Potassium nitrate liquid fertilizer and method for improving stability thereof

Abstract

The application discloses a kind of liquid potassium nitrate fertilizer and its stability improvement method, to improve the problem that high-concentration liquid potassium nitrate fertilizer is easy to crystallize, precipitate and effective component is unevenly distributed during low-temperature storage and long-term placement.The liquid fertilizer includes: 20%~35% of potassium nitrate, 5%~15% of ammonium nitrate, 0.5%~3% of polyethylene glycol, 0.01%~0.10% of 3,4-dimethyl pyrazole phosphate, 5%~12% of glycerol, 1%~3% of chelating stabilizer and deionized water by total weight.Polyethylene glycol number average molecular weight is 200~1000, and chelating stabilizer is sodium citrate and EDTA disodium compound.Preparation first disperses DMPP in glycerol to form mother liquor, then successively prepares chelating stabilizer aqueous solution, adds polyethylene glycol, batch dissolves potassium nitrate and ammonium nitrate, and adds mother liquor after cooling.The fertilizer has good low-temperature anti-crystallization, long-term storage stability and effective component retention rate.

Description

Technical Field

[0001] This invention belongs to the field of liquid fertilizer technology, specifically relating to a high-concentration potassium nitrate liquid fertilizer and a method for improving its stability. Background Technology

[0002] Liquid fertilizers, due to their uniform nutrient content, convenient application, and compatibility with integrated water and fertilizer systems such as drip irrigation and sprinkler irrigation, have seen increasingly widespread use in facility agriculture, fruit tree cultivation, and field drip irrigation in recent years. Potassium nitrate (KNO3) provides both nitrate nitrogen and readily available potassium. Nitrate nitrogen can be directly absorbed by crops without conversion, while potassium ions contribute to fruit enlargement and quality improvement. Therefore, liquid fertilizers based on potassium nitrate occupy an important position in integrated water and fertilizer products.

[0003] However, high-concentration potassium nitrate systems are quite sensitive to temperature changes. During low-temperature storage, transportation, or field application, potassium nitrate is prone to crystallization due to changes in dissolution equilibrium. Crystallization not only reduces the uniformity of available nutrients in liquid fertilizers but can also clog drip irrigation pipes, filters, or sprinklers, thereby affecting the uniformity of fertilization and its actual utilization effect.

[0004] Therefore, it is necessary to provide a potassium nitrate liquid fertilizer system that combines high potassium nitrate content with low-temperature storage stability and long-term storage stability. Summary of the Invention

[0005] The purpose of this invention is to provide a potassium nitrate liquid fertilizer and a method for improving its stability, so as to solve the problems that existing high-concentration potassium nitrate liquid fertilizers are prone to crystallization, precipitation, stratification or uneven distribution of effective components during low-temperature storage, long-term storage or fertigation.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A potassium nitrate liquid fertilizer. Based on the total weight of the liquid fertilizer, the potassium nitrate liquid fertilizer comprises the following components: Potassium nitrate 20%–35%; Ammonium nitrate 5%–15%; Polyethylene glycol 0.5%–3%; 3,4-Dimethylpyrazole phosphate 0.01%–0.10%; Glycerol 5%–12%; Chelating stabilizer 1%–3%; Deionized water should be replenished to 100%. The number average molecular weight of polyethylene glycol is 200-1000; the chelating stabilizer is a compound of sodium citrate and disodium EDTA.

[0007] Compared with existing technologies, this invention achieves better low-temperature stability, storage stability and dispersion uniformity of liquid fertilizer by synergistically combining potassium nitrate, ammonium nitrate, glycerol, polyethylene glycol, DMPP and compound chelating stabilizers, while maintaining high nitrogen and potassium nutrient content.

[0008] Potassium nitrate, as the primary source of nitrogen and potassium, provides crops with nitrate nitrogen and available potassium. Ammonium nitrate, as an auxiliary nitrogen source, enhances the continuity of nitrogen supply in fertilizers, enabling the product to supply both nitrate and ammonium nitrogen. Glycerol molecules contain multiple hydroxyl groups, which can form hydrogen bonds with water molecules, reducing the free water activity in the system and improving the low-temperature fluidity of high-salt systems, thereby delaying the crystallization process of potassium nitrate at decreasing temperatures. Controlling the glycerol content within the range of 5%–12% improves the solubility and low-temperature storage stability of potassium nitrate while avoiding excessive addition of glycerol that could lead to excessively high system viscosity or adverse effects on dissolution equilibrium.

[0009] The introduction of polyethylene glycol (PEG) can further improve the crystal growth control effect in high-concentration salt solutions. The PEG backbone contains ether oxygen structures, which can interact with ions in the solution and the surface of primary crystal nuclei. When primary crystal nuclei form in the system under low-temperature or long-term storage conditions, PEG molecular chains can adsorb or entangle on the surface of the nuclei to a certain extent, forming a steric hindrance effect that inhibits further crystal growth, thereby reducing the risk of visible crystallization and precipitation. Limiting the number-average molecular weight of PEG to 200–1000 is beneficial for balancing molecular chain flexibility, solubility, and crystal growth inhibition, avoiding insufficient steric hindrance due to excessively low molecular weight, and avoiding increased system viscosity or decreased dispersibility due to excessively high molecular weight.

[0010] DMPP, as a nitrification inhibitor, can enter the crop root zone during fertigation with liquid fertilizer, inhibiting ammonia oxidation in the soil. This helps slow the conversion of ammonium nitrogen to nitrate nitrogen, reduces the risk of nitrogen leaching loss, and improves nitrogen retention when used in combination with urea, ammonium nitrogen fertilizer, or soil background ammonium nitrogen. This invention controls the DMPP dosage to 0.01%–0.10%, ensuring its nitrification inhibition function while reducing the risk of uneven dispersion or the formation of solid particles due to excessively high local concentrations in high-salinity water phases.

[0011] The chelating stabilizer is a compound of sodium citrate and disodium EDTA. Sodium citrate has good buffering and chelating properties, while disodium EDTA has a strong metal ion chelating ability. The combination of these two compounds can chelate trace metal ions such as calcium, magnesium, iron, and aluminum that may be present in the system, reducing the possibility of metal ions forming small insoluble substances with phosphate, carbonate, or other anions. This reduces the risk of heterogeneous nucleation and maintains the clarity and storage stability of the liquid fertilizer.

[0012] Furthermore, the number average molecular weight of polyethylene glycol is 200–600.

[0013] By adopting the above technical solution, polyethylene glycol can effectively adsorb and sterically hinder the formation of primary crystal nuclei of potassium nitrate while maintaining good water solubility and low system viscosity. Compared with higher molecular weight polyethylene glycol, polyethylene glycol with a number average molecular weight of 200-600 is more easily and uniformly distributed in high-salt water phase systems, which helps to reduce the tendency of liquid fertilizer to crystallize during low-temperature storage and recovery to room temperature, and maintains good fluidity and drip irrigation suitability.

[0014] Furthermore, the amount of polyethylene glycol used is 1.0% to 2.0% of the total weight of the fertilizer.

[0015] After adopting the above technical solution, polyethylene glycol can form a relatively stable crystal growth inhibition environment in the system. When the amount of polyethylene glycol is lower than this range, the inhibition effect on crystal nucleus growth may be insufficient; when the amount is too high, it may increase the viscosity of the system and affect the fluidity of the fertilizer at low temperatures and the convenience of subsequent dilution and use. Therefore, controlling the polyethylene glycol content at 1.0% to 2.0% is beneficial to achieving a better balance between anti-crystallization performance, system viscosity, and fertigation delivery performance.

[0016] Furthermore, the mass ratio of polyethylene glycol to glycerol is 1:3 to 1:8.

[0017] After adopting the above technical solution, the regulatory effect of glycerol on the aqueous environment and the dissolution balance of potassium nitrate can be combined with the inhibitory effect of polyethylene glycol on the growth of primary crystal nuclei. Glycerol mainly improves the low-temperature stability of the high-salt potassium nitrate system by regulating the hydrogen bond network of water molecules and the free water activity through its polyhydroxy structure; while polyethylene glycol mainly inhibits the continued growth of crystals through molecular chain adsorption and steric hindrance. Controlling the mass ratio of the two to 1:3 to 1:8 can avoid insufficient low-temperature protection due to too low a proportion of glycerol, and can also avoid the increase in system viscosity or the impact on dissolution balance due to too high a proportion of glycerol, thereby improving the stability of fertilizer under low-temperature storage and long-term storage conditions.

[0018] Furthermore, the mass ratio of sodium citrate to disodium EDTA is 1:1 to 4:1.

[0019] Using the above technical solution, sodium citrate and disodium EDTA can form a compound chelate system that combines buffering, complexation, and system stabilization functions. Disodium EDTA preferentially complexes polyvalent metal ions in the system, reducing metal ion-induced precipitation and heterogeneous nucleation; sodium citrate helps maintain the stability of the system in a weakly acidic to neutral environment and improves the compatibility between the chelate system and the fertilizer salt system. Limiting the mass ratio of sodium citrate to disodium EDTA to 1:1 to 4:1 is beneficial for balancing complexation strength, pH stability, and cost control, and reduces the compatibility problems that may arise from excessive use of a single chelating agent.

[0020] Furthermore, the amount of DMPP used is 0.015% to 0.07% of the total fertilizer weight.

[0021] By adopting the above technical solution, DMPP can exert its nitrification inhibition effect at a relatively low addition amount and is evenly distributed into the crop root zone during drip irrigation or fertigation with potassium nitrate liquid fertilizer. This dosage range is beneficial for slowing down the conversion of ammonium nitrogen to nitrate nitrogen in the soil, reducing the risk of nitrogen leaching, and also avoids the problems of poor dispersion, local enrichment, or decreased storage stability caused by excessive DMPP addition in high-salt liquid fertilizer systems. Therefore, this preferred range helps to balance nitrification inhibition effect and liquid fertilizer system stability.

[0022] Furthermore, the liquid fertilizer has a pH of 5.5–7.5 and a density of 1.25–1.45 g / mL at 25°C.

[0023] After adopting the above technical solution, the liquid fertilizer is in a weakly acidic to neutral environment, which is conducive to the compatibility and stability of DMPP, potassium nitrate, ammonium nitrate, glycerol, polyethylene glycol, and chelating stabilizers in the same system. If the pH is too low, it may affect the chemical stability and application safety of some components; if the pH is too high, it may increase the risk of metal ion precipitation, system turbidity, or decreased DMPP stability. Maintaining the pH between 5.5 and 7.5 is beneficial for maintaining the clarity and component stability of the fertilizer during storage. Maintaining the density between 1.25 and 1.45 g / mL is beneficial for increasing the nutrient content per unit volume while maintaining suitable flowability, metering properties, and drip irrigation suitability.

[0024] The present invention also provides a method for improving the stability of the above-mentioned potassium nitrate liquid fertilizer, comprising the following steps: Step 1: Add DMPP to glycerol and stir at 30-50°C for 20-40 min to obtain DMPP-glycerol mother liquor; Step 2: Heat deionized water to 50-60°C, add chelating stabilizer and stir to obtain chelating stabilizer aqueous solution; Step 3: Add polyethylene glycol to the solution obtained in Step 2 and stir for 10-20 min to obtain an aqueous solution containing polyethylene glycol; Step 4: Add potassium nitrate in batches to the aqueous solution containing polyethylene glycol from Step 3, and stir to dissolve; Step 5: Add ammonium nitrate to the solution obtained in Step 4 in batches, stir to dissolve, and then cool to 25-30℃; Step 6: Add the DMPP-glycerol mother liquor obtained in Step 1 to the solution obtained in Step 5, stir for 20-30 minutes, and filter to obtain potassium nitrate liquid fertilizer.

[0025] By employing the aforementioned stability improvement method, the risks of uneven dispersion and heterogeneous nucleation caused by directly adding DMPP to a high-concentration potassium nitrate aqueous system can be avoided. DMPP has relatively poor dispersibility in water; if directly added to a high-concentration potassium nitrate solution, insufficiently dispersed DMPP particles may act as crystal nucleation sites, inducing potassium nitrate crystallization. This invention first utilizes the polar solvent properties of glycerol to pre-disperse DMPP, forming a DMPP-glycerol mother liquor, which is then added to the cooled high-salt water phase. This improves the uniformity of DMPP dispersion in liquid fertilizers and reduces the possibility of solid particles inducing crystallization.

[0026] Simultaneously, this invention first adds a chelating stabilizer to the aqueous phase to preferentially complex any trace metal ions that may be present in the system. Then, polyethylene glycol is added to form a stable aqueous protective environment, followed by the batch addition of potassium nitrate and ammonium nitrate. This order of addition helps reduce incomplete dissolution and localized supersaturation caused by localized aggregation of high-concentration salts, and also reduces the risk of crystallization during low-temperature cooling. After the potassium nitrate and ammonium nitrate dissolve, the temperature is lowered to 25–30°C, and the DMPP-glycerol mother liquor is introduced within this temperature range. This helps reduce the stability decrease caused by prolonged exposure of DMPP to high-temperature and high-salt environments.

[0027] Furthermore, in step four, the system temperature is not lower than 45°C when potassium nitrate is added, and potassium nitrate is added in batches at a rate of 5% to 20% of its total addition amount per minute; and / or, in step five, ammonium nitrate is added in batches at a rate of 3% to 15% of its total addition amount per minute.

[0028] By adopting the above technical solution, potassium nitrate can dissolve gradually at a relatively high system temperature, reducing the risk of excessively high local concentrations and undissolved solid residues caused by adding a large amount of potassium nitrate at once. A system temperature of at least 45°C is beneficial for increasing the potassium nitrate dissolution rate and reducing the risk of local supersaturation and crystal nucleation during dissolution. Controlling the potassium nitrate addition rate to 5%–20% of the total addition amount per minute achieves a balance between preparation efficiency and dissolution stability, avoiding both excessively rapid addition leading to local supersaturation and excessively slow addition resulting in a long preparation cycle.

[0029] Ammonium nitrate is added in batches at a rate of 3%–15% of its total addition volume per minute. This helps control the rate of change in the ionic strength of the system and reduces the impact of instantaneous salt enrichment on the system's stability. Both potassium nitrate and ammonium nitrate are added in batches, which allows the high-salt system to gradually reach the target concentration, improving the controllability of the preparation process and reducing the risk of crystallization or precipitation during later storage.

[0030] Furthermore, in step six, when adding the DMPP-glycerol mother liquor, the system temperature should not exceed 35°C.

[0031] By adopting the above technical solution, the risk of DMPP undergoing changes in dispersion or decreased stability under high temperature and high salt conditions can be reduced. Pre-dispersing DMPP in glycerol and adding it when the system is cooled to no higher than 35°C allows it to enter the potassium nitrate-ammonium nitrate high-salt system under milder conditions, which is beneficial for maintaining the uniform distribution of DMPP in the finished fertilizer. Simultaneously, this temperature condition also helps reduce the disturbance of dissolution equilibrium caused by local temperature changes after adding the mother liquor, thereby further improving the storage stability of the liquid fertilizer.

[0032] Based on the technical solution of the above invention, it can be seen that the invention is characterized by constructing a high-nitrogen-potassium liquid fertilizer system using potassium nitrate and ammonium nitrate. Through the synergistic effect of glycerol, polyethylene glycol of a specific molecular weight, and a sodium citrate-EDTA disodium compound chelating stabilizer, the risk of crystallization, precipitation, and turbidity in the high-salt system during low-temperature storage, long-term placement, and transportation is reduced from three aspects: free water state regulation, crystal growth inhibition, and metal ion complexation. Simultaneously, the introduction of DMPP gives the liquid fertilizer nitrification inhibition function, which helps improve nitrogen retention when applied in combination with urea, ammonium nitrogen fertilizer, or soil background ammonium nitrogen. Furthermore, the pre-dispersion of DMPP in glycerol and the addition of the high-salt water phase at a lower temperature improves its dispersion uniformity in the finished product, reducing the possibility of undispersed particles inducing heterogeneous nucleation of potassium nitrate. Therefore, the potassium nitrate liquid fertilizer obtained by this invention possesses high nutrient concentration, good low-temperature storage stability, long-term storage stability, and adaptability to fertigation. Attached Figure Description

[0033] Figure 1 The retention rate of active ingredients after high-temperature accelerated storage in Examples 1-5 and Comparative Examples 1-4. Detailed Implementation

[0034] The present invention will be further described below with reference to embodiments, but this does not limit the present invention.

[0035] Example 1

[0036] Add 0.2 g DMPP to 100.0 g glycerol and stir at 30 °C for 20 min to obtain DMPP-glycerol mother liquor. 1319.8 g of deionized water was heated to 50°C, and 10.0 g of sodium citrate and 10.0 g of disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 10.0 g of PEG with a number average molecular weight of 200 was added to the aqueous solution of chelating stabilizer and stirred for 10 min to obtain an aqueous solution containing PEG. Under the condition that the system temperature is not lower than 45°C, 400.0 g of potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition and stirred to dissolve. Then, 100.0 g of ammonium nitrate was added in batches at a rate of 5% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 25°C. Finally, DMPP-glycerol mother liquor was added and stirred for 20 min. The volume was made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, potassium nitrate liquid fertilizer was obtained.

[0037] In this embodiment, the potassium nitrate content is 20%, the ammonium nitrate content is 5%, the PEG content is 0.5%, the DMPP content is 0.01%, the glycerol content is 5%, the chelating stabilizer content is 1%, the PEG number average molecular weight is 200, and the mass ratio of sodium citrate to disodium EDTA is 1:1.

[0038] Example 2

[0039] Add 0.8 g DMPP to 150.0 g glycerol and stir at 40 °C for 30 min to obtain DMPP-glycerol mother liquor. 969.2 g of deionized water was heated to 55°C, and 28.0 g of sodium citrate and 12.0 g of disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 30.0 g of PEG with a number average molecular weight of 400 was added to the aqueous solution of chelating stabilizer and stirred for 15 min to obtain an aqueous solution containing PEG. Under the condition that the system temperature is not lower than 45°C, 560.0 g of potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition and stirred to dissolve. Then, 200.0 g of ammonium nitrate was added in batches at a rate of 8% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 28°C. Finally, DMPP-glycerol mother liquor was added and stirred for 25 min. The volume was made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, potassium nitrate liquid fertilizer was obtained.

[0040] In this embodiment, the potassium nitrate content is 28%, the ammonium nitrate content is 10%, the PEG content is 1.5%, the DMPP content is 0.04%, the glycerol content is 7.5%, the chelating stabilizer content is 2%, the PEG number average molecular weight is 400, the mass ratio of PEG to glycerol is 1:5, and the mass ratio of sodium citrate to disodium EDTA is 2.33:1.

[0041] Example 3

[0042] 1.4 g of DMPP was added to 240.0 g of glycerol and stirred at 50 °C for 40 min to obtain a DMPP-glycerol mother liquor. 608.6 g of deionized water was heated to 60 °C, and 48.0 g of sodium citrate and 12.0 g of disodium EDTA were added and stirred to obtain a chelating stabilizer aqueous solution. 40.0 g of PEG with a number-average molecular weight of 600 was added to the chelating stabilizer aqueous solution and stirred for 20 min to obtain a PEG-containing aqueous solution. Under conditions where the system temperature was not lower than 45 °C, 700.0 g of potassium nitrate was added in batches at a rate of 5% / min of the total potassium nitrate addition, and stirred until dissolved. Subsequently, 300.0 g of ammonium nitrate was added in batches at a rate of 3% / min of the total ammonium nitrate addition, and stirred until dissolved before cooling to 30 °C. Finally, the DMPP-glycerol mother liquor was added, stirred for 30 min, and the volume was brought to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, potassium nitrate liquid fertilizer was obtained.

[0043] In this embodiment, the potassium nitrate content is 35%, the ammonium nitrate content is 15%, the PEG content is 2%, the DMPP content is 0.07%, the glycerol content is 12%, the chelating stabilizer content is 3%, the PEG number average molecular weight is 600, the mass ratio of PEG to glycerol is 1:6, and the mass ratio of sodium citrate to disodium EDTA is 4:1.

[0044] Example 4

[0045] Add 2.0 g DMPP to 180.0 g glycerol and stir at 45 °C for 40 min to obtain DMPP-glycerol mother liquor. 828.0 g of deionized water was heated to 58℃, and 30.0 g of sodium citrate and 10.0 g of disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 60.0 g of PEG with a number average molecular weight of 1000 was added to the aqueous solution of chelating stabilizer and stirred for 20 min to obtain an aqueous solution containing PEG. Under the condition that the system temperature is not lower than 45℃, 600.0 g of potassium nitrate was added in batches at a rate of 15% / min of the total potassium nitrate addition and stirred to dissolve. Then, 240.0 g of ammonium nitrate was added in batches at a rate of 10% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 30℃. Finally, DMPP-glycerol mother liquor was added and stirred for 30 min. The volume was made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, potassium nitrate liquid fertilizer was obtained.

[0046] In this embodiment, the potassium nitrate content is 30%, the ammonium nitrate content is 12%, the PEG content is 3%, the DMPP content is 0.10%, the glycerol content is 9%, the chelating stabilizer content is 2%, the PEG number average molecular weight is 1000, the mass ratio of PEG to glycerol is 1:3, and the mass ratio of sodium citrate to disodium EDTA is 3:1.

[0047] Example 5

[0048] Add 0.3 g DMPP to 168.0 g glycerol and stir at 35 °C for 25 min to obtain DMPP-glycerol mother liquor. 927.7 g of deionized water was heated to 55°C, and 20.0 g of sodium citrate and 10.0 g of disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 24.0 g of PEG with a number average molecular weight of 400 was added to the aqueous solution of chelating stabilizer and stirred for 15 min to obtain an aqueous solution containing PEG. Under the condition that the system temperature is not lower than 45°C, 640.0 g of potassium nitrate was added in batches at a rate of 20% / min of the total potassium nitrate addition and stirred to dissolve. Then, 160.0 g of ammonium nitrate was added in batches at a rate of 15% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 25°C. Finally, DMPP-glycerol mother liquor was added and stirred for 20 min. The volume was made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, potassium nitrate liquid fertilizer was obtained.

[0049] In this embodiment, the potassium nitrate content is 32%, the ammonium nitrate content is 8%, the PEG content is 1.2%, the DMPP content is 0.015%, the glycerol content is 8.4%, the chelating stabilizer content is 1.5%, the PEG number average molecular weight is 400, the mass ratio of PEG to glycerol is 1:7, and the mass ratio of sodium citrate to disodium EDTA is 2:1.

[0050] Comparative Example 1 1119.2 g of deionized water was heated to 55°C, and 28.0 g of sodium citrate and 12.0 g of disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 30.0 g of PEG with a number average molecular weight of 400 was added to the aqueous solution of chelating stabilizer and stirred for 15 min to obtain an aqueous solution containing PEG. Under the condition that the system temperature is not lower than 45°C, 560.0 g of potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition and stirred to dissolve. Then, 200.0 g of ammonium nitrate was added in batches at a rate of 8% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 28°C. 0.8 g of DMPP was added and stirred for 25 min. The volume was made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, the liquid fertilizer of Comparative Example 1 was obtained.

[0051] Comparative Example 2 0.8 g DMPP was added to 150.0 g glycerol and stirred at 40 °C for 30 min to obtain DMPP-glycerol mother liquor. 999.2 g deionized water was heated to 55 °C, and 28.0 g sodium citrate and 12.0 g disodium EDTA were added and stirred to obtain a chelating stabilizer aqueous solution. While maintaining a system temperature not lower than 45 °C, 560.0 g potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition, and stirred until dissolved. Subsequently, 200.0 g ammonium nitrate was added in batches at a rate of 8% / min of the total ammonium nitrate addition, and stirred until dissolved before cooling to 28 °C. Finally, the DMPP-glycerol mother liquor was added, stirred for 25 min, and the volume was brought to 2.000 kg using the reserved 50.0 g deionized water. After filtration, the liquid fertilizer of Comparative Example 2 was obtained.

[0052] Comparative Example 3 0.8 g DMPP was added to 150.0 g glycerol and stirred at 40 °C for 30 min to obtain DMPP-glycerol mother liquor. 969.2 g deionized water was heated to 55 °C, and 28.0 g sodium citrate and 12.0 g disodium EDTA were added and stirred to obtain an aqueous solution of chelating stabilizer. 30.0 g PEG with a number average molecular weight of 2000 was added to the aqueous solution of chelating stabilizer and stirred for 15 min to obtain an aqueous solution containing PEG. Under conditions where the system temperature was not lower than 45 °C, 560.0 g potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition and stirred until dissolved. Subsequently, 200.0 g ammonium nitrate was added in batches at a rate of 8% / min of the total ammonium nitrate addition and stirred until dissolved, then cooled to 28 °C. Finally, the DMPP-glycerol mother liquor was added and stirred for 25 min. The volume was then brought to 2.000 kg using the reserved 50.0 g deionized water. After filtration, the liquid fertilizer of Comparative Example 3 was obtained.

[0053] Comparative Example 4 969.2 g of deionized water was heated to 55°C, and 28.0 g of sodium citrate and 12.0 g of disodium EDTA were added. The mixture was stirred to obtain an aqueous solution of chelating stabilizer. 30.0 g of PEG with a number average molecular weight of 400 was added to the aqueous solution of chelating stabilizer. After stirring for 15 min, 150.0 g of glycerol was added and stirring was continued. Under the condition that the system temperature was not lower than 45°C, 560.0 g of potassium nitrate was added in batches at a rate of 10% / min of the total potassium nitrate addition and stirred to dissolve. Then, 200.0 g of ammonium nitrate was added in batches at a rate of 8% / min of the total ammonium nitrate addition and stirred to dissolve. The temperature was then lowered to 28°C. 0.8 g of DMPP was added directly and stirred for 25 min. The volume was then made up to 2.000 kg using the reserved 50.0 g of deionized water. After filtration, the liquid fertilizer of Comparative Example 4 was obtained.

[0054] Test methods Stability test at room temperature: Take 500 mL of each example and comparative sample, place them in a transparent sealed polyethylene bottle, and store at 25°C in the dark for 6 months. During storage, observe the appearance of the samples every 30 days and record whether crystallization, precipitation, layering, turbidity, or obvious color change occurs. After 6 months, filter the samples through a 100-mesh sieve and observe whether there are visible crystals or precipitates on the sieve.

[0055] Low-temperature storage stability test: Take 500 mL of each sample and store it at 0℃ for 30 days. After storage, restore it to 25℃ and let it stand for 24 h to observe whether there is crystallization, precipitation, turbidity or stratification. Then filter 100 mL of the sample through a 0.45 μm filter membrane, dry the filter membrane at 60℃ to constant weight, and weigh the filter residue to evaluate the amount of crystallization at low temperature.

[0056] Thermal cycling stability test: Take 500 mL of each sample and perform thermal cycling test. Each cycle includes: placing at -5℃ for 12 h, placing at 25℃ for 12 h, for a total of 10 cycles. After the cycle is completed, return to 25℃, observe the appearance of the sample, and determine the mass of the filter residue.

[0057] Accelerated High-Temperature Storage Test: 500 mL of each sample was stored at 45°C in the dark for 30 days. The DMPP content, nitrate nitrogen content, and potassium content were measured before and after the test, and the retention rate of the active ingredient was calculated. The retention rate of the active ingredient was calculated using the following formula: Active ingredient retention rate (%) = (Actual content after storage / Actual content before storage) × 100% Among them, the DMPP content was determined by high performance liquid chromatography; the nitrate nitrogen content was determined by ion chromatography; and the potassium content was determined by flame photometry or inductively coupled plasma atomic emission spectrometry.

[0058] pH and density tests: pH was measured using a pH meter at 25°C; density was measured using a densitometer at 25°C.

[0059] Table 1. Results of appearance, pH and density tests for Examples 1-5 and Comparative Examples 1-4 As shown in the table, the liquid fertilizers obtained in Examples 1-5 were all clear and transparent at 25°C, with pH maintained in the range of 5.5-7.5 and density maintained in the range of 1.25-1.45 g / mL. In Comparative Example 1, due to the absence of glycerol, the dispersion of DMPP in the system was poor, and the initial sample showed slight turbidity and a small amount of fine suspended matter. Although Comparative Example 2 was initially basically clear, the lack of PEG resulted in insufficient inhibition of crystal growth under subsequent low-temperature and thermal cycling conditions. In Comparative Example 3, the high molecular weight of PEG increased the viscosity of the system. In Comparative Example 4, because DMPP was not pre-dispersed in glycerol, the initial sample showed a small amount of fine suspended matter.

[0060] Table 2. Stability test results of Examples 1-5 and Comparative Examples 1-4 after 6 months of storage at room temperature. The test results show that Examples 1-5 did not exhibit visible crystallization, precipitation, or stratification after 6 months of storage at 25°C, indicating that the combination of glycerol, specific molecular weight PEG, compound chelating stabilizer, and DMPP pre-dispersion process can effectively improve the long-term storage stability of high-concentration potassium nitrate liquid fertilizer. Comparative Example 1, without the addition of glycerol, had insufficient adjustment of the free water state, and DMPP struggled to form a stable pre-dispersion system, resulting in significant turbidity and fine crystals after storage. Comparative Example 2, without the addition of PEG, lacked steric hindrance to inhibit the continued growth of primary crystal nuclei, leading to the appearance of a small amount of fine crystals after storage. Comparative Example 3, using excessively high molecular weight PEG, did not form obvious crystals, but increased viscosity. In Comparative Example 4, DMPP was directly added to the high-salt water phase, easily forming unevenly dispersed particles and inducing potassium nitrate crystallization during storage.

[0061] Table 3. Results of Low-Temperature Storage Stability Tests for Examples 1-5 and Comparative Examples 1-4 Low-temperature test results showed that none of Examples 1-5 exhibited significant crystallization after storage at 0℃ for 30 days, and remained clear even after being restored to 25℃, indicating low filter residue quality. Comparative Example 1 showed the highest filter residue quality at low temperatures, demonstrating that glycerol plays a crucial role in reducing the risk of potassium nitrate crystallization under low-temperature conditions. Without glycerol, the free water state and low-temperature solubility stability of the system decreased, making potassium nitrate more prone to precipitation. Comparative Example 2, without PEG, lacked effective inhibition of primary crystal nuclei growth under low-temperature conditions, resulting in significantly higher filter residue quality. In Comparative Example 3, the PEG molecular weight was too high, leading to poor system fluidity at low temperatures and a higher filter residue quality than the examples. Comparative Example 4 indicated that directly adding DMPP to the high-salt water phase increases the risk of heterogeneous nucleation, but its deterioration degree is lower than that of the systems without glycerol or PEG.

[0062] Table 4. Results of thermal cycling stability tests for Examples 1-5 and Comparative Examples 1-4 The results of the thermal cycling further demonstrate that the examples still exhibit good anti-crystallization performance under repeated temperature fluctuations. Comparative Example 1 showed a significant increase in filter cake mass after thermal cycling, indicating that without the addition of glycerol, the system is unable to buffer the disturbances to the potassium nitrate dissolution equilibrium caused by repeated cooling and heating processes. Comparative Example 2, lacking PEG, allowed crystal nuclei to continue growing more easily during temperature fluctuations, resulting in a significant increase in filter cake mass. Comparative Example 3 did not show a large number of crystals, but its viscosity increased significantly, and its low-temperature fluidity was poor. In Comparative Example 4, because DMPP was not pre-dispersed, fine particles could act as heterogeneous nucleation sites during thermal cycling, inducing a small amount of potassium nitrate crystallization.

[0063] Depend on Figure 1The results show that after accelerated storage at 45°C for 30 days, the DMPP retention rate in Examples 1-5 was not less than 95%, and the retention rates of nitrate nitrogen and potassium content were all higher than 98%. In Comparative Example 1, due to the lack of glycerol, the DMPP lacked a suitable pre-dispersion medium, resulting in a significantly reduced retention rate after high-temperature storage. In Comparative Example 2, although the DMPP was pre-dispersed with glycerol, the lack of PEG resulted in insufficient anti-crystallization and dispersion stability, leading to a lower retention rate of effective components compared to Examples 1-5. In Comparative Example 3, the high molecular weight of PEG affected the viscosity and dispersion state of the system, resulting in a lower DMPP retention rate than Example 2. In Comparative Example 4, the direct addition of DMPP to the high-salt water phase increased the risk of local agglomeration or uneven distribution, leading to a significantly lower DMPP retention rate than Example 2, which used a DMPP-glycerol mother liquor.

[0064] As can be seen from Examples 1-5, potassium nitrate liquid fertilizers with clear appearance and good storage stability can be prepared when the potassium nitrate content is within the range of 20%-35%, ammonium nitrate content is within 5%-15%, PEG content is within 0.5%-3%, DMPP content is within 0.01%-0.10%, glycerol content is within 5%-12%, and chelating stabilizer content is within 1%-3%. No visible crystallization, precipitation, or stratification was observed in any of the examples after 6 months of storage at room temperature. After 30 days of storage at 0℃ and 10 cycles of thermal cycling at -5℃ / 25℃, the filter residue quality was significantly lower than that of the comparative example. After accelerated storage at 45℃ for 30 days, the DMPP retention rate was not less than 95%, and the nitrate nitrogen and potassium content retention rates were both higher than 98%.

[0065] Examples 2 and 5 exhibited better overall stability, indicating that when the molecular weight of PEG is 200-600, the amount of PEG is 1.0%-2.0%, the mass ratio of PEG to glycerol is 1:3-1:8, and the amount of DMPP is 0.015%-0.07%, the system can achieve a good balance between anti-crystallization performance, DMPP stability, flowability, and storage stability.

[0066] Comparative Example 1 shows that glycerol plays an important role in the low-temperature stability of high-concentration potassium nitrate liquid fertilizer and the dispersion stability of DMPP. Without the addition of glycerol, the system lacks regulation of the free water state and low-temperature dissolution equilibrium. At the same time, DMPP cannot form DMPP-glycerol mother liquor, which makes it more prone to turbidity, fine crystals and increased filter residue during storage and thermal cycling.

[0067] Comparative Example 2 shows that polyethylene glycol plays an important role in inhibiting the continued growth of primary potassium nitrate crystal nuclei. Without the addition of polyethylene glycol, although DMPP can be pre-dispersed in glycerol, the system lacks molecular chain adsorption and steric hindrance, resulting in a significant increase in crystallization after low-temperature storage and thermal cycling.

[0068] Comparative Example 3 shows that when the molecular weight of polyethylene glycol exceeds the range of 200-1000, the viscosity of the system increases, the low-temperature fluidity decreases, and the overall storage stability and application adaptability are lower than those of the Examples.

[0069] Comparative Example 4 shows that when DMPP is added directly to a high-salt water phase without being pre-dispersed in glycerol, it is easy to form unevenly dispersed fine particles. These particles can act as heterogeneous nucleation sites, increasing the risk of potassium nitrate crystallization and reducing the retention rate of DMPP after accelerated storage at high temperature.

[0070] In summary, this invention effectively improves the low-temperature stability, long-term storage stability, and effective component retention rate of high-concentration potassium nitrate liquid fertilizer through a combination of techniques including low-temperature stability regulation with glycerol, inhibition of crystal growth of polyethylene glycol with a specific molecular weight, complexation stabilization with sodium citrate-disodium EDTA, and pre-dispersion with DMPP-glycerol.

Claims

1. A potassium nitrate liquid fertilizer, characterized in that, Based on the total weight of the liquid fertilizer, it includes the following components: potassium nitrate 20%–35%; ammonium nitrate 5%–15%; polyethylene glycol 0.5%–3%; 3,4-dimethylpyrazole phosphate (DMPP) 0.01%–0.10%; glycerol 5%–12%; chelating stabilizer 1%–3%; deionized water to bring the total to 100%. The number-average molecular weight of the polyethylene glycol is 200–1000; The chelating stabilizer is a compound of sodium citrate and disodium EDTA.

2. The potassium nitrate liquid fertilizer according to claim 1, characterized in that, The number-average molecular weight of the polyethylene glycol is 200–600.

3. The potassium nitrate liquid fertilizer according to claim 2, characterized in that, The amount of polyethylene glycol used is 1.0% to 2.0% of the total weight of the fertilizer.

4. The potassium nitrate liquid fertilizer according to claim 3, characterized in that, The mass ratio of polyethylene glycol to glycerol is 1:3 to 1:

8.

5. The potassium nitrate liquid fertilizer according to any one of claims 1 to 4, characterized in that, The mass ratio of sodium citrate to disodium EDTA is 1:1 to 4:

1.

6. The potassium nitrate liquid fertilizer according to claim 1, characterized in that, The amount of DMPP used is 0.015% to 0.07% of the total fertilizer weight.

7. The potassium nitrate liquid fertilizer according to claim 1, characterized in that, The liquid fertilizer has a pH of 5.5–7.5 at 25°C and a density of 1.25–1.45 g / mL.

8. A method for improving the stability of potassium nitrate liquid fertilizer according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Add DMPP to glycerol and stir at 30-50°C for 20-40 min to obtain DMPP-glycerol mother liquor; Step 2: Heat deionized water to 50-60°C, add chelating stabilizer and stir to obtain chelating stabilizer aqueous solution; Step 3: Add polyethylene glycol to the solution obtained in Step 2 and stir for 10-20 min to obtain an aqueous solution containing polyethylene glycol; Step 4: Add potassium nitrate in batches to the aqueous solution containing polyethylene glycol from Step 3, and stir to dissolve; Step 5: Add ammonium nitrate to the solution obtained in Step 4 in batches, stir to dissolve, and then cool to 25-30℃; Step 6: Add the DMPP-glycerol mother liquor obtained in Step 1 to the solution obtained in Step 5, stir for 20-30 minutes, and filter to obtain the potassium nitrate liquid fertilizer.

9. The stability improvement method according to claim 8, characterized in that, When adding potassium nitrate in step four, the system temperature shall not be lower than 45°C, and the potassium nitrate shall be added in batches at a rate of 5% to 20% of the total amount added per minute. And / or, the ammonium nitrate mentioned in step five is added in batches at a rate of 3% to 15% of the total amount added per minute.

10. The stability improvement method according to claim 8, characterized in that, When adding DMPP-glycerol mother liquor in step six, the system temperature should not exceed 35℃.

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