High-purity oxamide with concentrated particle size and spherical morphology, preparation method of high-purity oxamide and application of high-purity oxamide in preparation of slow-release fertilizer

Abstract

The invention discloses high-purity oxamide with concentrated particle size and spherical morphology, a preparation method of the high-purity oxamide and application of the high-purity oxamide in preparation of a slow-release fertilizer, and belongs to the technical field of oxamide synthesis and application. The method comprises the following steps: mixing an ammonia-alcohol solution and an alcohol ester solution, conveying the mixed solution into a series tubular reactor for reaction, and then conveying the mixed solution into a parallel tubular reactor for reaction after pressure stabilization and flow equalization to obtain a reaction solution containing oxamide; performing flash evaporation on the reaction liquid to obtain flash evaporation liquid and flash evaporation gas, and performing solid-liquid separation on the flash evaporation liquid to obtain solid oxamide and filtrate; and drying the solid oxamide to obtain the oxamide. The method realizes high conversion rate and high purity, and the product has ultrafine particle size distribution. The purity of the prepared oxamide is greater than or equal to 99%, the D50 particle size is concentrated to 50 + / -15 microns, the spherical morphology is uniform, the release is stable, and the oxamide has a very good effect in preparation of long-acting slow-release fertilizers and combustion inhibitors and research on the nitrogen utilization rate.

Description

Technical Field

[0001] This invention belongs to the field of oxalamide synthesis and application technology, specifically relating to a high-purity oxalamide with concentrated particle size and spherical morphology, its preparation method, and its application in the preparation of slow-release fertilizers. Background Technology

[0002] Oxamide (OX) is an important organic compound with the molecular formula C2H4N2O2 and the structural formula (CONH2)2. Due to its high nitrogen content and efficient nitrogen-fixing properties, oxamide is widely used in the high-end fertilizer field as one of the main raw materials for slow-release fertilizers.

[0003] Currently, the main methods for synthesizing oxalamide include: 1. Hydrogen cyanide method: This method uses hydrogen cyanide as a raw material to synthesize oxalamide, but the cost of HCN is high, and HCN is a highly toxic chemical, making industrial production infeasible. 2. Ammonium oxalate or urea oxalate pyrolysis method: This method generates oxalamide by heating and dehydrating ammonium oxalate or urea oxalate. However, the yield of oxalamide preparation by pyrolysis is only 30-60%, resulting in high costs and hindering industrialization. 3. Oxalate ester method: This method generates oxalamide and the corresponding alcohol through the ammonolysis reaction of oxalate esters. Among these, the reaction of dimethyl oxalate with ammonia to prepare oxalamide is currently the most common industrial production method.

[0004] In the 1980s, Ube Industries of Japan adopted a discontinuous batch process to produce oxalamide via ammonolysis of dimethyl oxalate, achieving industrial-scale production of oxalamide. This method produces high-purity oxalamide, but the reaction time is long, lasting several hours, and the equipment consumes a lot of energy. Recent publicly disclosed oxalamide production technologies include patent applications CN111153823A, CN113045442A, and CN103242188A. CN113045442A discloses a method for preparing oxalamide using a microchannel reactor. Oxalate ester solution and ammonia are fed into the microchannel reactor in a specific ratio, reacting at 0–15°C for 5–60 seconds to obtain an aqueous oxalamide solution. Excess ammonia is then removed by continuous distillation. While this method improves reaction efficiency by utilizing a microchannel reactor, the use of ammonia as a reactant and the aqueous phase reaction complicate subsequent separation and purification processes. Furthermore, the oxalate ester is easily hydrolyzed to oxalic acid during the reaction, which is detrimental to obtaining high-purity oxalamide. Patent application CN119455856A discloses a process and apparatus for producing oxalamide. This involves mixing a methanol solution of liquid ammonia and a methanol solution of dimethyl oxalate to induce an ammonolysis reaction. After the reaction, the slurry is flash-evaporated, filtered, and dried to obtain solid oxalamide. Liquid-phase purification yields methanol as a byproduct. Ammonia in the tail gas is washed and absorbed by dimethyl oxalate and then returned to the reaction system. While this method achieves efficient utilization of ammonia, it still uses a traditional reactor, resulting in limited mass and heat transfer efficiency. Furthermore, the reaction temperature is high (40-130℃, 0.05-2.0 MPaG), and the reaction time is relatively long (0.2-1 h). Patent application CN115715962A discloses an apparatus and method for the continuous and efficient preparation of oxalamide. This method involves dissolving dimethyl oxalate and liquid ammonia separately in methanol solvent for a liquid-phase reaction. A steam drum is used to recover the reaction heat, and a multi-stage flash evaporation process is employed to reduce energy consumption. However, this method requires harsh reaction conditions (temperature 100-200℃, pressure 1.0-5.0MPa) and a long reaction time (10-60min), and it fails to solve the mass and heat transfer problems during the reaction process. Existing patents CN119455856A and CN115715962A both use liquid ammonia as a raw material to prepare an ammonia-methanol solution with methanol. However, because liquid ammonia is highly volatile and liquid at low temperatures, uneven mixing with methanol may lead to low ammonolysis reaction efficiency, numerous byproducts, and poor stability. Summary of the Invention

[0005] The technical problem to be solved by this invention is how to provide a high-purity oxalamide with concentrated particle size and spherical morphology, its preparation method and its application in the preparation of slow-release fertilizer.

[0006] The present invention solves the above-mentioned technical problems through the following technical means:

[0007] This invention proposes a method for preparing high-purity oxalamide with concentrated particle size and spherical morphology, comprising the following steps: (1) Dissolve ammonia gas in alcohols to obtain an ammonia-alcohol solution; (2) Mix alkyl oxalate with alcohol to obtain an alcohol ester solution; (3) After mixing the ammonia solution and the alcohol ester solution, the mixture is fed into a series tubular reactor for reaction. After pressure stabilization and flow equalization, the mixture is fed into a parallel tubular reactor for reaction to obtain a reaction solution containing oxalamide. (4) The reaction solution is flash evaporated to obtain flash liquid and flash vapor. The flash liquid is separated into solid and liquid to obtain solid oxalamide and filtrate. The solid oxalamide is dried (to evaporate the residual ammonia and alcohol) to obtain high-purity oxalamide with concentrated particle size and spherical morphology.

[0008] The obtained high-purity oxalamide has a purity of ≥99%, with particle size mainly concentrated in the range of 10-100 μm and D50 particle size concentrated in the range of 50±15 μm. The morphology is uniform and spherical.

[0009] Preferably, in step (1), the ammonia solution is a high-concentration ammonia solution, specifically, the concentration of ammonia in the ammonia solution is 6~8.5 mol / L. More preferably, it is 6.8~7.5 mol / L.

[0010] Preferably, in step (2), the alcohol ester solution is a low-concentration alcohol ester solution, specifically, the concentration of alkyl oxalate in the alcohol ester solution is 15~40% (w / w). More preferably, it is 15~20% (w / w).

[0011] Preferably, in steps (1) and (2), the alcohols include, but are not limited to, one or more of methanol and ethanol.

[0012] Preferably, in step (2), the alkyl oxalate includes, but is not limited to, one or more of dimethyl oxalate (DMO, CAS No. 553-90-2) and diethyl oxalate.

[0013] Preferably, in step (3), the mass ratio of alcohol ester solution to ammonia alcohol solution is 1:(1~2).

[0014] Preferably, in step (3), the reaction conditions are: 30~60℃, 0.3~0.8MPa, 2~6min; more preferably 40~50℃, 0.4~0.6MPa, 3~5min.

[0015] Preferably, the series-connected tubular reactor consists of multiple single tubes connected in series. More preferably, it consists of 3 to 12 single tubes connected in series.

[0016] Preferably, the parallel tubular reactor is composed of multiple single tubes connected in parallel; more preferably, it is composed of 20 to 32 single tubes connected in series.

[0017] Preferably, the inner diameter of a single tube in a series-connected tubular reactor or a parallel-connected tubular reactor is 15-500 mm, and the length of a single tube is 500-5000 cm.

[0018] Preferably, both the series tubular reactor and the parallel tubular reactor adopt a double-layer jacket structure design, including a reaction layer located in the inner layer and a heat exchange layer located in the outer layer, the heat exchange layer being used to control the reaction temperature.

[0019] Preferably, the material of the series tubular reactor and the parallel tubular reactor is one or more of stainless steel, glass, ceramic or polymer.

[0020] Preferably, in step (3), the tubular reactor is further equipped with packing or internal components to enhance shear mixing. The packing is flat ring packing. The internal components are any one or more selected from SMX, SMK, SMH, and SMV static mixers.

[0021] Preferably, the voltage stabilization and current equalization are achieved through a gradually expanding voltage stabilizer and current equalizer.

[0022] The gradually expanding pressure regulator includes a gradually expanding tube body and a flow equalization grid plate. The flow equalization grid plate is disposed inside the gradually expanding tube body, and the cross-section of the gradually expanding tube body gradually increases from the inlet to the outlet.

[0023] More preferably, the expanding tube is a hollow frustum shell type or a flared mouth type.

[0024] Preferably, in step (4), the flash evaporation adopts two-stage flash evaporation, with the first-stage flash evaporation temperature being 40~50℃ and the pressure being 0.05~0.1MPa; the second-stage flash evaporation temperature being 30~40℃ and the pressure being atmospheric pressure; more preferably, the first-stage flash evaporation temperature is 45℃ and the pressure is 0.08MPa; the second-stage flash evaporation temperature is 35℃.

[0025] Preferably, in step (4), the solid-liquid separation is performed by one or more of centrifugal separation, membrane filtration or sedimentation separation.

[0026] Preferably, in step (4), the drying temperature is 60~80℃ and the drying time is 0.5~2h.

[0027] Preferably, the method further includes condensing the flash vapor described in step (4) to recover unreacted alcohol and ammonia; then the filtrate, recovered alcohol and ammonia are separated by distillation and returned to steps (1) and (2) for recycling.

[0028] More preferably, the distillation column used for the distillation separation has a top temperature of 50~60℃, a bottom temperature of 65~75℃, and a reflux ratio of 1.5~2.5.

[0029] The present invention also proposes a high-purity oxalamide with concentrated particle size and spherical morphology prepared by the above preparation method.

[0030] The present invention also proposes any of the following applications of the high-purity oxalamide with concentrated particle size and spherical morphology: (a) Its application as a nitrogen fertilizer; (b) Application in the preparation of long-acting slow-release nitrogen fertilizer; (c) Application in the study of nitrogen release stability; (d) Application in experimental analysis for studying the level of nitrogen utilization; (e) Application as a combustion inhibitor.

[0031] The beneficial effects of this invention are: 1. This invention proposes a method for preparing high-purity oxamide with concentrated particle size and spherical morphology. This method is easily scaled up for industrial application. It utilizes an alcohol ester solution and an ammonia-alcohol solution prepared with high-efficiency gas-liquid distribution as reaction raw materials to efficiently and continuously synthesize oxamide under mild conditions. The resulting oxamide has an HPLC purity higher than 99% and can be directly applied in the field of combustion inhibitors.

[0032] 2. This invention uses ammonia gas to be bubbled into methanol under specific temperature and pressure conditions to form a stable, high-concentration and uniformly mixed ammonia-methanol microbubble solution, realizing a more controllable and safer ammonia-methanol system preparation process, which is especially suitable for efficient feeding in continuous tubular reaction mode.

[0033] 3. The reaction is carried out using tubular reactors, specifically through a series-then-parallel connection, cleverly combining the advantages of two fluid dynamics methods. Furthermore, a gradually expanding pressure-stabilizing and flow-equalizing device is creatively introduced between the series and parallel connections, allowing the mixed solution to smoothly enter the parallel tubular reactors for gentle maturation, eliminating 'flow deviation' or 'short-circuiting' phenomena. This results in high mass and heat transfer efficiency, a fast reaction rate, and a reduction in reaction time from the traditional hours to minutes, significantly improving production efficiency. 4. High conversion rate and high purity can be achieved under low temperature (30-60℃) and low pressure (0.3-0.8MPa) conditions, and the product exhibits a concentrated particle size distribution. OX purity ≥99%, particle size mainly concentrated in 10-100 μm, D50 particle size concentrated in 50±15 μm, and uniform morphology.

[0034] 5. Using ammonia-ethanol solution and alcohol-ester solution as reaction raw materials avoids aqueous phase reaction, reduces hydrolysis side reactions of raw materials, and simplifies subsequent separation and purification processes.

[0035] 6. The special design of the tubular reactor solves the clogging problem that may be caused by solid products, enabling long-term stable operation.

[0036] 7. The closed-loop recycling system enables the recovery and reuse of solvents and unreacted raw materials, reducing raw material consumption and emissions of waste. Attached Figure Description

[0037] Figure 1 This is a scanning electron microscope image of oxalamide prepared in Example 1 of the present invention; Figure 2 This is a scanning electron microscope image of oxalamide obtained in Example 2 of the present invention; Figure 3 This is a scanning electron microscope image of oxalamide obtained in Example 3 of the present invention; Figure 4 This is a particle size distribution diagram of oxalamide obtained in Example 1 of the present invention; Figure 5 This is a particle size distribution diagram of the oxalamide prepared in Comparative Example 1 of the present invention; Figure 6 This is the HPLC liquid chromatogram of oxalamide prepared in Example 1 of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art.

[0039] Unless otherwise specified, the test materials and reagents used in the following examples are commercially available or prepared by known methods.

[0040] Unless otherwise specified, all techniques or conditions described in the embodiments can be performed in accordance with the techniques or conditions described in the literature in this field or in the product manual. Unless otherwise specified, the quantitative experiments in the following embodiments are all repeated three times or more, and the results are averaged.

[0041] The nanobubble generator used in this embodiment is model NANO-UFB1000, which can generate nanobubbles with a diameter of 50~150nm.

[0042] Example 1: A method for preparing high-purity oxalamide with concentrated particle size and spherical morphology includes the following steps: (1) After ammonia gas is treated by a nanobubble generator, it is dissolved in methanol to obtain an ammonia-ethanol solution (the concentration of ammonia gas is 6.88 mol / L). (2) Mix dimethyl oxalate with methanol to obtain an alcohol ester solution (the concentration of dimethyl oxalate is 17% (v / v)). (3) The alcohol ester solution obtained in (2) and the ammonia alcohol solution obtained in (1) are mixed and then transported to a series tubular reactor for reaction. After pressure stabilization and flow equalization, the mixture is then transported to a parallel tubular reactor for reaction (the reaction conditions are 45℃, 0.5MPa, and 5min) to obtain a reaction solution containing oxalamide. The mass ratio of the alcohol ester solution to the ammonia alcohol solution is 1:1.5. The tubular reactor consists of 6 single tubes connected in series and adopts a double-layer jacket structure design, including a reaction layer located in the inner layer and a heat exchange layer located in the outer layer. The heat exchange layer is used to control the reaction temperature. The inner diameter of a single tube is 30mm and the length of a single tube is 800cm. The alcohol ester solution and the ammonia alcohol solution are mixed at a flow rate ratio of (0.1~60):(0.1~120); (4) The reaction solution was flash-evaporated (using two-stage flash evaporation, the first stage flash evaporation temperature was 45℃ and the pressure was 0.08MPa; the second stage flash evaporation temperature was 35℃ and the pressure was atmospheric pressure) to obtain flash liquid and flash vapor. The flash liquid was separated into solid and liquid to obtain solid oxamide and filtrate. The obtained solid oxamide was dried to obtain a high-purity oxamide product with concentrated particle size and spherical morphology. (The purity of oxamide OX obtained in this example was 99.02%, and the OX yield was 95.63%).

[0043] The obtained flash vapor is condensed to recover unreacted alcohol and ammonia; the filtrate and the alcohol and ammonia recovered from the flash vapor are separated by distillation and then returned to steps (1) and (2) for recycling.

[0044] The oxalamide prepared in this embodiment was characterized as follows: Scanning electron microscope image as follows Figure 1 As shown in the figure, the morphology of the prepared oxalamide is spherical, which is a relatively regular aggregated spherical morphology; like Figure 4 This is a particle size distribution diagram of the oxalamide prepared in Example 1 of the present invention. The particle size is mainly concentrated in the range of 10-100 μm, and the D50 particle size is concentrated in the range of 50±15 μm. like Figure 6 This is the HPLC liquid chromatogram of oxalamide prepared in Example 1 of the present invention.

[0045] Example 2: A method for preparing high-purity oxalamide with concentrated particle size and spherical morphology includes the following steps: (1) After ammonia is treated by a nanobubble generator, it is dissolved in methanol to obtain an ammonia-ethanol solution (the concentration of ammonia is 6 mol / L). (2) Mix dimethyl oxalate with methanol to obtain an alcohol ester solution (the concentration of dimethyl oxalate is 15% (v / v)). (3) The alcohol ester solution obtained in (2) and the ammonia alcohol solution obtained in (1) are mixed and then fed into a series tubular reactor for reaction. After pressure stabilization and flow equalization (the material flow is achieved through a gradually expanding pressure stabilizer and flow equalizer), the mixture is fed into a parallel tubular reactor for reaction (the reaction conditions are 40℃, 0.6MPa, and 3min) to obtain a reaction solution containing oxalamide. The mass ratio of the alcohol ester solution to the ammonia alcohol solution is 1:1. The tubular reactor consists of three single tubes connected in series and adopts a double-layer jacket structure design, including a reaction layer located in the inner layer and a heat exchange layer located in the outer layer. The heat exchange layer is used to control the reaction temperature. The inner diameter of a single tube is 15mm and the length of a single tube is 5000cm. (4) The reaction solution was flash-evaporated (using a two-stage flash evaporation method; the first-stage flash evaporation temperature was 40℃ and the pressure was 0.1MPa; the second-stage flash evaporation temperature was 40℃ and the pressure was atmospheric pressure) to obtain flash liquid and flash vapor. The flash liquid was then separated into solid and liquid phases to obtain solid oxalamide and filtrate. The obtained solid oxalamide was dried to obtain a high-purity oxalamide product with concentrated particle size and spherical morphology. (Scanning electron microscope image of the product is shown below) Figure 2 (As shown).

[0046] Example 3: A method for preparing high-purity oxalamide with concentrated particle size and spherical morphology includes the following steps: (1) After ammonia gas is treated by a nanobubble generator, it is dissolved in methanol to obtain an ammonia-ethanol solution (the concentration of ammonia gas is 8.5 mol / L). (2) Mix dimethyl oxalate with methanol to obtain an alcohol ester solution (the concentration of dimethyl oxalate is 20% (v / v)). (3) The alcohol ester solution obtained in (2) and the ammonia alcohol solution obtained in (1) are mixed and then fed into a series tubular reactor for reaction. After pressure stabilization and flow equalization (the material flow is achieved through a gradually expanding pressure stabilizer and flow equalizer), the mixture is fed into a parallel tubular reactor for reaction (the reaction conditions are 50℃, 0.4MPa, and 5min) to obtain a reaction solution containing oxalamide. The mass ratio of the alcohol ester solution to the ammonia alcohol solution is 1:2. The tubular reactor consists of 12 single tubes connected in series and adopts a double-layer jacket structure design, including a reaction layer located in the inner layer and a heat exchange layer located in the outer layer. The heat exchange layer is used to control the reaction temperature. The inner diameter of a single tube is 500mm and the length of a single tube is 500cm. (4) The reaction solution was flash-evaporated (using a two-stage flash evaporation method; the first-stage flash evaporation temperature was 50℃ and the pressure was 0.05MPa; the second-stage flash evaporation temperature was 30℃ and the pressure was atmospheric pressure) to obtain flash liquid and flash vapor. The flash liquid was then separated into solid and liquid phases to obtain solid oxalamide and filtrate. The obtained solid oxalamide was dried to obtain a high-purity oxalamide product with concentrated particle size and spherical morphology. (Scanning electron microscope image of the product is shown below) Figure 3 (As shown).

[0047] Comparative Example 1: The difference between this comparative example and Example 1 is that the reaction raw materials in step (1) are liquid ammonia and methanol mixed together, and the rest is the same as in Example 1.

[0048] Results: Oxalide had uneven morphology and a wider particle size distribution.

[0049] Comparative Example 2: The difference between this comparative example and Example 1 is that the reaction pressure in step (3) is 0.1 MPa, and the rest is the same as in Example 1.

[0050] Result: The low reaction pressure resulted in a slow reaction rate and incomplete reaction.

[0051] Comparative Example 3: The difference between this comparative example and Example 1 is that the reaction pressure in step (3) is 2 MPa, and the rest is the same as in Example 1.

[0052] Result: The reaction pressure was too high, which placed high demands on the quality of the equipment and resulted in high costs.

[0053] Comparative Example 4: The difference between this comparative example and Example 1 is that the reaction time in step (3) is 28 s, and the rest is the same as in Example 1.

[0054] Result: The reaction time was too short, and the reaction was incomplete.

[0055] Comparative Example 5: The difference between this comparative example and Example 1 is that the reaction time in step (3) is 9 min, and the rest is the same as in Example 1.

[0056] Result: The reaction time was too long, which led to an increase in byproducts.

[0057] Comparative Example 6: The difference between this comparative example and Example 1 is that the tubular reactor in step (3) is replaced with a batch reactor, and the rest is the same as in Example 1.

[0058] Results: The reaction took 5 hours to complete, indicating an excessively slow reaction rate. It is difficult to achieve continuous industrial reactions using a batch reactor. A tubular reactor with internal components essentially intensifies the reaction process, while also enabling continuous feeding and discharging.

[0059] Comparative Example 7: The difference between this comparative example and Example 1 is that only the first-stage flash evaporation is performed in step (4), and the second-stage flash evaporation is not performed. The rest is the same as in Example 1.

[0060] Result: The particle size of the prepared oxalamide product did not reach the micron level.

[0061] Comparative Example 8: The difference between this comparative example and Example 1 is that the temperature of the first-stage flash evaporation in step (4) is 30°C and the temperature of the second-stage flash evaporation is 25°C, while the rest is the same as in Example 1.

[0062] Result: The particle size of the prepared oxalamide product did not reach the micron level.

[0063] Comparative Example 9: The difference between this comparative example and Example 1 is that the pressure of the first-stage flash evaporation in step (4) is 0.8 MPa and the pressure of the second-stage flash evaporation is 0.5 MPa, while the rest is the same as in Example 1.

[0064] Result: The particle size of the prepared oxalamide product did not reach the micron level.

[0065] The oxamide product obtained in Example 1 was used in the following ways: Application Example 1: Study on the slow-release effect of high-purity oxalamide with spherical morphology as a fertilizer 1.1 Experimental Materials Fertilizers tested: Treatment A (Target Fertilizer - Apply Alone): High-purity oxalamide with spherical morphology Treatment B (Target Fertilizer - Compound with Binder): High-purity oxalamide with spherical morphology + bentonite binder (oxalamide to bentonite ratio 9:1, granulation into 2-3 mm particles). Treatment C (morphological control group): Conventional oxalamide (irregular morphology, wide particle size distribution, ordinary industrial grade) Treatment D (fast-acting nitrogen control group): regular urea Treatment E (blank control group): No nitrogen fertilizer applied. Test soil: air-dried and sieved loam with consistent background nitrogen content.

[0066] Cultivation conditions: constant temperature incubator (25℃), soil moisture maintained at 60% of field capacity.

[0067] 1.2 Experimental Methods Soil constant temperature incubation experiment: After mixing the soil and fertilizer, put them into a culture bottle and incubate at a constant temperature of 25°C.

[0068] Samples were taken on days 1, 3, 7, 14, 28, 56, and 84 after cultivation to determine the nitrogen content in the soil and calculate the cumulative nitrogen release rate.

[0069] Evaluation of sustained-release performance: According to the national standard for slow-release fertilizers (GB / T 23348-2009), the initial release rate and cumulative release rate are evaluated.

[0070] Table 1: Comparison of initial release rate and cumulative release rate among different treatment groups

[0071] 1.3 Conclusion As shown in the table above, compared with the irregular morphology of conventional oxalamide, the spherical high-purity oxalamide has shown excellent slow-release performance when applied alone, and can basically replace conventional nitrogen fertilizer for one-time basal application; when compounded with binder, the slow-release effect is further enhanced. High-purity oxalamide with a spherical morphology can be applied directly as a long-acting nitrogen source, or it can be compounded with binders as a slow-release functional material.

[0072] Application Example 2: Nitrogen Use Efficiency (NUE) Study of Spherical Oxalide 2.1 Experimental Materials Test crop: Maize (long growth period, nitrogen requirement is basically "S" shaped, nitrogen requirement pattern is typical and easy to sample).

[0073] Fertilizers tested: Treatment Group A (Target Fertilizer): Spherical oxalamide (particle size concentrated at 50 μm, high purity) Treatment group B (morphological control group): Conventional oxalamide (irregular morphology, wide particle size distribution, ordinary industrial grade) Treatment group C (quick-acting nitrogen control group): ordinary urea (as a control for routine fertilization). Control group D (blank control): No nitrogen fertilizer applied. Test soil: Field soil with moderate fertility and basically consistent nitrogen content.

[0074] Experimental period: from sowing to full maturity and harvest.

[0075] 2.2 Experimental Methods The field is divided into multiple micro-areas.

[0076] Grouping: Each process is set to 3 repetitions.

[0077] Fertilization: All nitrogen application treatments were designed with equal nitrogen application rates (pure nitrogen application rate of 180 kg / hm²).

[0078] Both A and B are applied as base fertilizer in one go.

[0079] Treatment C (urea) was applied in multiple applications according to local high-yield cultivation practices (50% basal fertilizer + 50% jointing fertilizer) to simulate optimal routine management.

[0080] Field management: Maintain consistent field management (irrigation, weeding, pest and disease control) across all treatments.

[0081] Sample collection and measurement: At harvest: Collect the above-ground parts of the corn plant (kernels, stems and leaves), dry and weigh them, grind them and then determine the total nitrogen content.

[0082] 2.3 Data Calculation Nitrogen fertilizer utilization rate (NUE, %) = [(Total nitrogen uptake by crops in the nitrogen-applied area - Total nitrogen uptake by crops in the control area) / Nitrogen application rate] × 100% Table 2: Comparison of crop nitrogen fertilizer utilization rate and yield response under different treatment groups

[0083] Results analysis: Based on the above field trials and table data, it can be concluded that spherical oxamide with a particle size concentrated at 50 μm, due to its special physical morphology (spherical + suitable particle size), can significantly reduce nitrogen loss and increase nitrogen fertilizer utilization rate to over 48% compared to conventional oxamide and ordinary urea.

[0084] Application Example 3: Performance Study of High-Purity Oxalide with Spherical Morphology as a Combustion Inhibitor 3.1 Experimental Background and Principles The mechanisms of action of combustion inhibitors typically include endothermic cooling, oxygen isolation, dilution of combustible gases, and free radical scavenging. Oxalide decomposes endothermically at high temperatures, which aligns with the characteristics of endothermic combustion inhibitors. The inhibitory effect of high-purity oxalamide with a spherical morphology was studied using common combustible materials (such as paper, wood, and polymers) through simple combustion tests.

[0085] 3.2 Experimental Materials Basic material: Qualitative filter paper (cut into uniform size, such as 2 cm × 10 cm strips) Treatment group A (target inhibitor): spherical oxalamide (particle size concentrated at 50 μm, high purity). Treatment group B (control inhibitor): Conventional oxalamide (irregular morphology, wide particle size distribution, ordinary industrial grade) Treatment Group C (Conventional Flame Retardant): Ammonium Dihydrogen Phosphate Blank control group: No treatment Dispersion medium: Deionized water + a small amount of detergent (as a surfactant to help disperse oxalamide evenly). Tools: stopwatch, lighter, iron stand, tweezers, drying oven, electronic balance 3.3 Experimental Methods Vertical burning test method: Sample preparation: Cut the filter paper into strips of 2 cm × 10 cm, and prepare 3 parallel samples for each group. Prepare oxalamide suspensions at different concentrations (e.g., 0%, 5%, 10%, 15%). Immerse the filter paper strips in the suspension for 30 seconds, ensuring a consistent immersion depth (8 cm). After removing from the oven, air dry naturally or dry at 60℃. Combustion test: The prepared filter paper strips are vertically fixed on the iron frame, with the lower end 2 cm above the horizontal plane. Light the bottom of the filter paper strip with a lighter and start the stopwatch at the same time. Record the following metrics: Burning time (s): The time from ignition to flame extinction. Burning length (mm): The actual length burned Combustion rate (mm / s) = Combustion length / Combustion time Self-extinguishing: Does the flame extinguish before reaching the top? Charcoal residue state: The form of residue after combustion (the char formation reflects the flame retardant effect). 3.4 Data Recording and Expected Results Table 3: Effects of different concentrations of spherical oxalamide on paper combustion behavior

[0086] 3.5 Results Analysis: As the concentration of spherical oxamide increases, the combustion rate gradually decreases. The sample treated with a 15% concentration exhibits self-extinguishing behavior after being removed from the flame, indicating that the spherical oxamide of this application has a significant combustion inhibition effect.

[0087] The obvious carbonization phenomenon indicates that oxalamide promotes the formation of a protective carbon layer in the material.

[0088] Table 4: Comparison of flame retardant effects of 10% concentration of different morphologies of oxalamide

[0089] Results analysis: The combustion rate of spherical oxalamide (3.32 mm / s) was significantly lower than that of conventional oxalamide with irregular morphology (4.93 mm / s), demonstrating the advantage of spherical morphology.

[0090] The carbon layer formed by spherical oxalamide is more continuous and dense, while the carbon layer of needle-shaped oxalamide has cracks, indicating that the spherical particles are more uniformly dispersed in the substrate.

[0091] Compared to the conventional flame retardant ammonium dihydrogen phosphate, spherical oxamide is slightly less effective but still significantly so, and its environmental friendliness may be superior (oxamide decomposes into ammonia and carbon dioxide).

[0092] Therefore, it is evident that spherical oxalamide has great potential as a combustion inhibitor, and it can be speculated that it may also be applicable to the field of combustion rate inhibitors for rocket fuel and small aerial vehicles, which requires further research.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing high-purity oxalamide with concentrated particle size and spherical morphology, characterized in that, Includes the following steps: (1) Dissolve ammonia gas in alcohols to obtain an ammonia-alcohol solution; (2) Mix alkyl oxalate with alcohol to obtain an alcohol ester solution; (3) After mixing the ammonia solution and the alcohol ester solution, the mixture is fed into a series tubular reactor for reaction. After pressure stabilization and flow equalization, the mixture is fed into a parallel tubular reactor for reaction to obtain a reaction solution containing oxalamide. The mass ratio of alcohol ester solution to ammonia alcohol solution is 1:(1~2); the reaction conditions are 30~60℃, 0.3~0.8MPa, 2~6min; (4) The reaction solution is flash evaporated to obtain flash liquid and flash vapor. The flash liquid is separated into solid and liquid to obtain solid oxalamide and filtrate. The solid oxalamide is dried to obtain the final product.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of ammonia in the ammonia-alcohol solution is 6~8.5 mol / L; in step (2), the concentration of alkyl oxalate in the alcohol ester solution is 15~40%.

3. The preparation method according to claim 1, characterized in that, In steps (1) and (2), the alcohol is one or more of methanol and ethanol; in step (2), the alkyl oxalate is one or more of dimethyl oxalate and diethyl oxalate.

4. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of alcohol ester solution to ammonia alcohol solution is 1:1.5; the reaction conditions are 40~50℃, 0.4~0.6MPa, 3~5min.

5. The preparation method according to claim 1, characterized in that, In step (3), the series tubular reactor is composed of multiple single tubes connected in series; the parallel tubular reactor is composed of multiple single tubes connected in parallel.

6. The preparation method according to claim 1, characterized in that, In step (4), the flash evaporation adopts a two-stage flash evaporation. The first stage flash evaporation temperature is 40~50℃ and the pressure is 0.05~0.1MPa; the second stage flash evaporation temperature is 30~40℃ and the pressure is atmospheric pressure.

7. The preparation method according to claim 1, characterized in that, In step (4), the drying temperature is 60~80℃ and the drying time is 0.5~2h.

8. The preparation method according to claim 1, characterized in that, It also includes condensing the flash vapor described in step (4) to recover unreacted alcohol and ammonia; then the filtrate, recovered alcohol and ammonia are separated by distillation and then returned to steps (1) and (2) for recycling.

9. High-purity oxalamide with concentrated particle size and spherical morphology prepared by the preparation method according to any one of claims 1-8.

10. The high-purity oxalamide with concentrated particle size and spherical morphology as described in claim 9, in any of the following applications: (a) Its application as a nitrogen fertilizer; (b) Application in the preparation of long-acting slow-release nitrogen fertilizer; (c) Application in the study of nitrogen release stability; (d) Application in experimental analysis for studying the level of nitrogen utilization; (e) Application as a combustion inhibitor.

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