High-purity all-water-soluble potassium sulfate fertilizer and preparation method thereof

Abstract

The application discloses a high-purity full-water-soluble potassium sulfate fertilizer and a preparation method thereof, and belongs to the technical field of potassium sulfate fertilizers. The high-purity full-water-soluble potassium sulfate fertilizer is composed of the following components in parts by weight: 960-980 parts of industrial-grade potassium sulfate, 10-20 parts of urea phosphate, 5-15 parts of potassium nitrate, 1-3 parts of sodium hexametaphosphate, 0.5-1.5 parts of sodium cocoyl glutamate and 1-5 parts of citric acid-polyethylene glycol modified polyaspartic acid. The problems of slow dissolution and false saturation of traditional potassium sulfate fertilizers in low-temperature cold water are effectively eliminated.

Description

A high-purity, fully water-soluble potassium sulfate fertilizer and its preparation method Technical Field

[0001] This invention belongs to the field of potassium sulfate fertilizer technology, specifically, it relates to a high-purity, fully water-soluble potassium sulfate fertilizer and its preparation method. Background Technology

[0002] Potassium sulfate (K2SO4), as a high-quality chlorine-free potassium fertilizer, boasts advantages such as a low salt index, high nutrient content, and the simultaneous provision of sulfur. It is widely used in the cultivation of high-value crops such as tobacco, grapes, melons, and vegetables, and is a core fertilizer in modern facility agriculture and integrated water and fertilizer management technologies. With the popularization of precision fertilization technologies such as drip irrigation and sprinkler irrigation, the market has placed extremely high demands on the solubility, solution stability, and hard water resistance of potassium sulfate products.

[0003] However, existing agricultural potassium sulfate products face multiple technical bottlenecks in practical applications. Firstly, there is the problem of delayed dissolution at low temperatures. According to thermodynamic principles, the dissolution process of potassium sulfate is endothermic, and its solubility decreases significantly in winter and spring or in cold water environments. The dense surface structure of ordinary potassium sulfate crystals restricts the diffusion rate of solvent molecules, causing the fertilizer to easily form a dense, difficult-to-disperse layer at the bottom after being added to water, resulting in extremely low dissolution efficiency and severely impacting fertilization efficiency.

[0004] Secondly, there is the problem of secondary precipitation induced by trace impurities. Potassium sulfate produced by the traditional Mannheim process or metathesis process often leaves trace amounts of silicate, calcium, magnesium, iron, aluminum ions, and other impurities. Although these impurities are invisible to the naked eye at low concentrations, they are very easy to precipitate and form flocculent matter or turbidity when preparing high-concentration mother liquor due to the common ion effect and salting-out effect, which can clog the precision drip irrigation system.

[0005] Furthermore, considering the actual situation, irrigation water in northern my country and most facility agriculture areas has high hardness. Calcium and magnesium ions in the water readily combine with high-concentration sulfate ions released from potassium sulfate. Under heterogeneous nucleation, a calcium sulfate microcrystalline scale layer rapidly grows at the interface between fertilizer pipes and drippers. This interface mineralization and scaling problem under hard water irrigation environment will also affect the operation of drip irrigation equipment.

[0006] To improve the performance of potassium sulfate fertilizer, numerous technological explorations have been undertaken. For example, Chinese patent application CN111187110A discloses a clear liquid-type urea-based granular water-soluble fertilizer, which uses urea, fully water-soluble potassium sulfate, and potassium dihydrogen phosphate as raw materials. It adjusts the pH value by adding additives such as urea phosphate, solid sulfuric acid, and citric acid, and also adds a water-soluble anti-caking agent. While the above technical solutions utilize acidic additives to improve hard water adaptability to some extent, their essence remains the physical mixing and granulation of the components. This physical mixing method cannot change the microscopic surface properties of the potassium sulfate crystals themselves, and dissolution lag still exists at low temperatures. Furthermore, the added additives are prone to stratification during storage and transportation, leading to uneven local concentrations during use and unstable hard water resistance.

[0007] To address the issues of fertilizer caking and dissolution, Chinese patent application CN107188656A discloses a powdered organic-inorganic water-soluble fertilizer that prevents caking by adding water-soluble anti-caking agents such as sodium dodecylbenzene sulfonate and polyethylene glycol. However, this technical solution merely mixes surfactants into the powder. According to crystal growth kinetics theory, this simple physical adsorption binding force is weak. When the fertilizer is added to water, the surfactant often dissolves and detaches before the crystals, failing to continuously act on the crystal interface to guide water molecule penetration. Therefore, it has limited effect on improving the low-temperature dissolution rate. Furthermore, the above technical solution does not address the lattice blocking mechanism for hard water scaling, and cannot solve the problem of calcium and magnesium ion mineralization and deposition on the drip irrigation pipe wall.

[0008] For example, Chinese patent application CN105347906A discloses an enhanced liquid fertilizer containing potassium humate and selenium, which improves nutrient utilization by adding polyaspartic acid as an enhancer and utilizing its chelation effect on metal ions. However, this solution is aimed at liquid fertilizer systems and focuses on nutrient enhancement. In the preparation of solid potassium sulfate fertilizer, if polyaspartic acid is added physically, due to its uneven distribution among solid particles, it is difficult to form an effective scale-inhibiting microenvironment around each potassium sulfate crystal nucleus at the microscale. When the fertilizer dissolves in hard water, the local supersaturation of calcium sulfate bursts instantly, and the free polyaspartic acid often does not have time to exert the threshold scale inhibition effect before precipitation has already formed.

[0009] In summary, existing technologies mostly employ physical mixing or granulation methods to add additives, failing to achieve in-situ modification of potassium sulfate crystals from the microscopic dimensions of crystal engineering and materials chemistry. Furthermore, simple physical mixing leads to uneven distribution and weak binding of functional additives, making it impossible for existing products to simultaneously address the three major pain points: low-temperature insolubility, impurity-induced turbidity, and hard water scaling.

[0010] Therefore, developing a high-purity, fully water-soluble potassium sulfate fertilizer and its preparation method is of great significance for promoting the technological upgrading of high-end water-soluble fertilizers. Summary of the Invention

[0011] To address the deficiencies in the aforementioned technical solutions, the present invention aims to provide a high-purity, fully water-soluble potassium sulfate fertilizer and its preparation method.

[0012] To achieve the above objectives, the present invention provides a high-purity, fully water-soluble potassium sulfate fertilizer, which, by weight, comprises the following: 960-980 parts of industrial-grade potassium sulfate, 10-20 parts of urea phosphate, 5-15 parts of potassium nitrate, 1-3 parts of sodium hexametaphosphate, 0.5-1.5 parts of sodium cocoyl glutamate, and 1-5 parts of citric acid-polyethylene glycol modified polyaspartic acid.

[0013] The citric acid-polyethylene glycol modified polyaspartic acid is an amphiphilic polymer with a three-dimensional network structure prepared by chemical grafting and condensation reaction.

[0014] The industrial-grade potassium sulfate is industrial-grade potassium sulfate produced by the Mannheim process or the metathesis process.

[0015] The urea phosphate is a crystalline solid with a pH of 1.6-2.0 in a 1% aqueous solution.

[0016] The sodium hexametaphosphate is food grade or industrial grade and is used as a dispersing and chelating agent.

[0017] The sodium cocoyl glutamate is an amino acid-type surfactant used as an interface wetting agent.

[0018] The preparation method of the citric acid-polyethylene glycol modified polyaspartic acid includes the following steps:

[0019] Step 1: In a reactor equipped with a mechanical stirrer and a reflux device, add solvent and polysuccinimide, heat to 60-70℃, and stir until completely dissolved to form a homogeneous solution;

[0020] Step 2: Add monomethoxy polyethylene glycol amine dropwise to the solution, with the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine controlled at 10:(0.8-1.2); add catalyst, heat to 80-85℃, and react at a constant temperature for 4-5 hours;

[0021] Step 3: Add anhydrous citric acid and a dehydrating condensing agent to the reaction system from Step 2, raise the temperature to 90-95℃, and react for 6-8 hours.

[0022] Step 4: After the reaction is complete, deionized water is added to adjust the pH to 10.0-11.0. Hydrolysis is carried out at 50℃ for 2 hours. Then, the product is precipitated with ethanol, centrifuged, dried and pulverized to obtain a light yellow powder of citric acid-polyethylene glycol modified polyaspartic acid.

[0023] Furthermore, the molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1000-2000 Daltons;

[0024] Furthermore, the catalyst in step 2 is 4-dimethylaminopyridine, and the amount of catalyst added is 0.5% of the total mass of the reactants;

[0025] Furthermore, in step 3, the amount of citric acid added is 5-10% of the mass of polysuccinimide;

[0026] Furthermore, the dehydrating condensing agent in step 3 is dicyclohexylcarbodiimide, and its addition amount is twice the mass of anhydrous citric acid.

[0027] A method for preparing a high-purity, fully water-soluble potassium sulfate fertilizer includes the following steps:

[0028] Step A: Dissolution and purification of raw materials: Add deionized water to the dissolving vessel, heat to 90-95℃, add the prescribed amount of industrial-grade potassium sulfate to prepare a near-saturated solution; then add ammonium sulfide, activated carbon powder and polyacrylamide in sequence, and stir at a constant temperature for 30-40 minutes; while hot, pass through a plate and frame filter press connected in series with a 0.5-micron ceramic membrane filter to obtain potassium sulfate mother liquor;

[0029] Step B: Dispersion treatment: Pump the potassium sulfate mother liquor prepared in step A into a crystallization kettle equipped with an ultrasonic generator; while stirring, add the formulated amounts of urea phosphate, potassium nitrate, sodium hexametaphosphate and sodium cocoyl glutamate in sequence, keep the temperature at 80-85℃, stir for 15-20 minutes to form a homogeneous modified mother liquor.

[0030] Step C: Ultrasonic-assisted antisolvent-controlled crystallization: Turn on the jacket cooling water to slowly lower the temperature of the feed solution to 60°C, then add a pre-dissolved aqueous solution of citric acid-polyethylene glycol modified polyaspartic acid; then turn on the ultrasound to induce crystallization, and simultaneously slowly add anhydrous ethanol as an antisolvent to the crystallization vessel at a flow rate of 1-2% / min of the mother liquor volume; and while adding the antisolvent, start the cooling program.

[0031] Step D: Separation and drying: When the temperature drops to 30℃, stop the ultrasonication, cooling and adding of the antisolvent, and continue stirring for 20-30 minutes; finally, send the crystal slurry into a centrifuge for solid-liquid separation, and after washing, send the filter cake into a vibrating fluidized bed dryer, control the inlet air temperature to 95-105℃, dry until the moisture content is ≤0.5%, and sieve to obtain a high-purity, fully water-soluble potassium sulfate fertilizer.

[0032] Furthermore, in the preparation process, the amount of ammonium sulfide added in step A is 0.05% of the mass of industrial-grade potassium sulfate, the amount of activated carbon powder added is 0.1% of the mass of industrial-grade potassium sulfate, and the amount of polyacrylamide added is 0.01% of the mass of industrial-grade potassium sulfate.

[0033] Furthermore, in the preparation process, the ultrasonic waves in step C, after being turned on, have a frequency of 20-28 kHz and a power density of 0.3-0.5 W / cm². 2 ;

[0034] Furthermore, in the preparation process, the cooling rate of the programmed cooling in step C is a precise cooling rate of 0.5-0.8℃ / min.

[0035] The beneficial effects of this invention are:

[0036] 1. This invention constructs a micro-core-shell structure with citric acid-polyethylene glycol modified polyaspartic acid as the hydrophilic shell and high-purity, fully water-soluble potassium sulfate as the functional core. By utilizing the strong capture and wetting effect of the high-density grafted PEG segments in the shell on water molecules under low-temperature conditions, the surface tension and dissolution activation energy of the solid-liquid interface are significantly reduced, thereby enabling water molecules to quickly penetrate the dense layer of the crystal surface and induce internal disintegration. This effectively eliminates the problems of delayed dissolution and false saturation commonly found in traditional potassium sulfate fertilizers in low-temperature cold water, and opens up a physical channel for efficient fertilization in facility agriculture during the low-temperature winter and spring seasons.

[0037] 2. This invention constructs a dual scale inhibition mechanism of urea phosphate / potassium sulfate solid solution and citric acid-polyethylene glycol modified polyaspartic acid. It utilizes the high concentration of protons released by the solid solution core at the dissolution micro-interface to build an acidic barrier to neutralize the alkalinity of hard water. At the same time, it utilizes the carboxyl functional groups enriched in the side chains of citric acid-polyethylene glycol modified polyaspartic acid to cause lattice distortion and threshold scale inhibition effect on the microcrystal nuclei, thus doubly blocking the thermodynamic path of calcium and magnesium ions combining with sulfate ions to form nuclei and grow. As a result, high concentration potassium sulfate fertilizer can still maintain a clear and transparent true solution state in high hardness water, solving the common industry problem of drip irrigation systems being blocked and affecting efficiency due to interface mineralization and scaling.

[0038] 3. The high-purity, fully water-soluble potassium sulfate fertilizer and its preparation method of the present invention utilize a pretreatment process coupled with a metathesis reaction and deep purification as the cornerstone to ensure the high purity of the product. In addition, it combines an explosive nucleation mechanism induced by ultrasonic cavitation and utilizes the impurity removal effect during the rapid crystallization process to effectively squeeze residual trace impurity ions out of the crystal lattice. Combined with the strong chelating effect of citric acid-polyethylene glycol modified polyaspartic acid on residual metal ions, the product is endowed with excellent hard water resistance and rapid solubility, while effectively avoiding salting out and secondary precipitation caused by the enrichment of trace metal impurities in the mother liquor. This ensures that the product fully meets and exceeds the physicochemical index requirements of GB / T20406-2017 for superior grade potassium sulfate for agricultural use, and is suitable for large-scale industrial continuous production. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The illustrative embodiments and descriptions of this invention are for explanation only and are not intended to limit the invention. Furthermore, regarding numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0040] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0041] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0042] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0043] Example 1

[0044] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 980 parts of industrial-grade potassium sulfate, 10 parts of urea phosphate, 5 parts of potassium nitrate, 1 part of citric acid-polyethylene glycol modified polyaspartic acid, 1 part of sodium hexametaphosphate, and 0.5 parts of sodium cocoyl glutamate.

[0045] The citric acid-polyethylene glycol modified polyaspartic acid is an amphiphilic polymer with a three-dimensional network structure prepared by chemical grafting and condensation reaction.

[0046] The industrial-grade potassium sulfate is industrial-grade potassium sulfate produced by the Mannheim process or the metathesis process.

[0047] The urea phosphate is a crystalline solid with a pH of 1.8 in a 1% aqueous solution.

[0048] The sodium hexametaphosphate is of industrial grade and is used as a dispersing and chelating agent;

[0049] The sodium cocoyl glutamate is an amino acid-type surfactant used as an interface wetting agent.

[0050] The preparation method of the citric acid-polyethylene glycol modified polyaspartic acid includes the following steps:

[0051] Step 1: In a reactor equipped with a mechanical stirrer and a reflux device, add dimethylformamide solvent, add polysuccinimide, heat to 60-70℃, and stir until completely dissolved to form a homogeneous solution;

[0052] Step 2: Add monomethoxy polyethylene glycolamine dropwise to the solution, with the mass ratio of polysuccinimide to monomethoxy polyethylene glycolamine controlled at 10:0.8; add catalyst, heat to 80-85℃, and react at a constant temperature for 4-5 hours to perform ring-opening grafting by nucleophilic attack of the amino group on the succinimide ring.

[0053] Step 3: Add anhydrous citric acid and a dehydrating condensing agent to the reaction system of Step 2, raise the temperature to 90-95℃, and react for 6-8 hours. Utilize the polycarboxyl structure of citric acid to undergo condensation crosslinking with the active sites on the main chain to construct a hyperbranched structure; wherein the amount of citric acid added is 5% of the mass of polysuccinimide.

[0054] Step 4: After the reaction is complete, deionized water is added to adjust the pH to 10.0-11.0. Hydrolysis is carried out at 50℃ for 2 hours to convert the remaining succinimide ring into aspartic acid units. Subsequently, the mixture is precipitated with ethanol, centrifuged, dried and pulverized to obtain a light yellow powder of hyperbranched citric acid-polyethylene glycol co-modified polyaspartic acid.

[0055] Furthermore, the molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1000 Daltons;

[0056] Furthermore, the catalyst in step 2 is 4-dimethylaminopyridine, and the amount of catalyst added is 0.5% of the total mass of the reactants;

[0057] Furthermore, the dehydrating condensing agent in step 3 is dicyclohexylcarbodiimide, and its addition amount is twice the mass of anhydrous citric acid;

[0058] A method for preparing a high-purity, fully water-soluble potassium sulfate fertilizer includes the following steps:

[0059] Step A: Dissolution and purification of raw materials: Add deionized water to a dissolving vessel, heat to 90-95℃, and add the prescribed amount of industrial-grade potassium sulfate to prepare a near-saturated solution; then add ammonium sulfide, activated carbon powder, and polyacrylamide in sequence, and stir at a constant temperature for 30-40 minutes to allow heavy metal ions to precipitate and organic colloids to adsorb and flocculate; while hot, pass the solution through a plate and frame filter press connected in series with a 0.5-micron ceramic membrane filter to remove all insoluble impurities and flocculants, obtaining a clear and transparent potassium sulfate mother liquor; through deep purification, crystal nuclei that may cause secondary precipitation are eliminated;

[0060] Step B: Dispersion treatment: The clarified potassium sulfate mother liquor obtained in step A is pumped into a crystallization kettle equipped with an ultrasonic generator; under stirring, the formulated amounts of urea phosphate, potassium nitrate, sodium hexametaphosphate and sodium cocoyl glutamate are added sequentially, the temperature is maintained at 80-85℃, and the mixture is stirred for 15-20 minutes to form a homogeneous modified mother liquor; an acidic microenvironment and a multi-level chelation dispersion system are constructed;

[0061] Step C: Ultrasonic-assisted antisolvent-controlled crystallization: Turn on the jacket cooling water to slowly lower the temperature of the feed solution to 60°C, then add a pre-dissolved aqueous solution of citric acid-polyethylene glycol modified polyaspartic acid; then turn on the ultrasound to induce crystallization, and simultaneously slowly add anhydrous ethanol as an antisolvent to the crystallization vessel at a flow rate of 1-2% / min of the mother liquor volume; while adding the antisolvent, start the cooling program to precisely lower the temperature of the feed solution at a rate of 0.5-0.8°C / min.

[0062] Step D: Separation and Drying: When the temperature drops to 30℃, stop the ultrasonic treatment, cooling and adding the antisolvent, and continue stirring to grow crystals for 20-30 minutes; finally, send the crystal slurry into a centrifuge for solid-liquid separation, and after washing, send the filter cake into a vibrating fluidized bed dryer, control the inlet air temperature to 95-105℃, dry until the moisture content is ≤0.5%, and sieve to obtain a high-purity, fully water-soluble potassium sulfate fertilizer.

[0063] Furthermore, in the preparation process, the amount of ammonium sulfide added in step A is 0.05% of the mass of industrial-grade potassium sulfate, the amount of activated carbon powder added is 0.1% of the mass of industrial-grade potassium sulfate, and the amount of polyacrylamide added is 0.01% of the mass of industrial-grade potassium sulfate.

[0064] Furthermore, in the preparation process, the ultrasonic waves in step C, after being turned on, have a frequency of 20-28 kHz and a power density of 0.3-0.5 W / cm². 2 .

[0065] Example 2

[0066] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 960 parts of industrial-grade potassium sulfate, 20 parts of urea phosphate, 10 parts of potassium nitrate, 5 parts of citric acid-polyethylene glycol modified polyaspartic acid, 3 parts of sodium hexametaphosphate, and 1.5 parts of sodium cocoyl glutamate.

[0067] In the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0068] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:1.2;

[0069] The molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 2000 Daltons;

[0070] In step 3, the amount of citric acid added is 10% of the mass of polysuccinimide;

[0071] The preparation methods of citric acid-polyethylene glycol modified polyaspartic acid and high-purity fully water-soluble potassium sulfate fertilizer in Example 2 are the same as those in Example 1.

[0072] Example 3

[0073] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0074] In the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0075] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:1.0;

[0076] The molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1500 Daltons;

[0077] In step 3, the amount of citric acid added is 8% of the mass of polysuccinimide;

[0078] The preparation methods of citric acid-polyethylene glycol modified polyaspartic acid and high-purity fully water-soluble potassium sulfate fertilizer in Example 3 are the same as those in Example 1.

[0079] Example 4

[0080] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 965 parts of industrial-grade potassium sulfate, 12 parts of urea phosphate, 15 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1.5 parts of sodium cocoyl glutamate.

[0081] In the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0082] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:0.9;

[0083] The molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1200 Daltons;

[0084] In step 3, the amount of citric acid added is 6% of the mass of polysuccinimide;

[0085] The preparation methods of citric acid-polyethylene glycol modified polyaspartic acid and high-purity fully water-soluble potassium sulfate fertilizer in Example 4 are the same as those in Example 1.

[0086] Example 5

[0087] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 960 parts of industrial-grade potassium sulfate, 10 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0088] In the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0089] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:1.1;

[0090] The molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1800 Daltons;

[0091] In step 3, the amount of citric acid added is 9% of the mass of polysuccinimide;

[0092] The preparation methods of citric acid-polyethylene glycol modified polyaspartic acid and high-purity fully water-soluble potassium sulfate fertilizer in Example 5 are the same as those in Example 1.

[0093] Comparative Example 1

[0094] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0095] The difference between this comparative example and Example 3 is that in the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0096] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:0.4.

[0097] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0098] Comparative Example 2

[0099] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0100] The difference between this comparative example and Example 3 is that in the preparation process of the citric acid-polyethylene glycol modified polyaspartic acid:

[0101] In step 2, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:2.0.

[0102] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0103] Comparative Example 3

[0104] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0105] The difference between this comparative example and Example 3 is that in the preparation process of citric acid-polyethylene glycol modified polyaspartic acid, the amount of citric acid added in step 3 is 1% of the mass of polysuccinimide.

[0106] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0107] Comparative Example 4

[0108] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0109] The difference between this comparative example and Example 3 is that in the preparation process of citric acid-polyethylene glycol modified polyaspartic acid, the amount of citric acid added in step 3 is 20% of the mass of polysuccinimide.

[0110] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0111] Comparative Example 5

[0112] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0113] The difference between this comparative example and Example 3 is that in the preparation process of the modified polyaspartic acid, step 3 is omitted, that is, citric acid is not added for cross-linking reaction to obtain modified polyaspartic acid, and the modified polyaspartic acid prepared is used to replace the citric acid-polyethylene glycol modified polyaspartic acid in Example 3.

[0114] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0115] Comparative Example 6

[0116] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0117] The difference between this comparative example and Example 3 is that in the preparation process of the citric acid modified polyaspartic acid, in step 2, monomethoxy polyethylene glycol amine is not added, but the mixture is kept at 80-85°C and stirred in the air for 4-5 hours, and then the reactions in steps 3 and 4 are continued; the citric acid modified polyaspartic acid prepared is used to replace the citric acid-polyethylene glycol modified polyaspartic acid in Example 3.

[0118] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0119] Comparative Example 7

[0120] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of a blend of sodium polyaspartate, polyethylene glycol, and citric acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0121] The difference between this comparative example and Example 3 is that this comparative example uses a physical blend of sodium polyaspartate, polyethylene glycol, and citric acid, replacing the citric acid-polyethylene glycol modified polyaspartate in Example 3; the proportions of each component in the physical mixture are consistent with the raw material feeding ratio of the synthesized modifier in Example 3.

[0122] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0123] Comparative Example 8

[0124] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 0.1 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0125] The difference between this comparative example and Example 3 is that the amount of citric acid-polyethylene glycol modified polyaspartic acid added in the component formulation is reduced to 0.1 parts.

[0126] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0127] Comparative Example 9

[0128] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 985 parts of industrial-grade potassium sulfate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0129] The difference between this comparative example and Example 3 is that the addition of urea phosphate is omitted, and the amount of industrial-grade potassium sulfate is increased to make up the difference.

[0130] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0131] Comparative Example 10

[0132] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0133] The difference between this comparative example and Example 3 lies in the different process parameters in the preparation method: In step C of the preparation method of a high-purity fully water-soluble potassium sulfate fertilizer, the temperature of the liquid is controlled to decrease at a rate of 3.0℃ / min.

[0134] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0135] Comparative Example 11

[0136] A high-purity, fully water-soluble potassium sulfate fertilizer, by weight, comprises the following raw materials: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

[0137] The difference between this comparative example and Example 3 lies in the different process parameters in the preparation method: In step C of the preparation method of a high-purity fully water-soluble potassium sulfate fertilizer, the temperature of the liquid is controlled to decrease at a rate of 0.1℃ / min.

[0138] Apart from the above, the other components, their amounts, and preparation methods in this comparative example are the same as in Example 3.

[0139] Test case

[0140] Low-temperature dissolution rate test: A constant temperature water bath was set up, with the temperature controlled at 5℃±0.5℃; 500mL of deionized water was placed in a beaker, and a magnetic stirrer was turned on at a speed of 200rpm; then, a total of 10.0g of fertilizer samples prepared in Examples 1-5 and Comparative Examples 1-11 were quickly added, and timing was started simultaneously. The solution state was observed, and the time (in seconds) required from the addition of the sample to the point where no visible solid particles remained was recorded as the complete dissolution time.

[0141] Hard water stability and transmittance test: Referring to the relevant provisions in NY / T1973-2010 "Determination of Water-Insoluble Matter Content and pH of Water-Soluble Fertilizers", artificial hard water containing CaCl2 and MgSO4 was prepared, with the hardness uniformly adjusted to 500 mg / L (calculated as CaCO3) to simulate harsh water quality. Subsequently, using the above-mentioned artificial hard water, the fertilizer samples of Examples 1-5 and Comparative Examples 1-11 were uniformly prepared into 5% (w / w) fertilizer solutions. The solutions were placed in transparent glass colorimetric tubes, sealed, and allowed to stand for 24 hours at a constant temperature of 25°C.

[0142] Visual inspection: First, observe whether there is any visible sediment at the bottom of the solution or whether there is any scaling on the tube wall; if there is a large amount of sediment, it is directly judged as unqualified;

[0143] Transmittance determination: Under the premise of no obvious precipitation and stratification, the supernatant was taken and its transmittance (T%) was measured at a wavelength of 600 nm using a UV-Vis spectrophotometer, with pure artificial hard water as a reference (T=100%); the transmittance was obtained.

[0144] Stability test of high-concentration mother liquor: First, the mother liquor was prepared. The fertilizer samples prepared in Examples 1-5 and Comparative Examples 1-11 were used to prepare a high-concentration mother liquor with a mass fraction of 20% using deionized water. The prepared mother liquor was placed in a centrifuge tube and centrifuged at 5000 rpm for 10 minutes using a high-speed centrifuge to simulate the gravity sedimentation effect of long-term static placement. After centrifugation, the percentage of mother liquor precipitate was observed and measured.

[0145] The test results are shown in Table 1:

[0146] Table 1

[0147]

[0148] Performance test data analysis: As can be seen from the data in Table 1, the high-purity, fully water-soluble potassium sulfate fertilizer prepared in Examples 1-5 of this invention showed excellent performance in all performance tests; taking Example 3 as an example, its dissolution time in cold water at 5℃ was only 55s, far lower than 282s in Comparative Example 6 and 212s in Comparative Example 11, achieving true low-temperature rapid dissolution; after standing in simulated hard water for 24 hours, the light transmittance was as high as 98.8% and there was no visible sediment. At the same time, the sedimentation amount of its 20% high-concentration mother liquor was only 0.01%, almost zero, proving that the fertilizer products prepared in Examples 1-5 have extremely high purity and excellent resistance to salt precipitation.

[0149] The technical solution of this invention constructs a microstructure with a hydrophilic shell of citric acid-polyethylene glycol modified polyaspartic acid and a functional core of high-purity, fully water-soluble potassium sulfate. Utilizing the strong water-capturing and wetting effect of the high-density grafted PEG segments in the shell at low temperatures, it significantly reduces the surface tension and dissolution activation energy at the solid-liquid interface, allowing water molecules to rapidly penetrate the dense layer of the crystal surface and induce internal disintegration. In contrast, Comparative Example 1 had a low PEG grafting ratio, resulting in a dissolution time of 134 s, while Comparative Example 2 had a high PEG grafting ratio, leading to a dissolution time of 108 s, both inferior to Example 3. This indicates that there is an optimal wetting threshold for the amount of PEG grafted; if it is too low, continuous water channels cannot be formed, while if it is too high, molecular chain entanglement may occur; neither can effectively achieve rapid dissolution at low temperatures.

[0150] The physical blend of sodium polyaspartate, polyethylene glycol, and citric acid used in Comparative Example 7 had a dissolution time of 198 s, which was far inferior to that of Example 3. This indicates that simple mixing cannot form a uniform hydrophilic modified layer on the crystal surface, and it must be achieved through the corresponding preparation method and process of this example.

[0151] Although Comparative Example 6 contains a polyaspartic acid backbone, the lack of PEG hydrophilic side chains resulted in a dramatic increase in its low-temperature dissolution time to 282 s, exhibiting significant dissolution lag. This clearly demonstrates that the strong capture of water molecules by high-density grafted PEG segments at the microscopic interface is key to overcoming the low surface energy of ordinary potassium sulfate crystals and reducing the dissolution activation energy. Although Comparative Example 11 has a complete formulation, the extremely slow cooling rate of 0.1 °C / min led to excessively large crystal size and a sharp decrease in specific surface area, resulting in a dissolution time deteriorating to 212 s. This clearly demonstrates that the formulation of this application can only construct a microcrystalline structure with the optimal specific surface area and transform the hydrophilic advantage of chemical modification into a macroscopic rapid dissolution effect when combined with the controlled cooling process within the specific range of 0.5-0.8 °C / min.

[0152] After standing in simulated hard water for 24 hours, Example 3 showed a transmittance of up to 98.8% and no visible sediment, demonstrating excellent scale inhibition performance. In contrast, Comparative Example 9, lacking urea phosphate as an acid barrier, showed severe turbidity and a large amount of white precipitate, which was visually unacceptable. Therefore, transmittance testing was not conducted, and transmittance data was not obtained. This clearly demonstrates that a single scale inhibitor is insufficient to resist high-hardness water; urea phosphate must be relied upon to release protons at the dissolution micro-interface to neutralize the alkalinity of the hard water.

[0153] Comparative Example 5 omitted step 3, i.e., did not add citric acid for cross-linking reaction, and obtained modified polyaspartic acid with a transmittance of only 82.4%, which was significantly lower than that of Example 3. This shows that the citric acid-polyethylene glycol modified polyaspartic acid constructed in this application can more effectively adsorb and distort calcium sulfate microcrystals through carboxyl functional groups, preventing their growth.

[0154] The difference between Comparative Examples 3 and 4 and Example 3 lies in the following: In the preparation process of citric acid-polyethylene glycol modified polyaspartic acid, the amount of citric acid added in step 3 is the same as the mass of polysuccinimide, which is too low and too high, respectively. The transmittance of Comparative Examples 3 and 4 is 85.3% and 90.1%, respectively, which do not reach the level of Example 3. This indicates that the network structure exhibited by citric acid-polyethylene glycol modified polyaspartic acid must be moderate in order to maximize its function of blocking the thermodynamic pathway of calcium and magnesium ions combining with sulfate ions to form nuclei and grow.

[0155] In contrast, the amount of citric acid-polyethylene glycol modified polyaspartic acid in Comparative Example 8 was too small, with a light transmittance of 92.3%, which was acceptable but not as good as in Example 3. This indicates that a sufficient amount must be applied to be effective in blocking the nucleation pathway of impurities. Furthermore, Comparative Example 7 could not achieve a good scale inhibition effect by relying solely on physical mixing.

[0156] In Example 3, the 20% high-concentration mother liquor centrifuged sedimentation amount was only 0.01%, almost zero; while in Comparative Example 10, a rapid cooling process of 3.0℃ / min was used, and the mother liquor centrifuged sedimentation amount was as high as 1.84%. The possible reason is that the excessively fast crystallization rate led to a severe mother liquor encapsulation effect, which caused impurities to be sealed inside the crystals and released after resolution, causing secondary pollution. The process parameters of this application successfully utilized the impurity removal effect during the crystallization process to achieve deep purification.

[0157] Furthermore, although the amount of citric acid added in Comparative Example 4 was increased, its precipitation amount actually increased to 1.15%; the possible reason is that the excessive degree of crosslinking caused the modifier to undergo self-aggregation.

[0158] In summary, this invention creatively solves three major pain points: poor solubility at low temperatures, turbidity caused by impurities, and scaling in hard water, and is suitable for large-scale industrial continuous production.

[0159] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-purity, fully water-soluble potassium sulfate fertilizer, characterized in that, The composition, by weight, is as follows: 960-980 parts of industrial-grade potassium sulfate, 10-20 parts of urea phosphate, 5-15 parts of potassium nitrate, 1-3 parts of sodium hexametaphosphate, 0.5-1.5 parts of sodium cocoyl glutamate, and 1-5 parts of citric acid-polyethylene glycol modified polyaspartic acid; the preparation method of the citric acid-polyethylene glycol modified polyaspartic acid includes the following steps: Step 1: In a reaction vessel equipped with a mechanical stirrer and a reflux device, add solvent, add polysuccinimide, heat to 60-70℃, and stir until completely dissolved to form a homogeneous solution; Step 2: Add monomethoxy polyethylene glycol amine dropwise to the solution, the mass ratio of polysuccinimide to monomethoxy polyethylene glycol amine is controlled at 10:(0.8-1.2); add catalyst, heat to 80℃; The reaction is carried out at 0-85℃ for 4-5 hours; Step 3: Anhydrous citric acid and dehydrating condensing agent are added to the reaction system of Step 2, the temperature is raised to 90-95℃, and the reaction is carried out for 6-8 hours; Step 4: After the reaction is completed, deionized water is added, the pH is adjusted to 10.0-11.0, and hydrolysis is carried out at 50℃ for 2 hours. Then, the mixture is precipitated with ethanol, centrifuged, dried, and pulverized to obtain citric acid-polyethylene glycol modified polyaspartic acid; The amount of citric acid added in Step 3 is 5-10% of the mass of polysuccinimide; The preparation method of a high-purity fully water-soluble potassium sulfate fertilizer includes the following steps: Step A: Dissolution and purification of raw materials: Deionized water is added to the dissolving kettle, the temperature is raised to 90-95℃, and industrial grade potassium sulfate is added to prepare a near-water-soluble potassium sulfate solution. Saturated solution; then ammonium sulfide, activated carbon powder and polyacrylamide are added sequentially, and stirred at a constant temperature for 30-40 minutes; while hot, it is passed through a plate and frame filter press with a 0.5-micron ceramic membrane filter in series to obtain potassium sulfate mother liquor; Step B: Dispersion treatment: the potassium sulfate mother liquor prepared in step A is pumped into a crystallization kettle equipped with an ultrasonic generator; under stirring, the formulated amounts of urea phosphate, potassium nitrate, sodium hexametaphosphate and sodium cocoyl glutamate are added sequentially, the temperature is maintained at 80-85℃, and stirred for 15-20 minutes to form a homogeneous modified mother liquor; Step C: Ultrasonic-assisted antisolvent controlled crystallization: the jacket cooling water is turned on to slowly reduce the temperature of the feed solution to 60℃, and then a pre-dissolved aqueous solution of citric acid-polyethylene glycol modified polyaspartic acid is added; then the ultrasonic generator is turned on. Induced crystallization is performed while anhydrous ethanol, the antisolvent, is slowly added to the crystallization vessel at a rate of 1-2% / min of the mother liquor volume. Simultaneously, a programmed cooling process is initiated during the addition of the antisolvent. Step D: Separation and drying: When the temperature drops to 30℃, ultrasonication, cooling, and the addition of the antisolvent are stopped, and stirring continues for 20-30 minutes. Finally, the crystal slurry is sent to a centrifuge for solid-liquid separation. The filter cake is washed and then sent to a vibrating fluidized bed dryer, with the inlet air temperature controlled at 95-105℃, dried until the moisture content is ≤0.5%, and sieved to obtain a high-purity, fully water-soluble potassium sulfate fertilizer. In step C of the method for preparing this high-purity, fully water-soluble potassium sulfate fertilizer, the programmed cooling rate is 0.5-0.8℃ / min for precise cooling.

2. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The high-purity, fully water-soluble potassium sulfate fertilizer comprises the following components by weight: 970 parts of industrial-grade potassium sulfate, 15 parts of urea phosphate, 8 parts of potassium nitrate, 3 parts of citric acid-polyethylene glycol modified polyaspartic acid, 2 parts of sodium hexametaphosphate, and 1 part of sodium cocoyl glutamate.

3. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The industrial-grade potassium sulfate is produced by the Mannheim process or the metathesis process.

4. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The urea phosphate is a crystalline solid with a pH of 1.6-2.0 in a 1% aqueous solution.

5. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The sodium hexametaphosphate is food-grade or industrial-grade sodium hexametaphosphate; the sodium cocoyl glutamate is an amino acid-type surfactant.

6. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The molecular weight of the monomethoxy polyethylene glycolamine in step 2 is 1000-2000 Daltons.

7. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The catalyst in step 2 is 4-dimethylaminopyridine.

8. A high-purity, fully water-soluble potassium sulfate fertilizer according to claim 7, characterized in that, In step 2, the amount of catalyst added is 0.5% of the total mass of the reactants.

9. A high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, The dehydrating condensing agent in step 3 is dicyclohexylcarbodiimide.

10. A high-purity, fully water-soluble potassium sulfate fertilizer according to claim 9, characterized in that, In step 3, the amount of dehydrating condensing agent added is twice the mass of anhydrous citric acid.

11. A high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, In step A of the preparation method of the high-purity fully water-soluble potassium sulfate fertilizer, the amount of ammonium sulfide added is 0.05% of the mass of industrial-grade potassium sulfate, the amount of activated carbon powder added is 0.1% of the mass of industrial-grade potassium sulfate, and the amount of polyacrylamide added is 0.01% of the mass of industrial-grade potassium sulfate.

12. The high-purity, fully water-soluble potassium sulfate fertilizer according to claim 1, characterized in that, In step C of the method for preparing a high-purity, fully water-soluble potassium sulfate fertilizer, the ultrasonic wave, after being activated, has a frequency of 20-28 kHz and a power density of 0.3-0.5 W / cm³. 2 .

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