A high tower sulfur-based total water-soluble compound fertilizer and a preparation method thereof

Abstract

This invention relates to the field of agricultural fertilizer preparation technology, and discloses a high-tower sulfur-based fully water-soluble compound fertilizer and its preparation method, comprising: compounding urea, urea phosphate, and choline chloride to form a eutectic solvent continuous phase; dry-coating pulverized solid monoammonium phosphate and potassium sulfate with sodium hexametaphosphate, and preheating to obtain a coated premix; uniformly adding the coated premix to the eutectic solvent continuous phase, mixing under high shear to obtain a suspension slurry, then pumping it to a high tower for spray granulation, and naturally cooling and crystallizing to obtain the finished product. This invention lowers the melting point of the slurry by constructing a eutectic system, allowing the material to maintain good fluidity at lower temperatures, effectively suppressing the side reaction of urea deammoniation to biuret upon heating; simultaneously, the coating pretreatment isolates the direct contact between the solid and liquid phases, avoiding interfacial side reactions and particle aggregation between salts, ensuring the suspension stability and rheological properties of the high-solids-content system.

Description

Technical Field

[0001] This invention relates to the field of agricultural fertilizer preparation technology, specifically to a high-tower sulfur-based fully water-soluble compound fertilizer and its preparation method. Background Technology

[0002] High-tower granulation technology has been widely used in compound fertilizer production due to its ability to produce fertilizer granules with a smooth, round appearance, uniform nutrient distribution, and the elimination of the need for drying. In recent years, with the popularization of fertigation technology, the market has placed higher demands on the water solubility of fertilizers. Among these, sulfur-based compound fertilizers, using potassium sulfate as the potassium source, are particularly suitable for chlorine-sensitive and sulfur-loving cash crops, representing an important development direction for fully water-soluble compound fertilizers.

[0003] However, the existing high-tower granulation process for preparing high-concentration sulfur-based fully water-soluble compound fertilizer faces intractable technological contradictions. Because the melting points of potassium sulfate and monoammonium phosphate are much higher than the conventional operating temperature of high-tower granulation, these raw materials can only be dispersed as solid powders in the molten liquid, which is primarily composed of urea. As the proportion of solid components in the formula increases, the viscosity of the solid-liquid mixture rises sharply, leading to a severe deterioration in slurry fluidity and easily causing overload of the slurry pump and blockage of the granulation nozzles during production.

[0004] To improve slurry flowability and maintain production continuity, the conventional approach is to further increase the system's heating temperature. However, this triggers a series of negative consequences: sustained high temperatures not only increase energy consumption but, more seriously, accelerate urea condensation at high temperatures, generating biuret, which is toxic to crop seeds and seedlings. Furthermore, in traditional high-temperature melt environments, untreated solid powders come into direct contact with the molten liquid. Due to the lack of effective interfacial buffers, localized physical aggregation or chemical side reactions easily occur between different salts. Conventional additives often degrade and become ineffective at high temperatures, causing solid particles to settle in the melt and the suspension system to rapidly destabilize. This not only reduces the quality of droplets produced by high-tower spraying, resulting in low pelleting rate and easy agglomeration of the final product, but also compromises the complete water solubility of the finished fertilizer.

[0005] In summary, how to overcome the technical bottleneck of high viscosity and easy sedimentation of high solid content slurry under low and controllable operating temperatures, and obtain a solid-liquid suspension system with excellent rheological properties and long-term stability, so as to balance the smoothness of high-tower granulation and the quality of sulfur-based fully water-soluble fertilizer, is a technical problem that urgently needs to be solved in the current fertilizer production field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high-tower sulfur-based fully water-soluble compound fertilizer and its preparation method, which solves the problems of biuret byproducts, poor water solubility due to salt metadecomposition, and easy settling and clogging of equipment by high-solids slurry during conventional high-tower production of sulfur-based compound fertilizers.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a high-tower sulfur-based fully water-soluble compound fertilizer, which adopts the following technical solution: A high-tower sulfur-based fully water-soluble compound fertilizer is produced by high-tower granulation of the following raw materials in parts by weight: 15.0-25.0 parts monoammonium phosphate, 25.0-35.0 parts potassium sulfate, 0.5-1.0 parts sodium hexametaphosphate; 33.0-43.0 parts urea, 3.0-5.0 parts urea phosphate, 2.0-3.0 parts choline chloride, 0.1-0.3 parts polyethylene glycol 400, and 0.2-0.4 parts sodium lignosulfonate; The preparation process of the compound fertilizer includes: crushing and classifying the monoammonium phosphate and potassium sulfate to obtain a mixed powder; pre-coating and mixing the mixed powder with sodium hexametaphosphate to obtain a coated premix; mixing urea, urea phosphate, choline chloride, polyethylene glycol 400 and sodium lignosulfonate to form a eutectic solvent continuous phase; dispersing the coated premix in the eutectic solvent continuous phase to form a suspension slurry, and then spraying and cooling it in a high tower to form granules.

[0008] By adopting the above technical solution, and by using a eutectic solvent continuous phase to replace the traditional pure molten urea liquid phase, and by using sodium hexametaphosphate to physically isolate the inorganic salt powder on its surface, the granulation system's operating temperature is reduced, side reactions are suppressed, and slurry fluidity is improved. The specific principle is as follows: Urea in the system provides a large number of amide groups, urea phosphate contains protonated phosphate groups, and choline chloride contains hydroxyl groups and quaternary ammonium salt structures. During the melt mixing process, a small amount of urea phosphate and choline chloride act as hydrogen bond donors / acceptors dispersed in the urea. The carbonyl oxygen atoms of urea form hydrogen bonds with the protons dissociated from urea phosphate, while the ionic network of choline chloride is embedded in the system. This interaction locally disrupts the regular lattice arrangement of pure urea molecules, causing the overall macroscopic melting endothermic peak of the mixture to shift significantly towards lower temperatures, thereby constructing a modified urea liquid phase with low melting point characteristics, namely the eutectic solvent continuous phase of this invention. This modified continuous phase does not require a fixed molar ratio phase equilibrium condition to maintain excellent liquid flowability at relatively low operating temperatures.

[0009] Sodium hexametaphosphate, a polyphosphate, is physically embedded on the surface of monoammonium phosphate and potassium sulfate during dry mixing using the high shear force of a plow-type mixer, forming a micro-dispersion barrier layer. When the coated premix is ​​dispersed into the continuous phase of a eutectic solvent, on the one hand, thanks to the low-temperature operating range of 115 to 118°C constructed by this invention, the deammoniation condensation side reaction that easily occurs between monoammonium phosphate and urea at high temperatures above 140°C is avoided from a thermodynamic perspective; on the other hand, this dispersion barrier layer effectively delays the surface dissolution and ion exchange rate of inorganic salt powder in the liquid phase system, inhibits the premature occurrence of metathesis reaction, and, together with the steric hindrance agent in the liquid phase, maintains the suspension stability and physicochemical stability of the slurry.

[0010] Meanwhile, polyethylene glycol 400 plays a steric hindrance lubricating role, and sodium lignosulfonate is adsorbed at the solid-liquid interface to improve the interfacial tension of the system. The synergistic effect of the two enables the inorganic salt powder to form a rheologically stable homogeneous suspension system in the eutectic solvent.

[0011] Preferably, the particle size distribution of the mixed powder satisfies the following: the median diameter of the powder is 80-120 μm, the mass fraction passing through a 60-mesh standard sieve is 95%-99%, and the mass fraction passing through a 325-mesh standard sieve (approximately 45 μm) is 5%-15%.

[0012] By adopting the above technical solutions, the median diameter of the powder is controlled within 80–120 μm, avoiding gravity settling and granulation nozzle clogging caused by excessively large particles. By limiting the mass fraction of 325-mesh ultrafine powder to within 15%, the total specific surface area of ​​the powder is controlled, preventing excessive absorption of the continuous phase liquid by ultrafine particles and thus avoiding a sharp increase in the viscosity of the suspended slurry. This ensures that the slurry has suitable shear-thinning rheological properties during high-tower centrifugal spraying.

[0013] Preferably, sodium hexametaphosphate is coated on the surface of the mixed powder particles. By adopting the above technical solution, the site-specific coating structure maximizes the isolation effectiveness of sodium hexametaphosphate, ensuring that the continuous phase of the eutectic solvent maintains chemical stability before cooling and crystallization.

[0014] Secondly, the present invention provides a method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer, which adopts the following technical solution: A method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer includes the following steps: Monoammonium phosphate and potassium sulfate are pulverized and particle size classified to a median diameter of 80-120 μm to obtain a mixed powder. The mixed powder is then dry-mixed with sodium hexametaphosphate at 20-25°C for 5-10 min to obtain a coated premix. The coated premix is ​​then heated to 80-85°C and sealed for heat preservation for later use. Urea was mixed with urea phosphate, choline chloride, polyethylene glycol 400 and sodium lignosulfonate. The mixture was then heated to 115-118℃ to form a eutectic liquid phase and kept at that temperature for later use, thus obtaining a continuous eutectic solvent phase. Under the condition of maintaining a constant temperature of the eutectic solvent continuous phase material, the premixed material that has been kept at a constant temperature is added to the eutectic solvent continuous phase at a constant rate for high-shear mixing to obtain a suspension slurry. The suspended slurry is pumped to the top of the high-tower granulator, centrifugally sprayed into droplets by a rotating nozzle, and then falls freely inside the granulation tower to exchange heat with the cold air at the bottom of the tower. After cooling and crystallization, the particles are collected at the bottom of the tower and screened to obtain the finished high-tower sulfur-based fully water-soluble compound fertilizer.

[0015] By employing the above technical solution, the surface pretreatment of the solid powder and the preparation of the eutectic solvent in the liquid phase are separated. The premixed coating, heated and kept at a constant temperature, reduces the temperature gradient during solid-liquid mixing when added to the liquid phase, preventing needle-like crystal precipitation caused by localized supercooling of the continuous phase. High-shear mixing forcibly breaks down powder agglomerates, allowing the sodium hexametaphosphate particles to be rapidly and uniformly dispersed within the eutectic system. Finally, the surface tension contraction and rapid phase change crystallization of the droplets are completed using cold air from a high-tower system, resulting in a finished product with rounded particles and no moisture-absorbing surface defects.

[0016] Preferably, the mixed powder and sodium hexametaphosphate are dry mixed at 20-25°C in a plow-type dry powder mixer, and the stirring speed is set to 120-150 r / min. The coated premix is ​​transferred into a screw conveyor with jacketed heat-conducting oil for dynamic heating and heat preservation.

[0017] By adopting the above technical solution, the high-shear flying blade of the plow can quickly break up the agglomeration of inorganic salt powder, and a very small amount of sodium hexametaphosphate can achieve uniform physical integration in a short time; the screw conveyor with heat tracing realizes continuous dynamic heating of materials, which is suitable for large-scale industrial continuous production.

[0018] Preferably, the mixture is heated to 115–118°C to eutecticly form a liquid phase, specifically: Place the mixture in a temperature-controlled reactor, turn on slow stirring and slowly heat to 115-118°C to allow the material to eutecticly melt and liquefy.

[0019] By adopting the above technical solution, the high-temperature environment above 130°C required for heating pure urea alone is avoided. All components are pre-mixed and then eutectic is induced directly at a lower temperature, completely eliminating the high-temperature melt stage and cutting off the heat-sensitive range for biuret formation from the source of the process.

[0020] Preferably, under the condition of maintaining a constant temperature of the eutectic solvent continuous phase material, the premixed coating material that has been kept at a constant temperature is uniformly added to the eutectic solvent continuous phase for high-shear mixing. Specifically, under the condition of maintaining the material temperature at 115-118°C, the high-shear stirring device is started and the speed is set to 400-600 r / min, and high-shear mixing is carried out continuously for 8-10 min.

[0021] By employing the above technical solution, high-speed shearing at 400–600 r / min inputs sufficient mechanical energy into the high-viscosity slurry, disrupting the van der Waals forces between inorganic particles and promoting the rapid directional alignment of sodium lignosulfonate at the solid-liquid interface. The controlled time of 8–10 min ensures both uniform dispersion and prevents prolonged mechanical shearing from generating heat that could damage the eutectic structure of the system.

[0022] Preferably, the suspended slurry is pumped to the top of the high tower granulator, specifically by pumping the suspended slurry to the top of the high tower granulator through a mud pump that maintains the temperature at 115-118°C by heating the pipe wall.

[0023] By adopting the above technical solution, the boundary layer temperature drop of the slurry in the long-distance conveying pipeline is eliminated by full-process heating, which prevents the fluid near the pipe wall from undergoing phase change and crystallization due to temperature reduction, and ensures the conveying efficiency of the mud pump and the continuity of the nozzle discharge.

[0024] Preferably, the slow heating to 115-118°C specifically involves: Warm water or low-pressure steam is introduced into the jacket of the reactor to provide a heat source, and the heat transfer boundary temperature is controlled to not exceed 120°C to prevent local overheating.

[0025] By adopting the above technical solution, the jacket heat transfer boundary temperature is strictly limited to within 120℃, which not only ensures that sufficient latent heat of eutectic is provided to the solid mixture, but also prevents the local material close to the reactor wall from undergoing urea deammoniation side reaction due to instantaneous overheating, thus ensuring the chemical stability of the eutectic continuous phase during the preparation process.

[0026] Preferably, the mixture of monoammonium phosphate and potassium sulfate is pulverized and classified to a median diameter of 80–120 μm to obtain a mixed powder using an air jet mill classifier.

[0027] By adopting the above technical solution, the expansion of compressed air during the air jet milling process generates an adiabatic cooling effect, which counteracts the heat generated by the milling collision and prevents low-melting-point inorganic salts from thermally melting and sticking to the walls in the milling chamber. At the same time, the air jet classifier can accurately cut off excessively large and small particles to obtain target powder with a concentrated particle size distribution.

[0028] This invention provides a high-tower sulfur-based fully water-soluble compound fertilizer and its preparation method. It has the following beneficial effects: 1. This invention constructs a low-eutectic solvent continuous phase containing electrostatic ion pairs in an acidic microenvironment by compounding urea with components such as urea phosphate and choline chloride. This effectively reduces the overall melting point of the system, allowing the slurry to maintain good fluidity at lower operating temperatures. This not only avoids the risk of urea condensation to form biuret in traditional high-temperature processes, but also reduces production energy consumption and directly solves the equipment operation problems of excessive load on high-tower mud pumps and easy clogging of granulation nozzles.

[0029] 2. This invention involves pulverizing and particle size classifying solid monoammonium phosphate and potassium sulfate, and then dry-coating them with sodium hexametaphosphate to obtain a coated premix. This pretreatment method isolates the solid components from direct contact with the liquid environment, inhibits interfacial side reactions and particle aggregation between different salts, and the particle size distribution design allows the solid powder to be uniformly dispersed in the continuous phase without sedimentation, thus improving the long-term stability of the high-concentration solid-liquid mixed suspension system.

[0030] 3. This invention involves uniformly adding the above-mentioned coated premix to a continuous phase of a eutectic solvent for high-shear mixing to obtain a suspension slurry with stable rheological properties, which is then subjected to high-tower spraying and granulation. Under mild process conditions, the homogeneity of the slurry is ensured, guaranteeing the uniform distribution of nutrients within the compound fertilizer granules. Furthermore, by blocking the formation pathway of insoluble impurities, the resulting compound fertilizer granules are round in appearance, not prone to clumping, and can be rapidly and completely dissolved in water, meeting the application requirements of high-quality, fully water-soluble fertilizers for fertigation and drip irrigation systems. Attached Figure Description

[0031] Figure 1 These are differential scanning calorimetry (DSC) curves of pure urea and the eutectic solvent continuous phases obtained in Examples 4 to 6 of this invention; Figure 2 This is a rheological curve of the apparent viscosity of the suspension slurry obtained in Examples 1 to 3 of the present invention as a function of shear rate; Figure 3 These are the Fourier transform infrared spectra of pure urea of ​​the present invention and the eutectic solvent continuous phase obtained in Preparation Example 4; Figure 4 The X-ray diffraction patterns of pure monoammonium phosphate, pure potassium sulfate, and the compound fertilizer product obtained in Example 1 are shown below. Figure 5 This is a graph showing the change of static settling rate of the suspended slurry obtained in Example 1 of the present invention and Comparative Examples 1 and 3 over time. Figure 6 This is a graph showing the change in light transmittance of the solution over time during the water dissolution process of the finished products in Example 1 and Comparative Examples 1 to 3 of the present invention. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Preparation Example 1: This preparation example provides a method for preparing a coated premix, including the following steps: Weigh out 20.0 parts by weight of industrial grade monoammonium phosphate, 30.0 parts by weight of industrial grade potassium sulfate, and 0.8 parts by weight of industrial grade sodium hexametaphosphate; The weighed monoammonium phosphate and potassium sulfate were fed into an air classifier for pulverization and particle size classification. The speed and air volume of the classifier were adjusted to ensure that the particle size distribution of the mixed powder after classification met the following requirements: 98% of the mass fraction passing through a 60-mesh standard sieve, the median diameter of the powder was 100 μm, and 8% of the mass fraction passing through a 325-mesh standard sieve. The graded mixed powder is added together with the weighed sodium hexametaphosphate into a plow-type dry powder mixer and dry-mixed at 150 r / min for 8 minutes at room temperature (20-25℃) to make the sodium hexametaphosphate uniformly coat the surface of monoammonium phosphate and potassium sulfate particles, thus obtaining the coated premix. The resulting coated premix is ​​transferred into a screw conveyor with jacketed heat transfer oil and heated to raise and stabilize the powder temperature at 82°C. The powder is then sealed and kept warm for later use.

[0034] Preparation Example 2: This preparation example provides a method for preparing a coated premix, including the following steps: Weigh out 15.0 parts by weight of industrial grade monoammonium phosphate, 25.0 parts by weight of industrial grade potassium sulfate, and 0.5 parts by weight of industrial grade sodium hexametaphosphate; The weighed monoammonium phosphate and potassium sulfate were fed into an air classifier and the parameters were adjusted so that the particle size distribution of the mixed powder after classification met the following requirements: 95% of the mass fraction passing through a 60-mesh standard sieve, the median diameter of the powder was 80 μm, and the mass fraction of ultrafine powder passing through a 325-mesh standard sieve was 15%. The graded mixed powder is added together with the weighed sodium hexametaphosphate into a mixer and dry-mixed at room temperature (20-25℃) for 5 minutes to obtain the coated premix. The resulting coated premix is ​​transferred into a heating device to raise the powder temperature and keep it constant at 80°C. It is then sealed and kept warm for later use.

[0035] Preparation Example 3: This preparation example provides a method for preparing a coated premix, including the following steps: Weigh out 25.0 parts by weight of industrial grade monoammonium phosphate, 35.0 parts by weight of industrial grade potassium sulfate, and 1.0 part by weight of industrial grade sodium hexametaphosphate; The weighed monoammonium phosphate and potassium sulfate were fed into a pulverizer and classifier. The parameters were adjusted so that the particle size distribution of the mixed powder after classification met the following requirements: 99% of the mass fraction passing through a 60-mesh standard sieve, the median diameter of the powder was 120 μm, and the mass fraction of ultrafine powder passing through a 325-mesh standard sieve was 5%. The graded mixed powder is added together with the weighed sodium hexametaphosphate into a mixer and dry-mixed at room temperature (20-25℃) for 10 minutes to obtain the coated premix. The resulting coated premix is ​​transferred into a heating device to raise the powder temperature and keep it constant at 85°C. It is then sealed and kept warm for later use.

[0036] Preparation Example 4: This preparation example provides a method for preparing a continuous phase in a eutectic solvent, comprising the following steps: Weigh out 38.0 parts by weight of industrial grade urea, 4.0 parts by weight of industrial grade urea phosphate, 2.5 parts by weight of industrial grade choline chloride, 0.2 parts by weight of polyethylene glycol 400, and 0.3 parts by weight of industrial grade sodium lignosulfonate. All the weighed raw materials were put into a reaction vessel equipped with a jacketed temperature control and a high-shear dispersing emulsifier for solid-phase mixing. A heat source is introduced into the jacket, and slow stirring is started to slowly heat the mixture in the vessel to 116°C. After the material undergoes eutectic melting in an acidic microenvironment and is completely liquefied to form a continuous phase of eutectic solvent containing electrostatic ion pairs, the material in the vessel is kept constant at 116°C by adjusting the jacket temperature control for later use.

[0037] Preparation Example 5: This preparation example provides a method for preparing a continuous phase in a eutectic solvent, comprising the following steps: Weigh out 33.0 parts by weight of industrial grade urea, 3.0 parts by weight of industrial grade urea phosphate, 2.0 parts by weight of industrial grade choline chloride, 0.1 parts by weight of polyethylene glycol 400, and 0.2 parts by weight of industrial grade sodium lignosulfonate. All the weighed raw materials were put into the jacketed reactor for solid-phase mixing. A heat source is introduced into the jacket, and slow stirring is started to slowly heat the mixture in the vessel to 115°C. After the material undergoes eutectic melting in an acidic microenvironment and is completely liquefied to form a continuous phase of eutectic solvent containing electrostatic ion pairs, the material in the vessel is kept constant at 115°C by adjusting the jacket temperature control for later use.

[0038] Preparation Example 6: This preparation example provides a method for preparing a continuous phase in a eutectic solvent, comprising the following steps: Weigh out 43.0 parts by weight of industrial grade urea, 5.0 parts by weight of industrial grade urea phosphate, 3.0 parts by weight of industrial grade choline chloride, 0.3 parts by weight of polyethylene glycol 400, and 0.4 parts by weight of industrial grade sodium lignosulfonate. All the weighed raw materials were put into the jacketed reactor for solid-phase mixing. A heat source is introduced into the jacket, and slow stirring is started to slowly heat the mixture in the vessel to 118°C. After the material undergoes eutectic melting in an acidic microenvironment and is completely liquefied to form a continuous phase of eutectic solvent containing electrostatic ion pairs, the material in the vessel is kept constant at 118°C by adjusting the jacket temperature control for later use. Example 1

[0039] This embodiment provides a method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer, including the following steps: Obtain the coated premix prepared at constant temperature in Preparation Example 1, and the eutectic solvent continuous phase prepared at constant temperature in Preparation Example 4; While maintaining the material temperature of the aforementioned eutectic solvent continuous phase at 116°C, the high-shear stirring device equipped in the reactor was started and the rotation speed was set to 500 r / min. The preheated coating premix obtained at 82°C was added at a uniform speed to the eutectic solvent continuous phase in the aforementioned reactor through a uniform feeding device. After the feeding was completed, the current speed was maintained and high shear mixing was continued for 9 minutes to obtain a suspension slurry with stable rheological properties. The obtained suspension slurry is pumped to the top of the high tower granulator through a mud pump that maintains the temperature at 116°C by heating the pipe wall. The slurry is centrifugally sprayed into droplets through a rotating nozzle, and falls freely in the granulation tower to exchange heat with the cold air at the bottom of the tower. After natural cooling and crystallization, the particles are collected at the bottom of the tower and screened to obtain the finished high tower sulfur-based fully water-soluble compound fertilizer. Example 2

[0040] This embodiment provides a method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer, including the following steps: Obtain the coated premix prepared at constant temperature in Preparation Example 3, and the eutectic solvent continuous phase prepared at constant temperature in Preparation Example 5; While maintaining the material temperature of the aforementioned eutectic solvent continuous phase at 115°C, the high-shear stirring device equipped in the reactor was started and the rotation speed was set to 600 r / min. The preheated coating premix obtained at 85°C was added at a uniform speed to the eutectic solvent continuous phase in the aforementioned reactor through a uniform feeding device. After the feeding was completed, the current speed was maintained and high shear mixing was continued for 10 minutes to obtain a suspension slurry with stable rheological properties. The obtained suspension slurry is pumped to the top of the high tower granulator through a mud pump that maintains the temperature at 115°C by heating the pipe wall. The slurry is centrifugally sprayed into droplets through a rotating nozzle, and falls freely in the granulation tower to exchange heat with the cold air at the bottom of the tower. After natural cooling and crystallization, the particles are collected at the bottom of the tower and screened to obtain the finished high tower sulfur-based fully water-soluble compound fertilizer. Example 3

[0041] This embodiment provides a method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer, including the following steps: Obtain the coated premix prepared at constant temperature in Preparation Example 2, and the eutectic solvent continuous phase prepared at constant temperature in Preparation Example 6; While maintaining the material temperature of the aforementioned eutectic solvent continuous phase at 118°C, the high-shear stirring device equipped in the reactor was started and the rotation speed was set to 400 r / min. The preheated coating premix obtained at 80°C was added at a uniform speed to the eutectic solvent continuous phase in the aforementioned reactor through a uniform feeding device. After the feeding was completed, the current speed was maintained and high shear mixing was continued for 8 minutes to obtain a suspension slurry with stable rheological properties. The obtained suspension slurry is pumped to the top of the high tower granulator through a mud pump that maintains the temperature at 118°C by heating the pipe wall. The slurry is centrifugally sprayed into droplets through a rotating nozzle, and falls freely in the granulation tower to exchange heat with the cold air at the bottom of the tower. After natural cooling and crystallization, the particles are collected at the bottom of the tower and screened to obtain the finished high tower sulfur-based fully water-soluble compound fertilizer.

[0042] Comparative Example 1: The difference from Example 1 is that no industrial-grade sodium hexametaphosphate was added when preparing the coating premix; all other aspects are the same.

[0043] Comparative Example 2: Compared with Example 1, the difference is that no industrial-grade urea phosphate was added when preparing the eutectic solvent continuous phase, and the material insulation and granulation temperature was increased to 135°C in order to maintain the flowability and pumpability of the slurry. All other aspects are the same.

[0044] Comparative Example 3: Compared with Example 1, the difference is that the coating premix was not preheated and was directly added to the continuous phase of the eutectic solvent at room temperature; all other aspects are the same.

[0045] Test Example 1: Pure urea and the eutectic solvent continuous phases prepared in Preparation Examples 4, 5, and 6 were selected as thermodynamic test samples. Differential scanning calorimetry was used for testing. Approximately 6 to 8 mg of sample was weighed and placed in an aluminum standard crucible, which was then covered and pressed tightly. Under a nitrogen protective atmosphere with a flow rate of 50 mL / min, the temperature was scanned from 30 °C to 150 °C at a heating rate of 10 °C / min, and the peak melting endothermic temperature of the sample was recorded.

[0046] The suspension slurries prepared in Examples 1, 2, and 3 were selected as rheological test samples. Before the test, each slurry sample was placed in a corresponding constant temperature device for heat preservation. The temperature of the sample in Example 1 was maintained at 116℃, the temperature of the sample in Example 2 was maintained at 115℃, and the temperature of the sample in Example 3 was maintained at 118℃.

[0047] Steady-state shear scan tests were performed on the suspended slurry using a rotational rheometer equipped with a coaxial cylindrical test fixture. The isothermal slurry was transferred into a test cylinder preheated to the same temperature and allowed to stand for 3 minutes to eliminate residual stress. The shear rate range was set to 0.1 s. -1 Increase continuously to 100s -1 The data acquisition time for each test point was 15 seconds. The apparent viscosity data of the system under different shear rates were recorded and the rheological curves were fitted.

[0048]

[0049] In Table 1, "-" indicates that the test was not tested or that the test specification is not applicable to the current physical state of the substance.

[0050] According to the data in Table 1, the peak endothermic melting temperature of pure urea is 133.4℃; combined with... Figure 1 The trend of the differential scanning calorimetry curves shows that after adding urea phosphate and choline chloride, the endothermic peak of the continuous phases obtained in Examples 4 to 6 shifts significantly towards lower temperatures, with the peak temperature dropping to the range of 105.1℃ to 111.9℃.

[0051] Test results show that electrostatic ion pairs are formed within the mixed system, disrupting the original regular lattice array of urea and producing a significant eutectic effect. This thermodynamic characteristic changes the limitation that conventional urea molten systems must be processed at temperatures above 130°C, allowing the material to maintain a liquid-phase flow state at an operating temperature of 115-118°C. This physically breaks down the reaction pathway of urea deammoniation condensation to form biuret at high temperatures.

[0052] Meanwhile, rheological test data showed that the apparent viscosity of the suspensions obtained in Examples 1 to 3 remained between 16.02 Pa·s and 22.31 Pa·s at a low shear rate of 0.1 s⁻¹; combined with Figure 2The rheological curves show that the system exhibits typical pseudoplastic fluid characteristics. The high initial static viscosity indicates that the encapsulated solid powder constructs a spatial structure with a certain yield stress within the continuous phase, restricting the gravitational sedimentation of the particles and ensuring the suspension stability of the high-solids-content system under static heat preservation or low-speed stirring. When the shear rate increases to 100 s⁻¹... -1 At this point, the apparent viscosity rapidly decreased from 1.76 Pa·s to 2.83 Pa·s. During the mud pump delivery and centrifugal scattering stages using the rotating nozzle at the high tower, the slurry was subjected to high-frequency shearing action applied by the equipment, causing the internal rheological structure to undergo orientational disintegration and viscosity reduction. This shear-thinning mechanism eliminates the engineering limitations of high-solids-content slurries easily causing mud pump overload and nozzle blockage, allowing the system to meet both the process requirements of static anti-settling and dynamic smooth flow.

[0053] Test Example 2: Pure urea and the eutectic solvent continuous phase prepared in Preparation Example 4 were selected as the infrared spectroscopy test samples. Transmission scanning was performed using a Fourier transform infrared spectrometer. Liquid samples were coated onto potassium bromide pellets, or solid samples were ground and pressed with potassium bromide to form pellets. The scanning range was set to 4000 cm⁻¹. -1 Up to 400cm -1 The resolution is 4cm. -1 The ambient temperature was controlled at 25℃ and the relative humidity was below 40%. The wavenumbers of the characteristic absorption peaks of carbonyl and amino groups in the system were recorded.

[0054] Industrial-grade pure monoammonium phosphate, industrial-grade pure potassium sulfate, and the finished product particles obtained after natural cooling and crystallization in Example 1 were selected as samples for phase structure analysis. The finished product of Example 1 was ground into powder in an agate mortar and passed through a 200-mesh standard sieve. Phase scanning was performed using an X-ray diffractometer with a CuKα ray source, tube voltage set to 40kV, tube current set to 40mA, scanning step size set to 0.02°, scanning rate set to 5° / min, and testing range set to 10° to 60°. The position and intensity of the characteristic diffraction peaks of each sample were recorded.

[0055]

[0056] In Table 2, "-" indicates that the substance was not tested or that the current substance does not contain the infrared / XRD characteristic peak.

[0057] Based on the data in Table 2, and combined with Figure 3The Fourier transform infrared spectrum shows that, compared with the infrared absorption of pure urea, the carbonyl stretching vibration peak and the amino stretching vibration peak in the liquid continuous phase of Preparation Example 4 both red-shifted towards lower wavenumbers, and the characteristic absorption peaks showed a broadening trend. This regular shift in the infrared absorption bands reflects the formation of new hydrogen bond networks between urea molecules and components such as urea phosphate and choline chloride, altering the original electron cloud distribution between molecules. This structural change in the chemical microenvironment confirms the formation of electrostatic ion pairs in the eutectic solvent continuous phase, explaining the thermodynamic mechanism of the system's macroscopic melting point reduction from the perspective of intermolecular forces.

[0058] Based on the data in Table 2, and combined with Figure 4 The X-ray diffraction pattern of the compound fertilizer product obtained in Example 1 shows that the diffraction patterns of the main crystal form characteristic diffraction peaks corresponding to pure monoammonium phosphate and pure potassium sulfate are well preserved. No significant angular shift was observed in the positions of the diffraction peaks, and no impurity phase diffraction peaks generated by the metathesis reaction were detected throughout the entire scanning range. The test results indicate that the pre-dry coating of sodium hexametaphosphate on the surface of the solid particles forms an effective physical and chemical barrier, preventing direct contact and side reactions between monoammonium phosphate and potassium sulfate in the urea melt environment. This test data confirms that the product maintains the phase independence of the original salts during processing and after molding, supporting the conclusion from a crystal structure perspective that the finished compound fertilizer possesses completely water-soluble properties and that there is no loss of nutrients as byproducts.

[0059] It should be noted that the microstructure of the sodium hexametaphosphate coating layer and the eutectic solvent continuous phase described in this invention mainly functions in the high-tower granulation preparation stage of compound fertilizer, i.e., the high-temperature suspension slurry state. During this intermediate liquid processing, the coating layer effectively isolates the solid and liquid from direct contact, and the eutectic system significantly reduces the granulation operation temperature. After the suspension slurry is cooled and solidified by high-tower spraying into the final finished particles, the above structure is transformed into a uniformly distributed solid solution or co-crystallized macroscopic matrix.

[0060] Although it is difficult to directly peel off and observe a pure liquid continuous phase or a clearly defined physical coating layer inside the finally cooled and solidified compound fertilizer granules using conventional methods such as XRD, the physicochemical property test data in Table 3 below show that the finished product of Example 1 of this invention has a lower mass fraction of water-insoluble matter, a shorter total water solubility time, and a lower biuret content than Comparative Example 2 of the high-temperature process. This macroscopic performance result, combined with the conclusion that no metathesis impurity phase was detected in the XRD pattern above, indirectly confirms from the two dimensions of macroscopic performance and phase stability that: in the high-temperature slurry preparation stage, the coating layer and the eutectic system effectively construct and play a protective role in suppressing salt interface side reactions and reducing thermal deammoniation side reactions.

[0061] Test Example 3: The suspended slurries prepared in Example 1, Comparative Example 1, and Comparative Example 3 were selected as static sedimentation test samples. Each slurry sample was injected into a constant temperature sedimentation tube with volume graduations. The sedimentation tube temperature of Example 1 and Comparative Example 1 was maintained at 116°C, and that of Comparative Example 3 was maintained at its actual equilibrium temperature after mixing. The volume of the clear liquid layer precipitated on the upper layer of the system was observed and recorded within 0 to 60 minutes. The percentage of the clear liquid layer to the total volume was calculated to evaluate the suspension stability of the slurry.

[0062] The finished compound fertilizer granules obtained after natural cooling and crystallization in Example 1 and Comparative Examples 1 to 3 were selected as samples for biuret content testing. Each group of samples was weighed, dissolved in deionized water and brought to a final volume. The absorbance was measured using a UV-Vis spectrophotometer within the required wavelength range. The mass fraction of biuret in each sample was calculated using the standard curve.

[0063] The finished compound fertilizer granules of Example 1 and Comparative Examples 1 to 3 were selected as samples for water solubility index testing. An equal amount of finished granules were weighed and put into deionized water at a constant temperature of 25°C. The mixture was stirred at a fixed speed, and the time required for the solid particles to completely disappear was recorded. The solution was then filtered through a pre-weighed microporous membrane. The membrane and the retained filter residue were dried to a constant weight, and the mass fraction of water-insoluble matter was calculated.

[0064]

[0065] Based on the data in Table 3, comparing the physicochemical properties of Example 1 and Comparative Example 2, Comparative Example 2, due to the absence of urea phosphate, failed to construct a continuous eutectic solvent phase, resulting in insufficient fluidity of the system at lower temperatures. Therefore, the processing temperature needed to be increased to maintain the pumping and granulation of the slurry. High-temperature processing conditions promoted the thermal deammoniation reaction of urea, leading to a biuret mass fraction of 1.58% in the finished product; in Example 1, the biuret mass fraction was 0.63%. The test results indicate that this invention, by constructing a eutectic system, reduced the processing temperature and suppressed the side reaction of urea thermal decomposition to generate biuret.

[0066] Based on the data in Table 3, and combined with Figure 5 and Figure 6 The trend of change is that, as in Comparative Example 1, the system lacks the core micro-dispersion and physical interlocking effect because it has not undergone dry mixing treatment with sodium hexametaphosphate. After entering the continuous phase, the solid powder is very prone to severe physical agglomeration and local caking. Figure 5 In Comparative Example 1, the volume ratio of the clear liquid layer increased sharply during the settling process, indicating that the system experienced severe solid-liquid phase separation and a significant decrease in suspension stability. Figure 6In Comparative Example 1, the solution transmittance increased more slowly during the dissolution process, corresponding to a water-insoluble matter mass fraction of 2.45% and a complete dissolution time extended to 342 s in Table 3. Example 1, through the high-shear micro-dispersion and physical isolation effect of sodium hexametaphosphate, effectively suppressed the physical aggregation of solid particles, maintained the high dispersion and homogeneity of the slurry, thereby ensuring that the finished product has a faster dissolution rate and higher solution transmittance.

[0067] Based on the data in Table 3, and combined with Figure 5 and Figure 6 The test results of Comparative Example 3 showed that when the premixed material coated at room temperature without preheating was directly added to the continuous phase of the eutectic solvent, a temperature difference was generated at the interface during solid-liquid mixing, which affected the rheological structure of the continuous phase, resulting in uneven coating on the surface of some particles, and causing particle sedimentation and side reactions. Figure 5 The static settlement rate of Comparative Example 3 was higher than that of Example 1. Figure 6 The rate of increase in the transmittance of the solution was lower than that in Example 1. Test data show that preheating the solid phase before feeding can reduce the temperature gradient during solid-liquid mixing, improve the uniformity of the flow field and temperature field in the high-shear mixing process, and help maintain the processing stability of the suspension slurry and the solubility of the finished product.

Claims

1. A high-tower sulfur-based fully water-soluble compound fertilizer, characterized in that, It is made by high-tower granulation of the following raw materials in parts by weight: 15.0-25.0 parts monoammonium phosphate, 25.0-35.0 parts potassium sulfate, and 0.5-1.0 parts sodium hexametaphosphate; 33.0-43.0 parts urea, 3.0-5.0 parts urea phosphate, 2.0-3.0 parts choline chloride, 0.1-0.3 parts polyethylene glycol 400, 0.2-0.4 parts sodium lignosulfonate; The preparation process of the compound fertilizer includes: crushing and classifying the monoammonium phosphate and potassium sulfate to obtain a mixed powder, and pre-coating and mixing the mixed powder with the sodium hexametaphosphate to obtain a coated premix. The urea, urea phosphate, choline chloride, polyethylene glycol 400 and sodium lignosulfonate are mixed to form a eutectic solvent continuous phase; the coated premix is ​​dispersed in the eutectic solvent continuous phase to form a suspension slurry, which is then sprayed and cooled in a high tower to form granules.

2. The high-tower sulfur-based fully water-soluble compound fertilizer according to claim 1, characterized in that, The particle size distribution of the mixed powder satisfies the following: the median diameter of the powder is 80-120 μm, the mass fraction passing through a 60-mesh standard sieve is 95%-99%, and the mass fraction passing through a 325-mesh standard sieve is 5%-15%.

3. The high-tower sulfur-based fully water-soluble compound fertilizer according to claim 2, characterized in that, The sodium hexametaphosphate is coated on the particle surface of the mixed powder.

4. A method for preparing a high-tower sulfur-based fully water-soluble compound fertilizer, characterized in that, The preparation of a high-tower sulfur-based fully water-soluble compound fertilizer as described in any one of claims 1 to 3 includes the following steps: Monoammonium phosphate and potassium sulfate are pulverized and particle size classified to a median diameter of 80-120 μm to obtain a mixed powder. The mixed powder is then dry-mixed with sodium hexametaphosphate at 20-25°C for 5-10 min to obtain a coated premix. The coated premix is ​​then heated to 80-85°C and sealed for heat preservation for later use. Urea was mixed with urea phosphate, choline chloride, polyethylene glycol 400 and sodium lignosulfonate. The mixture was then heated to 115-118°C to form a eutectic liquid phase and kept at a constant temperature for later use, thus obtaining a continuous phase of eutectic solvent. While maintaining a constant temperature of the eutectic solvent continuous phase material, the premixed material that has been kept at a constant temperature is uniformly added to the eutectic solvent continuous phase for high-shear mixing to obtain a suspension slurry. The suspended slurry is pumped to the top of the high-tower granulator, centrifugally sprayed into droplets by a rotating nozzle, and falls freely in the granulation tower to exchange heat with the cold air at the bottom of the tower. After cooling and crystallization, the particles are collected at the bottom of the tower and screened to obtain the finished high-tower sulfur-based fully water-soluble compound fertilizer.

5. The preparation method according to claim 4, characterized in that, The process of dry mixing the mixed powder with sodium hexametaphosphate at 20-25°C is carried out in a plow-type dry powder mixer, and the stirring speed is set to 120-150 r / min. The step of heating the coated premix to 80-85°C and sealing it for heat preservation involves transferring the coated premix into a screw conveyor with jacketed heat-conducting oil for dynamic heating and heat preservation.

6. The preparation method according to claim 4, characterized in that, The process of heating the mixture to 115–118°C to eutecticly form a liquid phase specifically involves: Place the mixture in a temperature-controlled reactor, turn on slow stirring and slowly heat to 115-118°C to allow the material to eutecticly melt and liquefy.

7. The preparation method according to claim 4, characterized in that, Under the condition of maintaining a constant temperature of the eutectic solvent continuous phase material, the premixed material that has been kept at a constant temperature is uniformly added to the eutectic solvent continuous phase for high-shear mixing. Specifically, under the condition of maintaining the material temperature at 115-118°C, the high-shear stirring device is started and the speed is set to 400-600 r / min, and high-shear mixing is performed continuously for 8-10 min.

8. The preparation method according to claim 4, characterized in that, The step of pumping the suspended slurry to the top of the high tower granulator specifically involves pumping the suspended slurry to the top of the high tower granulator using a mud pump that maintains the temperature at 115-118°C by heating the pipe wall.

9. The preparation method according to claim 6, characterized in that, The slow heating to 115-118°C specifically involves: Warm water or low-pressure steam is introduced into the jacket of the reactor to provide a heat source, and the heat transfer boundary temperature is controlled to not exceed 120°C to prevent local overheating.

10. The preparation method according to claim 4, characterized in that, The process of pulverizing and classifying monoammonium phosphate and potassium sulfate to a median diameter of 80–120 μm to obtain a mixed powder is carried out using an air jet mill classifier.

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