Low-temperature preparation method of high-activity chelated zinc fertilizer

Abstract

The invention relates to a low-temperature preparation method of a high-activity chelated zinc fertilizer, and belongs to the technical field of fertilizer chemical industry. Aiming at the problems of chelate decomposition and high energy consumption caused by an existing high-temperature process, ethylene diamine tetraacetic acid is added step by step in a low-temperature environment of 35-45 DEG C: firstly, an initial chelating solution is formed by adding 0.8-1.0 time of theoretical chelating molar weight, and then a sodium hydroxide solution is dropwise added to control the pH to be 3.5-4.5 within 60-120 minutes and is stable; and then supplementing ethylenediaminetetraacetic acid which is 0.1-0.2 time of the theoretical amount, reacting for 60-120 minutes to achieve chelation balance, and finally stirring, aging and naturally cooling. The method obviously improves the chelating efficiency and the product stability, reduces the energy consumption, and is used for preparing the efficient and stable liquid zinc fertilizer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fertilizers. More particularly, the present application relates to a low-temperature preparation method of high-activity chelated zinc fertilizer. BACKGROUND

[0002] In the field of chelated zinc fertilizer preparation, the existing technology has the following limitations: 1. The high-temperature reaction environment leads to insufficient stability of the product. The traditional process uses a temperature above 60℃ to accelerate the reaction, but high temperature promotes the decomposition of chelates, resulting in loss of effective ingredients and performance degradation during storage. Although reducing the temperature can alleviate the decomposition, it significantly prolongs the reaction time, and it is difficult to balance energy consumption and efficiency; 2. The problem of residual free zinc ions is prominent. The one-step chelation process is prone to zinc precipitation due to pH mutation, and even if the chelating agent is added in excess, it still cannot be completely complexed. The residual free zinc not only reduces the effectiveness of the product, but also causes precipitation and stratification during storage; 3. Impurities in raw materials restrict product purity. Impurity metals such as iron, manganese, and lead in industrial-grade zinc sulfate have a stronger binding ability with chelating agents than zinc ions, and they preferentially consume effective ingredients. Residual heavy metals not only reduce the zinc chelation rate, but also cause physiological disorders in plants. Conventional filtration methods are difficult to remove dissolved impurity complexes; 4. Physical properties and biological availability are limited. Low viscosity of liquid products leads to storage stratification, and poor leaf wetting affects crop absorption. At the same time, microbial reproduction easily causes product spoilage and clogs leaf stomata, further restricting nutrient utilization efficiency. SUMMARY

[0003] An object of the present application is to solve at least the above problems and to provide at least the advantages that will be mentioned later.

[0004] In order to achieve these objects and other advantages in accordance with the present application, a low-temperature preparation method of high-activity chelated zinc fertilizer is provided, comprising the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 35℃ to 45℃; Solid zinc sulfate is added to the reaction kettle, the amount of zinc sulfate added is such that its mass percentage concentration in water is 10% to 30%, and stirring is continued until the zinc sulfate is completely dissolved, obtaining a zinc sulfate solution; A total amount of 0.8 to 1.0 times the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added to the zinc sulfate solution, the theoretical chelation molar amount refers to the molar amount of ethylenediaminetetraacetic acid required to completely chelate with zinc ions in the added zinc sulfate, the temperature of the reaction system is controlled to maintain at 35℃ to 45℃, and the stirring reaction is continued for 10 min to 30 min, obtaining a preliminary chelation liquid; The temperature of the reaction system is maintained at 35℃ to 45℃, and a mass percentage concentration of 15% to 25% of sodium hydroxide solution is added dropwise to the preliminary chelation liquid, and the dropwise addition speed is controlled to make the pH value of the reaction system stable at 3.5 to 4.5 within 60 min to 120 min from the initial value; Under the conditions of maintaining the temperature of the reaction system at 35-45℃ and the pH value at 3.5-4.5, a total amount of 0.1-0.2 times the theoretical chelating molar amount of solid ethylenediaminetetraacetic acid is added to the reaction system, and the reaction is continuously stirred for 60-120 min to make the reaction system reach chelating equilibrium; The temperature of the reaction system is maintained at 35-45℃, and the stirring is continued for 30-90 min for aging; The reaction system is naturally cooled to room temperature to obtain a chelated zinc fertilizer solution.

[0005] Preferably, after the end of the aging step and before the start of the natural cooling step, potassium dihydrogen phosphate and sodium citrate are added to the reaction system; The amount of potassium dihydrogen phosphate added is 0.2-0.4 times the molar amount of zinc ions in the zinc sulfate; The amount of sodium citrate added is 0.3-0.5 times the mass of potassium dihydrogen phosphate; After the addition, the temperature of the reaction system is maintained at 35-45℃, and the stirring is continued for 30-60 min.

[0006] Preferably, after the addition of potassium dihydrogen phosphate and sodium citrate and the stirring for 30-60 min, xanthan gum is added to the reaction system; The amount of xanthan gum added is 0.05-0.15% of the total mass of the reaction system; After the addition of xanthan gum, the temperature of the reaction system is maintained at 45-55℃, and the stirring is continued for 60-90 min until a uniform colloid is formed.

[0007] Preferably, after the addition of xanthan gum and the stirring for 60-90 min, trisodium citrate is added to the reaction system; The amount of trisodium citrate added is 0.02-0.08% of the total mass of the reaction system; After the addition of trisodium citrate, the temperature of the reaction system is maintained at 35-45℃, and the stirring is continued for 30-45 min.

[0008] Preferably, after the natural cooling to room temperature, sodium benzoate is added to the obtained chelated zinc fertilizer solution; The amount of sodium benzoate added is 0.1-0.2% of the total mass of the reaction system; After the addition of sodium benzoate, the stirring is continued for 20-40 min at an environment of 25-30℃.

[0009] Preferably, after the sodium benzoate is added and stirred for 20 min to 40 min, an equivalent amount of hydrochloric acid is added dropwise to the reaction system to adjust the pH to 3.0-3.5, and then the β-cyclodextrin is added; The amount of the β-cyclodextrin added is 1.5 times to 3.0 times the mass of the sodium benzoate; After the β-cyclodextrin is added, the temperature of the reaction system is maintained at 40℃ to 45℃, and stirring is continued for 60 min to 90 min to form the inclusion compound.

[0010] Preferably, after the zinc sulfate solution is obtained, sodium hydroxide is added to the solution to adjust the pH to 5.0-6.0, and then ethylenediamine di-o-hydroxyphenyl acetic acid is added; The amount of the ethylenediamine di-o-hydroxyphenyl acetic acid added is 1.2 times to 1.8 times the total molar amount of the iron and manganese impurities in the zinc sulfate; The temperature of the reaction system is maintained at 35℃ to 45℃, and stirring is continued for 20 min to 40 min to form an impurity chelate precipitate; The impurity chelate precipitate is removed by filtration.

[0011] Preferably, in the filtration step, a nanofiltration membrane assembly with a molecular weight cut-off of 500 Da to 800 Da is used; The zinc sulfate solution is pumped into the nanofiltration membrane assembly at an operating pressure of 0.8 MPa to 1.2 MPa; The surface flow rate of the nanofiltration membrane is controlled at 1.5 m / s to 2.5 m / s, and the temperature is maintained at 35℃ to 45℃; The filtration cycle is stopped when the concentration of lead ions in the permeate is ≤0.1 ppm, and the permeate is collected as the purified zinc sulfate solution.

[0012] The present application at least includes the following beneficial effects: The present application improves reaction efficiency, reduces energy consumption and raw material consumption, and significantly enhances product stability through a low-temperature step-by-step chelation process. The free zinc ions are fixed by phosphate additives, the phosphorus nutrition is supplemented synchronously, and the chemical stability is strengthened. The solution dispersibility is improved by using a biological thickening agent to achieve zero stratification for long-term storage. The wetting agent is used to optimize the leaf surface adhesion performance, effectively promoting crop absorption. The preservative is used to inhibit microbial proliferation, maintain the patency of the stomata. The molecular inclusion technology is used to eliminate odor, and the leaf surface absorption sustainability is ensured. The selective chelating agent is used to deeply remove competitive metal impurities, and the chelation purity is improved. The membrane separation technology is used to target the removal of heavy metals, significantly reducing the risk of plant toxicity.

[0013] In general, the present application realizes four-dimensional performance breakthroughs of performance leap, long-term stability, biological synergism and green energy saving through the synergistic effect of low-temperature step-by-step chelation, deep impurity purification and composite functional modification: the low-temperature step-by-step process combined with phosphate stabilization enables the chelated zinc content to break through 13.0%, and the free zinc residue to be reduced to 0.10%; the synergistic effect of thickening agent and molecular inclusion corrosion protection ensures 90-day zero stratification, colony <100 CFU / mL, and decomposition rate as low as 1.2%; the wetting agent improves leaf adhesion (contact angle attenuation 0.17° / s), combined with heavy metal targeted removal, and the leaf zinc absorption amount is increased by 152%; the whole process is operated at low temperature, the energy consumption is reduced by 37.5%, and the EDTA dosage is reduced by 12.5%. Finally, a new type of chelated zinc fertilizer system with high activity, long-lasting effect, low toxicity and easy absorption is formed.

[0014] Other advantages, objects, and features of the present application will be apparent from the following specification, and will be understood by those skilled in the art. DETAILED DESCRIPTION

[0015] The present application will be further described in detail below with reference to the examples, so that those skilled in the art can implement it according to the description.

[0016] It should be noted that the experimental methods described in the following embodiments are conventional methods unless otherwise specified, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0017] <Example 1> A low-temperature preparation method of a high-activity chelated zinc fertilizer, comprising the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 40℃; Solid zinc sulfate is added to the reaction kettle, the amount of zinc sulfate added is such that its mass percentage concentration in water is 20%, and stirring is continued until the zinc sulfate is completely dissolved, obtaining a zinc sulfate solution; A total amount of 0.9 times the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added to the zinc sulfate solution, the theoretical chelation molar amount refers to the molar amount of ethylenediaminetetraacetic acid required for complete chelation with zinc ions in the added zinc sulfate, the reaction system temperature is controlled to maintain at 40℃, and the stirring reaction is continued for 20 min, obtaining a preliminary chelation liquid; The reaction system temperature is maintained at 40℃, and a 20% mass percentage concentration of sodium hydroxide solution is added dropwise to the preliminary chelation liquid, and the dropwise addition speed is controlled to stabilize the pH value of the reaction system at 4.0 within 90 min from the initial value; Under the conditions of maintaining the temperature of the reaction system at 40℃ and the pH value at 4.0, solid ethylenediaminetetraacetic acid with a total amount of 0.15 times the theoretical chelation molar amount is added to the reaction system, and the reaction is continuously stirred for 90 min to make the reaction system reach chelation equilibrium; The temperature of the reaction system is maintained at 40℃, and the aging is continuously stirred for 60 min; The reaction system is naturally cooled to room temperature to obtain a clear and transparent chelated zinc fertilizer solution.

[0018] <Embodiment 2> A low-temperature preparation method of a high-activity chelated zinc fertilizer includes the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 40℃; Solid zinc sulfate is added to the reaction kettle, the amount of zinc sulfate added is such that the mass percentage concentration of zinc sulfate in water is 20%, and the stirring is continuously performed until the zinc sulfate is completely dissolved to obtain a zinc sulfate solution; Solid ethylenediaminetetraacetic acid with a total amount of 0.9 times the theoretical chelation molar amount is added to the zinc sulfate solution, the temperature of the reaction system is controlled and maintained at 40℃, and the reaction is continuously stirred for 20 min to obtain a preliminary chelation solution; The temperature of the reaction system is maintained at 40℃, and a 20% mass percentage concentration sodium hydroxide solution is added dropwise to the preliminary chelation solution, and the dropwise addition speed is controlled to make the pH value of the reaction system stable at 4.0 from the initial value within 90 min; Under the conditions of maintaining 40℃ and pH 4.0, solid ethylenediaminetetraacetic acid with a total amount of 0.15 times the theoretical chelation molar amount is added, and the reaction is continuously stirred for 90 min; 40℃ is maintained, and the aging is continuously stirred for 60 min; Potassium dihydrogen phosphate and sodium citrate are added to the system after aging, the amount of potassium dihydrogen phosphate added is 0.3 times the molar amount of zinc ions in zinc sulfate, and the amount of sodium citrate added is 0.4 times the mass of potassium dihydrogen phosphate; 40℃ is maintained, and the stirring is continuously performed for 45 min; Xanthan gum is added to the system, and the amount of xanthan gum added is 0.1% of the total mass of the reaction system; 50℃ is maintained, and the stirring is continuously performed for 75 min until a uniform colloid is formed; Trisodium citrate is added to the system, and the amount of trisodium citrate added is 0.05% of the total mass of the reaction system; 40℃ is maintained, and the stirring is continuously performed for 38 min; The reaction system is naturally cooled to room temperature to obtain a uniform and stable chelated zinc fertilizer solution.

[0019] <Embodiment 3> A low-temperature preparation method of high-activity chelated zinc fertilizer, comprising the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 40℃; Solid zinc sulfate is added to the reaction kettle, and the amount of zinc sulfate added is such that the mass percentage concentration of zinc sulfate in water is 20%, and stirring is continued until complete dissolution, obtaining a zinc sulfate solution; A total amount of 0.9 times the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added to the zinc sulfate solution, the temperature of the reaction system is controlled at 40℃, and stirring is continued for 20 min to obtain a preliminary chelation solution; The temperature of the reaction system is maintained at 40℃, and a 20% mass percentage concentration of sodium hydroxide solution is added dropwise to the preliminary chelation solution, and the dropwise addition speed is controlled to stabilize the pH value of the reaction system at 4.0 within 90 min; Under the conditions of maintaining 40℃ and pH 4.0, a total amount of 0.15 times the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added, and stirring is continued for 90 min; Maintain 40℃ and continue to stir and age for 60 min; The reaction system is naturally cooled to 28℃ (room temperature); Sodium benzoate is added to the cooled solution, and the amount of sodium benzoate added is 0.15% of the total mass of the reaction system; Stir at 28℃ for 30 min; 6mol / L hydrochloric acid is added dropwise to the reaction system to adjust the pH to about 3.3, and then β-cyclodextrin is added, and the amount of β-cyclodextrin added is 2.0 times the mass of sodium benzoate; The temperature is raised to 42℃, and stirring is continued for 75 min to form an inclusion complex; Cool to room temperature to obtain a chelated zinc fertilizer solution with no irritating odor.

[0020] <Example 4> A low-temperature preparation method of high-activity chelated zinc fertilizer, comprising the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 40℃; Solid zinc sulfate is added to the reaction kettle, and the amount of zinc sulfate added is such that the mass percentage concentration of zinc sulfate in water is 20%, and stirring is continued until complete dissolution, obtaining a zinc sulfate solution; Sodium hydroxide is added to the zinc sulfate solution to adjust the pH to about 5.5, and then ethylenediamine di-o-hydroxyphenyl acetic acid (EDDHA) is added, and the amount of EDDHA added is 1.5 times the total molar amount of iron and manganese impurities in the zinc sulfate; The temperature of the reaction system is maintained at 40℃, and the reaction is continuously stirred for 30 min to form impurity chelate precipitate; The impurity chelate precipitate is removed by filtration using a nanofiltration membrane module with a molecular weight cut-off of 650 Da. The solution is pumped into the nanofiltration membrane module at an operating pressure of 1.0 MPa, and the surface flow rate of the nanofiltration membrane is controlled at 2.0 m / s. The temperature is maintained at 40℃. The filtration cycle is stopped when the lead ion concentration in the permeate is ≤0.1 ppm. The permeate is collected as the purified zinc sulfate solution. Solid ethylenediaminetetraacetic acid is added to the purified zinc sulfate solution in a total amount of 0.9 times the theoretical chelation molar amount. The temperature of the reaction system is maintained at 40℃, and the reaction is continuously stirred for 20 min to obtain a preliminary chelate solution. The preliminary chelate solution is transferred to a reaction kettle, and the temperature is maintained at 40℃. A 20% by mass hydroxide sodium solution is added dropwise to the preliminary chelate solution. The dropwise addition rate is controlled to stabilize the pH value of the reaction system at 4.0 within 90 min. Under the conditions of maintaining the temperature of the reaction system at 40℃ and the pH value at 4.0, solid ethylenediaminetetraacetic acid is added in a total amount of 0.15 times the theoretical chelation molar amount. The reaction is continuously stirred for 90 min. The temperature of the reaction system is maintained at 40℃, and the stirring is continued for 60 min. The reaction system is naturally cooled to room temperature to obtain a colorless and transparent chelated zinc fertilizer solution.

[0021] <Example 5> A low-temperature preparation method of a high-activity chelated zinc fertilizer includes the following steps: Deionized water is added to a reaction kettle, and the stirrer is started. The temperature is controlled at 40℃. Solid zinc sulfate is added to the reaction kettle. The amount of zinc sulfate added is such that the mass percentage concentration of zinc sulfate in water is 20%. The stirring is continued until the zinc sulfate is completely dissolved to obtain a zinc sulfate solution. Sodium hydroxide is added to the zinc sulfate solution to adjust the pH to about 5.5. Ethylenediamine di-o-hydroxyphenyl acetic acid is then added in an amount of 1.5 times the total molar amount of iron and manganese impurities in the zinc sulfate. The temperature of the reaction system is maintained at 40℃, and the reaction is continuously stirred for 30 min to form impurity chelate precipitate. The impurity chelate precipitate is removed by filtration using a nanofiltration membrane module with a molecular weight cut-off of 650 Da. The solution is pumped into the nanofiltration membrane module at an operating pressure of 1.0 MPa, and the surface flow rate of the nanofiltration membrane is controlled at 2.0 m / s. The temperature is maintained at 40℃. The filtration cycle is stopped when the lead ion concentration in the permeate is ≤0.1 ppm. The permeate is collected as the purified zinc sulfate solution. A total amount of 0.9 times of the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added to the purified zinc sulfate solution, the temperature of the reaction system is controlled to be maintained at 40°C, and the reaction is continuously stirred for 20 min to obtain a preliminary chelation liquid; The temperature of the reaction system is maintained at 40°C, and a 20% mass percentage concentration of sodium hydroxide solution is added dropwise to the preliminary chelation liquid, and the dropping speed is controlled to stabilize the pH value of the reaction system at 4.0 within 90 min; Under the conditions of maintaining the temperature of the reaction system at 40°C and the pH value at 4.0, a total amount of 0.15 times of the theoretical chelation molar amount of solid ethylenediaminetetraacetic acid is added to the reaction system, and the reaction is continuously stirred for 90 min; The temperature of the reaction system is maintained at 40°C, and the stirring is continued for 60 min; Potassium dihydrogen phosphate and sodium citrate are added to the system after aging, the amount of potassium dihydrogen phosphate added is 0.3 times the molar amount of zinc ions in zinc sulfate, and the amount of sodium citrate added is 0.4 times the mass of potassium dihydrogen phosphate; Maintain 40°C for 45 min of continuous stirring; Xanthan gum is added, and the amount added is 0.1% of the total mass of the reaction system; Maintain 50°C for 75 min of continuous stirring until a uniform colloid is formed; Trisodium citrate is added, and the amount added is 0.05% of the total mass of the reaction system; Maintain 40°C for 38 min of continuous stirring; The reaction system is naturally cooled to 28°C (room temperature); Sodium benzoate is added, and the amount added is 0.15% of the total mass of the reaction system; Maintain 28°C for 30 min of continuous stirring; 6 mol / L hydrochloric acid is added dropwise to the reaction system to adjust the pH to about 3.3, and then β-cyclodextrin is added, and the amount added is 2.0 times the mass of sodium benzoate; The temperature is raised to 42°C, and the stirring is continued for 75 min to form an inclusion compound; Cool to room temperature to obtain a transparent chelated zinc fertilizer solution that is uniform and odorless.

[0022] <Comparative Example 1> A method for preparing a chelated zinc fertilizer, comprising the following steps: Deionized water is added to the reaction kettle, the stirrer is started, and the temperature is controlled at 60°C; Solid zinc sulfate is added to the reaction kettle, and the amount of zinc sulfate added is such that the mass percentage concentration in water is 20%, and the stirring is continued until complete dissolution; The total amount of solid ethylenediaminetetraacetic acid added to the zinc sulfate solution at one time was 1.2 times the theoretical chelation molar amount (the definition of the theoretical chelation molar amount is the same as in Example 1); The pH was adjusted to 4.0 within 30 min by rapidly dropping a 25% mass percentage sodium hydroxide solution; The reaction was maintained at 60°C for 60 min under continuous stirring; The reaction system was naturally cooled to room temperature to obtain a chelated zinc fertilizer solution.

[0023] <Experiment and Conclusion> (1) Comparative Example 1 (traditional high-temperature process), Example 1 (basic low-temperature step-by-step process), and Example 5 (comprehensive intensified process) were selected for comparative testing. All samples used the same batch of raw materials: industrial-grade zinc sulfate (ZnSO4·7H2O≥99.0%, Fe≤100 ppm, Mn≤50 ppm, Pb≤2.5 ppm), ethylenediaminetetraacetic acid (EDTA≥99.0%). Three batches of samples were prepared in parallel, and were left to stand at room temperature for 7 days before testing.

[0024] The chelated zinc content was determined according to the NY 1110-2010 standard, using an atomic absorption spectrometer (PerkinElmer PinAAcle 900T) to detect the total amount of zinc elements, and the proportion of chelated zinc was calculated after deducting the free zinc. The free zinc ions were determined by potentiometric titration (GB / T23739-2009), using a 0.05 mol / L EDTA standard solution to titrate to the potential jump end point. The total amount of EDTA was calculated according to the ratio of the actual amount of material to the theoretical chelation molar amount. The energy consumption data was recorded in real time by a steam flow meter (Emerson Rosemount 8800D) to record the steam consumption for heating the reaction kettle to the target temperature and maintaining it throughout the process, which was converted to energy consumption per ton of product. The product stability was tested by a 40°C constant temperature accelerated experiment, and the decomposition rate was calculated by monitoring the change in peak area of EDTA-Zn chelate by high performance liquid chromatography (Agilent 1260Infinity II) every month.

[0025] The experimental results are shown in Table 1 below: Table 1 The high-temperature one-step process of Comparative Example 1 showed serious defects: although the reaction at 60°C accelerated the chelation kinetics, the free zinc residue was as high as 2.51%, proving that the excess EDTA (1.2 times) did not completely chelate the zinc ions. The chelated zinc content was only 11.2%, which was 17% lower than the theoretical value (13.5%), indicating that high temperature led to an increase in side reactions. The energy consumption of 285 MJ / t was 60% higher than that of the low-temperature process, and the decomposition rate reached 12.1% after accelerated storage, confirming that the high-temperature environment promoted the degradation of the chelate.

[0026] The low-temperature step-by-step process of Example 1 achieved a breakthrough improvement: under the condition of 40℃, through the synergistic effect of the first 0.9 times EDTA acidic pre-chelation and the second 0.15 times EDTA pH4.0 supplementary chelation, the free zinc was controlled at 0.83%, and the chelated zinc content was increased to 12.8%. The total EDTA dosage was reduced to 1.05 times, which was 12.5% less than the comparative example. The energy consumption of 178 MJ / t was significantly reduced, and the 6-month decomposition rate was only 2.9%, which verified that the low-temperature environment was conducive to the stability of the chelate.

[0027] Example 5, on the basis of maintaining the core process of Example 1, integrated phosphate stabilization and nanofiltration impurity removal: the free zinc was further reduced to 0.10%, which was due to the precipitation and fixation of residual zinc ions by potassium dihydrogen phosphate; the chelated zinc content reached 13.0% due to the deep purification of raw materials. The energy consumption increased slightly to 230 MJ / t due to the nanofiltration process, but the decomposition rate decreased to 1.2%, which proved that the comprehensive stability was improved.

[0028] (2) Select Example 1 (basic low-temperature process), Example 2 (add potassium dihydrogen phosphate + xanthan gum + trisodium citrate), Example 5 (comprehensive strengthening process: containing impurity removal and corrosion prevention) and Comparative Example 1 (traditional high-temperature process) for parallel test. The experiment used a unified batch of raw materials (industrial grade zinc sulfate: ZnSO4·7H2O≥99.0%, Fe 82±5 ppm, Mn 43±3 ppm, Pb 2.3±0.2 ppm; EDTA≥99.0%). The chelated zinc fertilizer solution was prepared according to the method of each example and comparative example, and three batches in parallel.

[0029] Chelated zinc content: according to the standard NY 1110-2010, the total zinc content was determined by atomic absorption spectrometry, the free zinc ion was determined by potentiometric titration method (GB / T 23739-2009), and the proportion of chelated zinc was calculated. EDTA utilization rate: according to the actual dosage, the ratio of total EDTA dosage to theoretical chelation molar quantity was calculated. Accelerated storage experiment: 40℃ constant temperature storage for 6 months, the peak area change of EDTA-Zn chelate was monitored by high performance liquid chromatography every month, and the decomposition rate was calculated. Dispersion stability: room temperature standing for 90 days, observing the precipitation layering phenomenon and recording the volume ratio of the precipitate. Viscosity change: the apparent viscosity was determined by rheometer (Brookfield DV2T) at 25℃ (spindle LV-63, 60 rpm). After 90 days of room temperature storage, the corn seedlings (variety Zhengdan 958, 5 leaf stage) were sprayed with 2 mL of the sample solution containing 50 mg / L of zinc, and the contact angle of the droplet on the corn leaf surface was recorded by contact angle measuring instrument (OCA20) within 10 seconds. After 24 hours, the leaves were collected, microwave digested, and the zinc content increment was determined by atomic absorption spectrometry.

[0030] The experimental results are shown in Table 2 below: Table 2 The free zinc ion residue of Comparative Example 1 reached 2.51%, resulting in only 11.2% of chelated zinc content. After 90 days of solution standing, serious stratification (precipitate volume 18.5%) occurred, low viscosity (1.8 mPa·s) could not maintain suspension stability, and the contact angle decay rate of 0.42° / s indicated poor wetting, resulting in only 15.1 mg / kg of final leaf zinc increment.

[0031] Example 1 used a low-temperature step-by-step chelation process, with free zinc reduced to 0.83% and chelated zinc content increased to 12.8%. The precipitate volume was significantly reduced to 5.2%, but the viscosity was still low (2.0 mPa·s). The contact angle decay rate of 0.45° / s was comparable to the comparative example, and the leaf zinc increment of 20.3 mg / kg was 34% higher than the comparative example.

[0032] Example 2 achieved triple optimization by adding potassium dihydrogen phosphate, xanthan gum and trisodium citrate: potassium dihydrogen phosphate further reduced free zinc to 0.35%; xanthan gum increased viscosity to 152 mPa·s, with only 0.8% of precipitate volume after 90 days; trisodium citrate improved leaf wetting (contact angle decay 0.18° / s), synergistically promoting leaf zinc absorption to 26.7 mg / kg, which was 31% higher than Example 1.

[0033] Example 5 integrated all the optimized processes: free zinc was reduced to the lowest value of 0.10%, and chelated zinc content reached 13.0%. Xanthan gum maintained high viscosity (155 mPa·s) to reduce the precipitate volume to 0.5%, and the contact angle decay rate of 0.17° / s was the best wetting performance, with a final leaf zinc increment of 38.1±1.8 mg / kg, which was 152% higher than Comparative Example 1.

[0034] (3) Comparative Example 1 (traditional non-preservation process), Example 1 (basic low-temperature process without preservation), Example 3 (containing sodium benzoate and β-cyclodextrin inclusion), and Example 5 (comprehensive preservation and strengthening) were selected for comparison. All samples were prepared with the same raw materials and stored at room temperature for 0 days, 30 days and 90 days, respectively.

[0035] Microbial total count was determined according to GB 20287-2006, using nutrient agar medium to cultivate at 37°C for 48 hours. Odor evaluation was performed by 6 panelists who were selected by olfactory sensitivity screening (0-5 score system: 0 = no odor, 5 = strong irritating odor). Leaf absorption experiment selected potted corn seedlings (variety Zhengdan 958, 5 leaf stage), sprayed 2 mL of the test solution containing 50 mg / L of zinc on the leaves, and observed the stomatal opening of the lower epidermis of the leaves by scanning electron microscopy (Hitachi SU8010) after 2 hours (50 stomata were measured for each sample, and the opening ratio was calculated). After 24 hours, the leaves were collected and digested by microwave, and the zinc content was determined by atomic absorption spectrometry.

[0036] The experimental data are shown in Table 3: Table 3 When no preservative was added in Comparative Example 1 and Example 1, the total number of colonies increased exponentially with the extension of storage time. After 90 days, the colonies of Comparative Example 1 reached 3.2 x 10 6 CFU / mL, producing strong rancid odor (score 4.2), microbial metabolites blocking the leaf stomata causing the opening degree to decrease from 78% to 38%, and the zinc absorption amount of the leaves decreased by 57%. Example 1 slowed down the microbial proliferation due to the low temperature process, but after 90 days, the colonies still exceeded 10 5 CFU / mL, and the zinc absorption amount decreased by 42%.

[0037] Example 3 achieved significant bacteriostasis by adding sodium benzoate, and the colonies were controlled at 85 CFU / mL after 90 days. However, the uncomplexed sodium benzoate made the fresh sample have a slight sour taste (score 0.2), and the score increased to 0.4 after long-term storage. After being complexed by β-cyclodextrin, the benzoic acid was molecularly encapsulated, and the odor score was always below 0.4 (close to no odor). The complex released benzoic acid on the leaf surface, avoiding stomatal blockage (the opening degree was maintained at >79%), and the zinc absorption amount reached 35.0 mg / kg after 90 days, which was increased by 72% compared with Example 1.

[0038] Example 5 integrated raw material purification and suspension stabilization technology, and was further optimized on the basis of preservative complexation. The total number of colonies was comparable to that of Example 3, but due to the reduction of impurities (lead residue 0.007 ppm) and the solubilizing effect of xanthan gum, the zinc absorption amount on the leaf surface reached 41.3 mg / kg (0 days) and 38.1 mg / kg (90 days), which was increased by 6-9% compared with Example 3. It was proved that the comprehensive process had synergistic gain for plant absorption.

[0039] (4) Example 1 (without impurity removal process), Example 4 (EDDHA + nanofiltration impurity removal), Example 5 (comprehensive impurity removal + additive reinforcement) and Comparative Example 1 (traditional high-temperature process) were selected for parallel testing. The same batch of industrial-grade zinc sulfate raw material (ZnS04·7H20≥99.0%, Fe 82±5 ppm, Mn 43±3 ppm, Pb 2.3±0.2 ppm), EDTA (≥99.0%) and EDDHA (≥90.0%) were used in the experiment. All samples were strictly prepared according to the methods of each example for three batches.

[0040] Lead ion concentration: Determined by graphite furnace atomic absorption spectrometry (GB / T 5009.12-2017) in the finished solution. Iron and manganese content: Inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 8300). Chelated zinc content and free zinc ion determination are the same as the previous scheme. Accelerated decomposition rate: Calculate the loss rate of EDTA-Zn chelate HPLC peak area after 6 months of storage at 40°C. The roots of wheat seedlings (variety Jimai 22, plant height 10 cm) were exposed to 100-fold diluted test solution for 48 hours, and the root length inhibition rate was determined. The leaf surface spraying experiment is the same as the scheme in (2) (corn seedlings, 50 mg / L zinc concentration), and the leaf zinc increment is measured after 24 hours.

[0041] The experimental data are shown in Table 4: Table 4 Examples 4 and 5 added EDDHA (1.5 times the molar amount of iron and manganese impurities) immediately after dissolving zinc sulfate, and formed stable iron / manganese-EDDHA chelate precipitates at 40°C. The data show that this step reduces the residual iron and manganese to 2.1 ppm and 1.5 ppm (Example 4), respectively, with a removal rate of more than 97%. This is because the binding constant of EDDHA to Fe 3+ (logK=33.9) and Mn²⁺ (logK=14.5) is much higher than that of EDTA-Zn (logK=16.5), which preferentially chelates and precipitates impurity metals. The iron and manganese concentrations in the solution after filtration are significantly lower than those in Example 1 (79 ppm Fe, 42 ppm Mn), which avoids impurity competition for chelation from the source.

[0042] Example 4 and 5 employed nanofiltration membrane with a cut-off molecular weight of 650 Da to achieve deep removal of lead ions at a pressure of 1.0 MPa. The residual lead in Example 4 was 0.08 ppm, while Example 5 was further reduced to 0.007 ppm by recycling filtration, much lower than 1.85 ppm in Comparative Example 1. This effect was attributed to two aspects: one, the nanofiltration membrane pore size (about 1 nm) could retain metal complexes with a molecular weight greater than 500 Da; the second, the operating conditions (40°C, 2.0 m / s surface flow rate) inhibited membrane fouling, ensuring sustained and efficient separation. The complete removal of lead was crucial to reduce phytotoxicity.

[0043] The accelerated decomposition rate of Example 4 (1.8%) was significantly lower than Example 1 (2.9%), demonstrating that iron and manganese impurities could catalyze the oxidative degradation of chelates. The elimination of lead further reduced the side reactions induced by heavy metals, reducing the decomposition rate of Example 5 to 1.2%. In addition, free zinc was better controlled in a pure system: Example 5 fixed residual zinc by potassium dihydrogen phosphate, with free zinc of only 0.10%, while Example 4 did not add phosphate, with free zinc of 0.81%.

[0044] Hydroponic experiments showed that the high residual heavy metals in Comparative Example 1 and Example 1 resulted in a wheat root length inhibition rate of 38-41%, while Examples 4 and 5 reduced the inhibition rate to 5% and 3%, respectively, due to deep removal of impurities. Leaf absorption data was more direct: the zinc increment of Example 4 (32.5 mg / kg) was 60% higher than that of Example 1 (20.3 mg / kg), demonstrating that the bioavailability of zinc was significantly improved after the removal of competitive metal ions. Example 5 had a zinc increment of 38.1 mg / kg due to the optimization of the integrated process, reflecting the synergistic gain of impurity removal and stabilizer addition.

[0045] Although embodiments of the present application have been disclosed as above, they are not limited to the application listed in the specification and embodiments, and can be fully applied to various fields suitable for the present application, and additional modifications can be easily made by those skilled in the art, and therefore the present application is not limited to specific details and the examples shown and described herein, without departing from the general concept defined by the claims and equivalent scope.

Claims

1. A low temperature process for the preparation of a high active chelated zinc fertilizer, characterized in that, The method comprises the following steps: Deionized water is added into a reaction kettle, a stirrer is started, and the temperature is controlled at 35-45 DEG C; Solid zinc sulfate is added into the reaction kettle, the amount of zinc sulfate added is such that the mass percentage concentration of zinc sulfate in water is 10-30%, and stirring is continued until the zinc sulfate is completely dissolved to obtain a zinc sulfate solution; Solid ethylenediaminetetraacetic acid is added into the zinc sulfate solution in a total amount of 0.8-1.0 times the theoretical chelation molar amount, the theoretical chelation molar amount refers to the molar amount of ethylenediaminetetraacetic acid required for complete chelation with zinc ions in the added zinc sulfate, the temperature of the reaction system is controlled at 35-45 DEG C, and stirring is continued for 10-30 min to obtain a preliminary chelation solution; The temperature of the reaction system is maintained at 35-45 DEG C, a 15-25% sodium hydroxide solution is added dropwise into the preliminary chelation solution, and the dropping speed is controlled such that the pH value of the reaction system is stabilized at 3.5-4.5 within 60-120 min from the initial value; Under the conditions that the temperature of the reaction system is maintained at 35-45 DEG C and the pH value is maintained at 3.5-4.5, solid ethylenediaminetetraacetic acid is added into the reaction system in a total amount of 0.1-0.2 times the theoretical chelation molar amount, and stirring is continued for 60-120 min to make the reaction system reach chelation equilibrium; The temperature of the reaction system is maintained at 35-45 DEG C, and stirring is continued for 30-90 min for aging; The reaction system is naturally cooled to room temperature to obtain a chelated zinc fertilizer solution.

2. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 1, characterized in that, After the end of the aging step and before the start of the natural cooling step, potassium dihydrogen phosphate and sodium citrate are added into the reaction system; The amount of potassium dihydrogen phosphate added is 0.2-0.4 times the molar amount of zinc ions in the zinc sulfate; The amount of sodium citrate added is 0.3-0.5 times the mass of potassium dihydrogen phosphate; After the addition, the temperature of the reaction system is maintained at 35-45 DEG C, and stirring is continued for 30-60 min.

3. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 2, characterized in that, After the addition of potassium dihydrogen phosphate and sodium citrate and stirring for 30-60 min, xanthan gum is added into the reaction system; The amount of xanthan gum added is 0.05-0.15% of the total mass of the reaction system; After the addition of xanthan gum, the temperature of the reaction system is maintained at 45-55 DEG C, and stirring is continued for 60-90 min until a uniform colloid is formed.

4. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 3, characterized in that, After the addition of xanthan gum and stirring for 60-90 min, trisodium citrate is added into the reaction system; The amount of trisodium citrate added is 0.02-0.08% of the total mass of the reaction system; After the addition of trisodium citrate, the temperature of the reaction system is maintained at 35-45 DEG C, and stirring is continued for 30-45 min.

5. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 1, characterized in that, After the natural cooling to room temperature, sodium benzoate is added into the obtained chelated zinc fertilizer solution; The amount of sodium benzoate added is 0.1-0.2% of the total mass of the reaction system; After the addition of sodium benzoate, stirring is continued for 20-40 min in an environment of 25-30 DEG C.

6. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 5, characterized in that, After the sodium benzoate is added and stirred for 20 min to 40 min, an equivalent amount of hydrochloric acid is added dropwise to the reaction system to adjust the pH to 3.0-3.5, and then β-cyclodextrin is added; The amount of β-cyclodextrin added is 1.5 to 3.0 times the mass of sodium benzoate; After the β-cyclodextrin is added, the temperature of the reaction system is maintained at 40-45°C, and stirring is continued for 60-90 min to form an inclusion complex.

7. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 1, characterized in that, After the zinc sulfate solution is obtained, sodium hydroxide is added to the solution to adjust the pH to 5.0-6.0, and then ethylenediamine di-o-hydroxyphenyl acetic acid is added; The amount of ethylenediamine di-o-hydroxyphenyl acetic acid added is 1.2 to 1.8 times the total molar amount of iron and manganese impurities in the zinc sulfate; The temperature of the reaction system is maintained at 35-45°C, and stirring is continued for 20-40 min to form an impurity chelate precipitate; The impurity chelate precipitate is removed by filtration.

8. The process for the low temperature preparation of highly active chelated zinc fertilizers according to claim 7, characterized in that, In the filtration step, a nanofiltration membrane module with a molecular weight cut-off of 500-800 Da is used; The zinc sulfate solution is pumped into the nanofiltration membrane module at an operating pressure of 0.8-1.2 MPa; The nanofiltration membrane surface flow rate is controlled at 1.5-2.5 m / s, and the temperature is maintained at 35-45°C; The filtration cycle is stopped when the lead ion concentration in the permeate is ≤0.1 ppm, and the permeate is collected as the purified zinc sulfate solution.

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