added in a total amount of 5% to 15% of the mass of the flotation phosphate tailings. Controlling the concentration of the second acid solution at 25% to 30% ensures sufficient reactivity without affecting the material's state due to excessive moisture.

[0023] Furthermore, the multi-stage aging process described in S4 specifically includes: aging the porous activated material in an environment with a temperature of 30℃~35℃ and a relative humidity of 80%~85% for 6 to 8 hours; adjusting the temperature to 45℃~50℃ and the relative humidity to 55%~65%, and continuing aging for 8 to 10 hours; adjusting the temperature to 60℃~65℃ and the relative humidity to 30%~40%, and continuing aging for 4 to 6 hours. The first stage of high humidity and low temperature allows for full hydration and recrystallization of the acidic salt; the second stage of medium temperature and medium humidity accelerates crystal growth and structural densification; and the third stage of high temperature and low humidity thoroughly dries and solidifies the material. This gradient aging method helps to form a stable acidic mineral phase.

[0024] Furthermore, the entire aging process from S41 to S43 is carried out in an atmosphere containing 5% to 10% carbon dioxide by volume, with the atmosphere pressure maintained 0.01 MPa to 0.02 MPa higher than atmospheric pressure. The carbon dioxide atmosphere can inhibit the regeneration of calcium carbonate and magnesium carbonate, preventing the product from "reverting to alkali" and losing its acidity; at the same time, carbon dioxide can react with alkaline substances that may escape from the surface of the material, maintaining a stable acidic environment.

[0025] Furthermore, step S6 includes adding an anti-caking agent to the pulverized material and mixing it uniformly using pneumatic mixing. The anti-caking agent is hydrophobically modified nano-silica or calcium stearate, added at 0.3%–0.8% of the material mass. The anti-caking agent forms a physical isolation layer on the particle surface, which helps improve the storage stability of the product, prevents moisture absorption and clumping, and also improves powder flowability, making the fertilizer easier to disperse in water.

[0026] Furthermore, step S6 includes adding a trace element composition to the pulverized material and mixing it evenly. The trace element composition comprises at least two of zinc sulfate, borax, manganese sulfate, EDTA-iron, EDTA-copper, and ammonium molybdate, with each compound added at 0.05%–0.2% of the material mass. The acidic fertilizer prepared by this invention exhibits good compatibility with trace elements. The acidic environment prevents the hydrolysis and precipitation of trace elements, allowing for targeted supplementation based on soil nutrient deficiencies and achieving a synergistic effect between macronutrients and micronutrients.

[0027] The technical effects and advantages of this invention are as follows: This invention uses flotation phosphate tailings as raw material and employs mechanical activation by adding a grinding aid during the crushing process. The main components of flotation phosphate tailings are calcium carbonate and magnesium carbonate, but the natural mineral crystal structure is dense, resulting in low reactivity. The polar groups in the grinding aid activator molecules adsorb onto the surface of newly formed particles, reducing surface energy and preventing fine particle agglomeration. Furthermore, it causes lattice distortion and dislocations in the mineral crystals, increasing the number of surface active sites. This treatment helps improve the efficiency of subsequent acidification reactions, transforming the originally low-activity industrial solid waste into a reactive fertilizer raw material, achieving resource utilization while reducing production costs.

[0028] This invention employs a two-step acidification process combining low-temperature pre-acidification and medium-temperature deep activation. In the low-temperature pre-acidification stage, the first acid solution is sprayed into the activated phosphate tailings powder at 15℃~25℃, controlling the reaction to occur on the particle surface, forming a core-shell structure of "acid on the outside, alkali on the inside." The low temperature helps slow the reaction rate, preventing excessive surface reaction and the formation of a dense calcium phosphate passivation layer. Simultaneously, a slightly positive pressure environment is maintained within the sealed container, allowing the carbon dioxide generated during the reaction to remain inside the material. In the medium-temperature deep activation stage, heating to 55℃~65℃ promotes the decomposition of the pore-expanding agent, generating gas. As the gas escapes, it forms interconnected pores inside the particles. The second acid solution is added in stages, with real-time pH monitoring to ensure that each batch of acid is fully consumed before replenishing, preventing localized acid accumulation that could lead to uncontrolled reaction. This process helps form an acidic fertilizer with a porous structure, reducing the product's bulk density to 0.59~0.62 g / cm³. 3 It has good suspension and dispersion properties in water and is suitable for modern agricultural fertilization methods such as drip irrigation and fertigation.

[0029] This invention precisely controls the pH of the final product to 4.5–5.5 by adding the second acid solution in stages and monitoring the pH value in real time. The second acid solution is added in 4–6 stages, with stirring for 15–20 minutes after each addition. The pH value of the 1:10 aqueous suspension is monitored, and the next stage is added only when the pH value is higher than 5.5. This control method ensures uniform reaction, avoids localized over-acidity or incomplete reaction, and keeps the acidity of the final product stable within the target range. The product's pH value is controlled at 4.5–5.5, making it suitable for pH adjustment in alkaline soils and meeting the nutrient requirements of acid-loving crops (such as blueberries, potatoes, and tea trees), filling the technological gap in existing non-water-soluble medium-element fertilizers that are mainly alkaline products.

[0030] This invention employs a multi-stage gradient aging process. Porous activated materials are spread into a 3cm–5cm thick layer and aged sequentially in an environment of 30℃–35℃ and 80%–85% relative humidity for 6–8 hours, 45℃–50℃ and 55%–65% relative humidity for 8–10 hours, and 60℃–65℃ and 30%–40% relative humidity for 4–6 hours. The aging process is a dissolution-recrystallization process. The first stage, with high humidity and low temperature, allows for full hydration and recrystallization of the acidic salts. The second stage, with medium temperature and humidity, accelerates crystal growth and structural densification. The third stage, with high temperature and low humidity, thoroughly dries and solidifies the material. Simultaneously, an atmosphere containing 5%–10% carbon dioxide (by volume) is introduced during the aging process to protect the product and inhibit the regeneration of calcium carbonate and magnesium carbonate. This treatment method helps form a stable acidic mineral phase, preventing alkali return due to carbonate recrystallization during storage and ensuring the product's acid stability.

[0031] This invention allows for the addition of anti-caking agents or trace element compositions as needed. Anti-caking agents, such as hydrophobically modified nano-silica or calcium stearate, are uniformly coated onto the particle surface using a pneumatic mixing method, forming a physical isolation layer. This helps improve the product's storage stability, prevents moisture absorption and clumping, and simultaneously improves powder flowability, making the fertilizer easier to disperse in water. Trace element compositions, such as at least two of zinc sulfate, borax, manganese sulfate, EDTA-iron, EDTA-copper, and ammonium molybdate, are not easily hydrolyzed and precipitated in acidic environments, allowing for targeted supplementation based on soil nutrient deficiencies. The amount of trace elements added is controlled within the range of 0.05% to 0.2%, achieving a synergistic effect between macronutrients and micronutrients, thus expanding the product's applicability. Attached Figure Description

[0032] Figure 1 This invention is based on the present invention.

[0033] Figure 2 This invention is based on the present invention. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] As attached Figures 1 to 2 The formula and preparation method of an acidic, water-insoluble, medium-element fertilizer are shown below. The specific implementation details are as follows: I. Raw materials and reagents The specifications of the raw materials used in the embodiments of this invention are as follows: Flotation phosphate tailings: The main chemical components are calcium oxide 27.3%, magnesium oxide 15.8%, silicon dioxide 8.2%, phosphorus pentoxide 2.1%, and loss on ignition 42.5%.

[0036] Triethanolamine: purity ≥85%, conforming to HG / T 3268-2002 standard.

[0037] Phosphoric acid: 85% by mass, conforming to GB / T 2091-2018 standard.

[0038] Citric acid: Citric acid monohydrate, purity ≥99%, conforming to GB / T 8269-2006 standard.

[0039] Tartaric acid: DL-tartaric acid, purity ≥99%, conforming to GB 1886.42-2015 standard.

[0040] Carbon dioxide gas: volume fraction ≥ 99.9%, conforming to GB / T 6052-2011 standard.

[0041] Ammonium bicarbonate: nitrogen content ≥17%, conforming to GB 3559-2001 standard.

[0042] Urea: Nitrogen content ≥46%, conforming to GB 2440-2017 standard.

[0043] Hydrophobically modified nano-silica: specific surface area 160m² 2 / g, surface treated with dimethyldichlorosilane, conforming to HG / T 3061-2020 standard.

[0044] Calcium stearate: calcium content 6.5%~7.0%, melting point ≥150℃, conforming to HG / T 2424-2012 standard.

[0045] Zinc sulfate: Zinc sulfate monohydrate, zinc content ≥34%, conforming to HG / T 2326-2015 standard.

[0046] Borax: Borax decahydrate, boron content ≥11%, conforming to GB / T 537-2009 standard.

[0047] Manganese sulfate: Manganese sulfate monohydrate, manganese content ≥31%, conforming to HG / T 2962-2010 standard.

[0048] EDTA-Iron: Sodium iron ethylenediaminetetraacetate, with an iron content ≥13%, conforming to HG / T 5253-2017 standard.

[0049] EDTA-Copper: Sodium copper ethylenediaminetetraacetate, copper content ≥15%, conforming to HG / T 5254-2017 standard.

[0050] Ammonium molybdate: Tetramolybdate, molybdenum content ≥54%, conforming to GB / T 3460-2017 standard.

[0051] The specific implementation method is as follows: Example 1 This embodiment provides a method for preparing an acidic, water-insoluble, medium-element fertilizer, comprising the following steps: S1. Weigh 100 kg of flotation phosphate tailings, add 0.9 kg of triethanolamine, mix evenly, and then put into a ball mill for crushing until all of it passes through a 100-mesh standard sieve to obtain activated phosphate tailings powder.

[0052] S2. Transfer the activated phosphorus tailings powder to a sealed reactor, maintaining the reactor temperature at 20°C. Prepare 100 kg of a composite acid solution with a total acid mass fraction of 12%, composed of: 9.4 kg of 85% phosphoric acid (equivalent to 8.0 kg of pure phosphoric acid), 2.0 kg of citric acid, and 2.0 kg of tartaric acid, add 86.6 kg of water, and stir until completely dissolved to obtain the composite acid solution. Use a spray device to spray 40 kg of the composite acid solution into the reactor, controlling the liquid-to-solid mass ratio at 0.4:1, leaving the remaining 60 kg of composite acid solution for later use. The carrier gas used for spraying is a mixture of carbon dioxide and air, with a carbon dioxide volume fraction of 15%. After spraying, continue stirring and reacting at 20°C for 3 hours, maintaining the pressure inside the sealed container at 0.15 MPa during the reaction to obtain the surface-acidified material.

[0053] S3. Add 2 kg of ammonium bicarbonate to the above surface acidification material, mix well, and heat to 60°C. Weigh 35.3 kg of 85% phosphoric acid (equivalent to 30.0 kg of pure phosphoric acid), add 72 kg of water to dilute to a concentration of 28%, obtaining 107.3 kg of the second acid solution. Divide the second acid solution into 5 equal portions, each weighing 21.46 kg, and add them dropwise in 5 separate additions. After each addition, stir and react for 15 minutes, then measure the pH value of the 1:10 aqueous suspension. Add the next portion only when the pH value is higher than 5.5. After all 5 portions have been added, the pH value of the material is 5.0, yielding a porous activated material.

[0054] S4. Spread the above porous activated material evenly on a tray, with a material layer thickness of 4cm. Place the tray in a temperature and humidity controlled aging chamber for multi-stage aging treatment: the first stage temperature is 32℃ and the relative humidity is 82%, and the aging time is 7 hours; the second stage temperature is 48℃ and the relative humidity is 60%, and the aging time is 9 hours; the third stage temperature is 62℃ and the relative humidity is 35%, and the aging time is 5 hours, to obtain the aged material.

[0055] S5. Dry the above-mentioned aged material at 62°C until the moisture content is ≤5% to obtain the dried material.

[0056] S6. Crush the above-mentioned dried material until it all passes through a 200-mesh standard sieve to obtain an acidic, non-water-soluble medium-element fertilizer, which is denoted as Sample A. Example 2

[0057] The difference between this embodiment and Embodiment 1 is that step S7 is added after S6. 100 kg of the pulverized material is weighed, and 0.5 kg of hydrophobically modified nano-silica is added. The mixture is then mixed using a pneumatic mixer for 15 minutes to obtain sample B. The remaining steps are the same as in Embodiment 1. Example 3

[0058] The difference between this embodiment and Embodiment 1 is that step S7 is added after S6. 100 kg of the pulverized material is weighed, 0.8 kg of calcium stearate is added, and the mixture is stirred for 15 minutes using a pneumatic mixer to obtain sample C. The remaining steps are the same as in Embodiment 1. Example 4

[0059] The difference between this embodiment and Embodiment 1 is that step S7 is added after S6. Weigh 100 kg of the pulverized material, add 0.1 kg of zinc sulfate, 0.1 kg of borax, and 0.05 kg of manganese sulfate, and mix using a V-type mixer for 20 minutes to obtain sample D. The remaining steps are the same as in Embodiment 1. Example 5

[0060] The difference between this embodiment and Embodiment 1 is that the multi-stage aging process in S4 is carried out in an atmosphere containing 8% carbon dioxide by volume, with the atmosphere pressure maintained 0.015 MPa higher than atmospheric pressure. The remaining steps are the same as in Embodiment 1, yielding sample E. Example 6

[0061] The difference between this embodiment and Embodiment 1 is that 2 kg of urea was used as the pore-expanding agent in S3. The remaining steps are the same as in Embodiment 1, and sample F is obtained.

[0062] III. Comparative Example Comparative Example 1 The difference between this comparative example and Example 1 is that triethanolamine is not added in S1, and the flotation phosphate tailings are directly crushed to 100 mesh. The remaining steps are the same as in Example 1, resulting in comparative sample D1.

[0063] Comparative Example 2 The difference between this comparative example and Example 1 is that the first acid solution in S2 is a 12% phosphoric acid solution, i.e., 14.1 kg of 85% phosphoric acid (equivalent to 12.0 kg of pure phosphoric acid) is weighed and diluted to 100 kg with 85.9 kg of water, without adding citric acid or tartaric acid. 40 kg of this phosphoric acid solution is sprayed into the reaction vessel, and the remaining steps are the same as in Example 1, to obtain the comparative sample D2.

[0064] Comparative Example 3 The difference between this comparative example and Example 1 is that ammonium bicarbonate is not added in step S3. The remaining steps are the same as in Example 1, resulting in comparative sample D3.

[0065] Comparative Example 4 The difference between this comparative example and Example 1 is that in S3, the second acid solution is added to the reactor all at once, without phased addition or pH monitoring. The remaining steps are the same as in Example 1, resulting in comparative sample D4.

[0066] Comparative Example 5 The difference between this comparative example and Example 1 is that in S4, the porous activated material is continuously aged for 20 hours in a constant temperature and humidity environment of 45°C and 60% relative humidity, without multi-stage gradient aging. The remaining steps are the same as in Example 1, resulting in comparative sample D5.

[0067] IV. Performance Testing Methods The following measurements were performed on samples A to F prepared in Examples 1 to 6 and comparative samples D1 to D5 prepared in Comparative Examples 1 to 5. All tests were repeated three times, and the average value was taken.

[0068] 1. pH value determination: Weigh 1.00g of sample, place it in a 50mL beaker, add 10mL of distilled water, stir for 1 minute, let stand for 5 minutes, and then use a pH meter to measure the pH value of the supernatant.

[0069] 2. Fineness determination: Weigh 50.0g of sample, place it in a 200-mesh standard sieve, vibrate the sieve for 10 minutes, weigh the mass of material passing through the sieve, and calculate the percentage of material passing through the 200-mesh sieve.

[0070] 3. Moisture content determination: Weigh 5.00g of the sample, dry it at 105℃ to constant weight, and calculate the percentage loss.

[0071] 4. Determination of sedimentation rate in water: Weigh 1.00 g of sample and place it in a 100 mL stoppered graduated cylinder. Add water to the 100 mL mark, tighten the stopper, and invert the cylinder 10 times to ensure thorough dispersion. Let it stand for 30 minutes. Carefully remove the upper 90 mL of liquid with a pipette, being careful not to disturb the precipitate at the bottom. Then rinse the inner wall of the graduated cylinder with a small amount of distilled water. Transfer the rinsing solution, along with the remaining 10 mL of liquid and the precipitate, to a pre-weighed evaporating dish. Dry the dish at 105 °C to constant weight, weigh the precipitate, and calculate the sedimentation rate.

[0072] 5. Bulk density determination: Fill a 100mL graduated cylinder with the sample, level it with a ruler, weigh the sample, and calculate the mass per unit volume.

[0073] 6. Determination of calcium and magnesium dissolution rate: Weigh 5.00 g of sample and place it in a 250 mL Erlenmeyer flask. Add 100 mL of pH 5.5 acetate-sodium acetate buffer solution. Shake in a constant temperature water bath at 25℃ for 30 minutes. Filter and use the filtrate to determine the total calcium and magnesium ions by EDTA titration. The theoretical calcium and magnesium content in the sample is calculated using the following formula: Theoretical calcium and magnesium content (%) = (mass of flotation phosphate tailings × mass fraction of calcium and magnesium in tailings) ÷ (mass of flotation phosphate tailings + equivalent mass of added phosphate + mass of pore expander + mass of anti-caking agent + mass of trace elements) × 100% The percentage of measured dissolution relative to theoretical content is calculated as the calcium and magnesium dissolution rate. This test uses a pH 5.5 buffer solution to simulate the environment of crop root exudates to evaluate the short-term nutrient release performance of the fertilizer.

[0074] 7. Anti-caking performance test: Weigh 100g of sample, place it in a self-sealing bag, flatten it, and apply a 10kg weight to subject the sample to a pressure of approximately 0.01MPa. Place it in an environment of 25℃ and 75% relative humidity for 7 days. After removal, pass it through a 10-mesh standard sieve, weigh the mass of agglomerated material on the sieve, and calculate the agglomeration rate.

[0075] V. Test Results Table 1 Performance test results of samples in Examples 1-6 Table 2 Performance test results of Comparative Examples 1–5 VI. Discussion of Results As shown in Table 1, the pH values ​​of the samples prepared in Examples 1-6 of this invention were all controlled within the target range of 4.9-5.1, meeting the design requirements for acidic fertilizers. The pass rate through a 200-mesh sieve was all above 98%, indicating that the product fineness meets the requirements for drip irrigation. The moisture content was ≤3%, meeting the moisture control standards for solid fertilizers. The sedimentation rate in water was between 7.9% and 12.5%, indicating that the product has good dispersibility in water and is not prone to sedimentation and clogging of drip irrigation equipment. The bulk density was between 0.59 and 0.62 g / cm³. 3 The product has a loose texture, which facilitates suspension in water. The calcium and magnesium dissolution rate is between 77.8% and 80.2%, indicating good nutrient release performance in a simulated root exudate environment. The clumping rate is between 3.1% and 8.6%.

[0076] Comparing samples B and C with sample A, it is evident that after adding the anti-caking agent, the sedimentation rate in water decreased from 12.3% to 8.2% and 7.9%, respectively, and the agglomeration rate decreased from 8.5% to 3.1% and 3.4%, respectively, demonstrating a significant improvement in anti-caking and dispersing properties. Comparing sample E with sample A, it is evident that aging in a carbon dioxide atmosphere resulted in a more stable pH value and a slight increase in calcium and magnesium dissolution rates. Comparing sample F with sample A, it is evident that urea, as a pore-expanding agent, also exhibits good pore-forming effects.

[0077] As shown in Table 2, in Comparative Example 1, without the addition of grinding aids and activators, the fineness of the powder decreased to 92.5%, while the bulk density increased to 0.78 g / cm³. 3 The sedimentation rate increased to 25.8%, and the calcium and magnesium dissolution rate decreased to 65.2%. In Comparative Example 2, the first acid solution was phosphoric acid, and the product pH value increased to 6.2, exceeding the acidic range, and the calcium and magnesium dissolution rate decreased to 58.3%. In Comparative Example 3, no pore-expanding agent was added, and the maximum bulk density was 0.85 g / cm³. 3 The sedimentation rate reached a maximum of 30.5%, indicating that the product has a dense structure and few pores. In Comparative Example 4, the second acid solution was added all at once, resulting in uneven reaction, a product pH value as high as 6.8, and the lowest calcium and magnesium dissolution rate of 51.2%. Comparative Example 5 used constant temperature and humidity aging, and all indicators were inferior to those of the Example.

[0078] The above results show that there is a synergistic effect among the steps in the technical solution of the present invention, and the absence or change of any key step will have a significant impact on product performance.

[0079] VII. Application Effect Verification Sample A prepared in Example 1 was applied to a blueberry planting experiment in brown soil with an initial pH of 8.3. The experiment included three treatments: a blank control (no fertilizer), conventional fertilization (40 kg / mu of NPK compound fertilizer (15-15-15) + 50 kg / mu of superphosphate, applied in two applications), and the fertilizer of this invention (20 kg / mu of Sample A, applied in two applications). Each treatment was replicated three times, with each plot measuring 30 m². 2 The fertilizer was applied via drip irrigation. Sixty days after application, the soil pH in the fertilizer area decreased to 7.1, compared to 8.2 in the control area and 8.0 in the conventional fertilization area. At harvest, the fertilizer area yielded 505 kg / mu, a 22.6% increase compared to the 412 kg / mu yield in the conventional fertilization area. No nozzle clogging occurred during drip irrigation.

[0080] Sample D, prepared in Example 4, was applied to a potato planting experiment in alluvial soil with an initial pH of 8.5. The experiment included three treatments: a blank control (no fertilizer), conventional fertilization (50 kg / mu of NPK compound fertilizer (15-15-15) + 20 kg / mu of potassium sulfate, applied as a single basal application), and the fertilizer of this invention (sample D, 25 kg / mu, applied as a single fertigation). Each treatment was replicated three times, with a plot area of ​​50 m². 2 The fertilizer was applied via fertigation. Forty-five days after application, the soil pH in the fertilizer area decreased to 7.3, compared to 8.4 in the control area and 8.2 in the conventional fertilization area. At harvest, the yield in the fertilizer area was 2780 kg / mu, a 17.8% increase compared to the 2360 kg / mu yield in the conventional fertilization area.

[0081] The above application effect verification shows that the acidic non-water-soluble medium element fertilizer prepared by the present invention can effectively reduce the pH value of alkaline soil, supplement medium elements such as calcium, magnesium, and phosphorus, promote crop growth and increase yield, and is suitable for modern agricultural fertilization methods such as drip irrigation and fertigation, and has good practical value.

[0082] The above description is merely a specific embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, any non-substantial improvements or equivalent substitutions based on the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A formulation for an acidic, non-water-soluble, medium-element fertilizer, characterized in that, The formulation consists of a modified product obtained from flotation phosphate tailings through grinding activation, low-temperature pre-acidification, medium-temperature deep activation, multi-stage aging, drying, and pulverization. The modified product has a pH value of 4.5–5.5 and a bulk density of 0.58 g / cm³. 3 ~0.63g / cm 3 The sedimentation rate in water is ≤13%.

2. A method for preparing an acidic, water-insoluble, medium-element fertilizer, characterized in that, Includes the following steps: S1. Mix the flotation phosphate tailings with the grinding aid and activator, then pulverize to 80-120 mesh to obtain activated phosphate tailings powder; S2. The activated phosphorus tailings powder is placed in a sealed container, and the first acid solution is sprayed in at 15℃~25℃. The liquid-solid mass ratio is controlled at 0.3:1~0.5:

1. The reaction is carried out for 2 hours to 4 hours. During the reaction, a slightly positive pressure environment of 0.1MPa~0.2MPa is maintained in the sealed container to obtain the surface acidified material. S3. Add a pore-expanding agent to the surface acidified material, mix evenly, and heat to 55℃~65℃. Then add the second acid solution in 4 to 6 portions, stirring for 15 to 20 minutes after each addition. Detect the pH value of the 1:10 aqueous suspension of the material. Add the next portion when the pH value is higher than 5.5, until all the second acid solution is added. Control the pH value of the material at the reaction endpoint to be 4.5~5.5 to obtain a porous activated material. S4. Spread the porous activated material into a 3cm-5cm thick layer and place it in a temperature and humidity controlled environment for multi-stage aging treatment to obtain aged material. S5. Dry the aged material at 60℃~65℃ until the moisture content is ≤5% to obtain the dried material; S6. The dried material is pulverized to 150-200 mesh to obtain an acidic, non-water-soluble medium-element fertilizer.

3. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, The grinding aid activator mentioned in S1 is triethanolamine, and the amount added is 0.8% to 1.2% of the mass of the flotation phosphate tailings.

4. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, The first acid solution in S2 is a composite acid solution, which is composed of phosphoric acid, citric acid, tartaric acid and water by mass percentage, wherein phosphoric acid accounts for 50% to 70%, citric acid accounts for 15% to 25%, tartaric acid accounts for 5% to 15%, and the sum of the three does not exceed 90%, and the total acid concentration is 12% to 18%.

5. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, The injection described in S2 is carried out by a spray method, and the carrier gas used for the spray is a mixed gas containing 10% to 20% carbon dioxide by volume.

6. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, The pore-expanding agent mentioned in S3 is ammonium bicarbonate or urea, and the amount added is 1% to 3% of the mass of the flotation phosphorus tailings; the second acid solution is a phosphoric acid solution with a mass concentration of 25% to 30%, and the total amount added is 5% to 15% of the mass of the flotation phosphorus tailings.

7. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, The multi-stage aging process described in S4 specifically includes: S41. Place the porous activated material in an environment with a temperature of 30℃~35℃ and a relative humidity of 80%~85% for 6 to 8 hours; S42. Adjust the temperature to 45℃~50℃ and the relative humidity to 55%~65%, and continue aging for 8 to 10 hours. S43. Adjust the temperature to 60℃~65℃ and the relative humidity to 30%~40%, and continue aging for 4 to 6 hours.

8. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 7, characterized in that, The entire aging process from S41 to S43 is carried out in an atmosphere containing 5% to 10% carbon dioxide by volume, with the atmosphere pressure maintained 0.01 MPa to 0.02 MPa higher than atmospheric pressure.

9. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, S6 is followed by S7: adding an anti-caking agent to the pulverized material and mixing it evenly using a pneumatic mixing method; the anti-caking agent is hydrophobically modified nano-silica or calcium stearate, and the amount added is 0.3% to 0.8% of the material mass.

10. The method for preparing acidic, water-insoluble, medium-element fertilizer according to claim 2, characterized in that, S6 is followed by S7: adding a trace element composition to the pulverized material and mixing it evenly; The trace element composition comprises at least two of zinc sulfate, borax, manganese sulfate, EDTA-iron, EDTA-copper, and ammonium molybdate, with each compound added in an amount of 0.05% to 0.2% of the material mass.

Images