Fe3O4 (at) N-P-HC multifunctional compound for concerted catalysis and nutrient recovery as well as preparation and application of Fe3O4 (at) N-P-HC multifunctional compound

Abstract

The invention relates to a multifunctional Fe3O4 (at) N-P-HC compound for concerted catalysis and nutrient recovery as well as preparation and application thereof. The preparation method comprises the steps of preparation of the Fe3O4 (at) N-P-HC compound, hydrothermal carbonization reaction of agricultural and forestry wastes, preparation of struvite crystals and preparation of a compound fertilizer. The prepared compound with magnetic FeO as a core and nitrogen and phosphorus doped hydrothermal carbon as a carrier realizes closed-loop circulation of catalysis, fertilizer supply and recovery. The compound fertilizer disclosed by the invention is a brand new two-in-one functional product of a slow release fertilizer and a soil conditioner, a synergistic interaction effect is realized, and the additional value of the product is greatly improved; according to the invention, integrated treatment of waste treatment, resource recovery and product high-value treatment is adopted, so that maximum utilization of resources is realized.

Description

Technical Field

[0001] This invention belongs to the field of agricultural waste resource utilization and fertilizer technology. More specifically, this invention relates to a Fe3O4@NP-HC multifunctional complex for synergistic catalysis and nutrient recovery, a method for preparing the complex, and a systematic method for using the complex to convert lignocellulosic agricultural and forestry waste into hydrothermal carbon-struvite compound fertilizer. Background Technology

[0002] my country possesses vast agricultural biomass resources, with existing data showing approximately 865 million tons of crop straw generated annually. Efficient utilization of these wastes is crucial for achieving sustainable agricultural development and environmental protection. Hydrothermal technology, as an effective thermochemical conversion method, can transform agricultural biomass into hydrothermal char and high-value-added chemicals, representing a new approach to the resource utilization of agricultural waste. Existing research has shown that adding catalysts can significantly improve the efficiency of hydrothermal reactions and the quality of products. CN202310556443.8 discloses a method and application of iron ion catalysis to promote the humification of biomass to synthesize artificial humic acid. Iron ion catalysis increases the yield of humic acid in hydrothermal char, but the catalytic effect is limited, and the reactivity of the raw materials and the complex can be further enhanced. CN119219448A discloses a method using nanobubble technology coupled with an external nitrogen source (ammonium sulfate and urea, etc.), where the nitrogen source acts as both a complex and a nitrogen source supplement for the production of nitrogen-rich liquid fertilizer. This method is innovative in enhancing nitrogen transfer and increasing the nitrogen content of solid products, but it has significant limitations: nitrogen in the liquid phase mainly exists in the form of NH4⁺, resulting in low utilization and easy volatilization loss. In nitrogen and phosphorus recovery, struvite (magnesium ammonium phosphate, MAP) crystallization has become a research hotspot due to its ability to simultaneously recover nitrogen and phosphorus elements and the slow-release fertilizer effect of the product. CN110078040A discloses a method and system for recovering struvite from wastewater from wood-based activated carbon production. Nitrogen and magnesium sources are added to the wastewater, and struvite is produced through a struvite crystallization process. However, the one-time addition of sodium hydroxide solution leads to excessive saturation and large fluctuations in the system, and the long discharge cycle results in uneven crystal morphology, affecting precipitation and separation efficiency. CN118289990A discloses an apparatus and method for recovering high-purity granular struvite from corn starch wastewater. The magnesium-phosphorus dosage ratio is controlled by a metering pump, and the pH is adjusted using an electromagnetic air pump and an alkali pump. However, this method has a single detection index, low pH detection frequency (once every 1-2 hours), requires timed adjustment of alkali addition, and fixed discharge time (discharge once every 24 hours), which may lead to low sludge removal efficiency, unstable crystallization process, and poor controllability of product quality.

[0003] While the aforementioned technologies each have their own focus, they all have limitations. For example, CN202310556443.8 primarily focuses on humic acid yield, and its iron salt catalyst has a single function, failing to address the problem of nitrogen and phosphorus nutrient enrichment and recovery in the product. Struvite recovery technologies such as CN110078040A rely on wastewater with fixed compositions as raw materials, and the crystallization process is poorly controlled, making it difficult to adapt to the complex and variable composition of hydrothermal liquefaction liquids from agricultural and forestry waste. More importantly, existing technologies generally treat catalytic conversion, nutrient release, and crystallization recovery as separate unit operations, lacking an integrated solution that can run through the entire process, efficiently catalyze biomass depolymerization, act as an internal nutrient reservoir, and ultimately achieve intelligent control of the entire process. Therefore, developing a multifunctional catalyst and a matching synergistic process is an urgent need to achieve high-value and full-scale utilization of agricultural and forestry waste. Summary of the Invention

[0004] The purpose of this invention is to improve the yield of hydrothermal char production methods, provide a hydrothermal char compound fertilizer with high nitrogen and phosphorus recovery, and ensure that the resulting compound fertilizer has higher fertilizer efficiency, better fertilizer utilization, and reduced leaching losses for crops.

[0005] Based on this, the present invention provides a method for preparing Fe3O4@NP-HC multifunctional complexes, characterized in that the method includes the following steps:

[0006] (1) Raw material pretreatment: After crushing the lignocellulosic agricultural and forestry waste, it was immersed in hydrochloric acid solution and heated to react. After solid-liquid separation, it was dried to obtain pretreated solid;

[0007] (2) Hydrothermal load: The pretreated solid is mixed with Fe 2+ and Fe 3+ The mixed aqueous solution was placed in a hydrothermal reactor and reacted at 180~240℃ for 2~6 hours under an inert atmosphere. After the reaction, the solid and liquid were separated and dried to obtain porous hydrothermal carbon.

[0008] (3) Co-precipitation to form magnetic centers: Ammonia water is added to the porous hydrothermal carbon until the system is alkaline. After the reaction, the mixture is separated by magnetic separation and dried to obtain Fe3O4@HC intermediate.

[0009] (4) Nitrogen and phosphorus co-doping modification: Ammonium dihydrogen phosphate and urea are dissolved in a solvent at a mass ratio of 1.5~2.5:1 to prepare a modification solution; the Fe3O4@HC intermediate is immersed in the modification solution, and after dispersion treatment, solid-liquid separation is performed.

[0010] (5) Stepwise pyrolysis activation: The solid obtained in step (4) is heated to 500-600℃ at a heating rate of 3-8℃ / min under an inert atmosphere and calcined for 2.5-3.5 hours; after calcination, the obtained solid is immersed in dilute sulfuric acid solution and stirred, then washed and dried; finally, it is treated at 350-450℃ for 0.8-1.2 hours in a mixed atmosphere of H2 and Ar to obtain the Fe3O4@NP-HC multifunctional composite.

[0011] According to a preferred embodiment, in step (1), the lignocellulosic agricultural and forestry waste powder and hydrochloric acid solution are stirred at 70-90°C for 1-3 hours. The separated solids are dried in a vacuum drying oven at 55-65°C for 10-14 hours, and the dried solids are added to a hydrothermal reactor. The lignocellulosic agricultural and forestry waste powder used is straw, forestry sawdust, or livestock manure powder with a particle size of 30-50 mesh. The straw is, for example, crop straw such as corn, rice, sesame, or sorghum. The forestry sawdust is, for example, various types of sawdust produced during forestry processing. The hydrochloric acid can be a 0.5-1.5 mol / L solution, and the preferred weight ratio of agricultural and forestry waste powder to hydrochloric acid is 1:8-12.

[0012] The purpose of immersing lignocellulosic agricultural and forestry waste powder in hydrochloric acid solution is to: 1. Remove ash and metal ions. Hydrochloric acid can dissolve alkali metals, alkaline earth metals, and some transition metals in the raw materials, reducing ash content and preventing these impurities from clogging pores or affecting catalytic activity during subsequent pyrolysis; 2. Promote pore development. Acid washing can partially hydrolyze glycosidic bonds in cellulose and hemicellulose, removing some amorphous components and forming a more developed porous structure after carbonization; 3. Increase surface acidic functional groups. Hydrochloric acid treatment can introduce -Cl, -COOH, and other groups onto the surface, enhancing its hydrophilicity and ion exchange capacity, which is beneficial for subsequent Fe... 2+ and Fe 3+ 4. Increase carbon content. By removing minerals, the carbon content is relatively increased, making the carbon skeleton of the final composite more stable.

[0013] Agricultural and forestry waste powder and hydrochloric acid solution are stirred at a weight ratio of 1:8-12 at 70-90℃ for 1-3 hours. When the treatment temperature and time are within the specified range, if the weight ratio of agricultural and forestry waste powder to hydrochloric acid solution is greater than 1:8, the excessive amount of hydrochloric acid may lead to excessive corrosion of the hydrothermal carbon, severe loss of organic matter, and serious damage to the pore structure. If the weight ratio is less than 1:12, the amount of hydrochloric acid is insufficient, impurities are not thoroughly removed, and pore development is inadequate. Therefore, a weight ratio of 1:8-12 is reasonable. When the weight ratio and treatment time are within the specified range, if the treatment temperature is below 70℃, the reaction rate is slow, the hydrochloric acid does not fully dissolve ash and metal ions, resulting in more impurities remaining and subsequent pore development of the hydrothermal carbon is hindered. The hydrochloric acid has a low specific surface area. If the processing temperature exceeds 90℃, the hydrochloric acid volatilization intensifies, producing a large amount of hydrogen chloride gas, creating a hazardous operating environment and potentially leading to excessive hydrolysis of biomass, severe loss of organic matter, and reduced carbon yield. Therefore, a processing temperature of 70~90℃ is suitable. When the weight ratio of agricultural and forestry waste powder to hydrochloric acid solution and the processing temperature are within the aforementioned range, if the processing time is less than 1 hour, the acid and raw materials will not be in sufficient contact, resulting in poor impurity removal and insufficient opening of the hydrothermal carbon pore structure. If the processing time is longer than 3 hours, energy consumption will increase, and excessive corrosion of the hydrothermal carbon skeleton may occur, leading to a decrease in mechanical strength, pore collapse, and a reduction in specific surface area. Therefore, a processing time of 1~3 hours is appropriate.

[0014] After being treated with hydrochloric acid solution, agricultural and forestry waste powder needs to be separated into solid and liquid by conventional filtration method. The separated solid is dried in a vacuum drying oven at 55~65℃ for 10~14h. The dried solid is then added to a hydrothermal reactor.

[0015] In a preferred embodiment, in step (2), FeCl2·4H2O and FeCl3·6H2O are used as raw materials according to Fe 2+ Fe 3+ The iron solution is dissolved in deionized water at a molar ratio of 1:1.5~2.5. The resulting iron solution is added to the hydrothermal reactor of the previous step. The dried solid and the iron solution are subjected to hydrothermal reaction in a N2 protective atmosphere at a weight ratio of 1:8~15 and a hydrothermal reaction temperature of 180~240℃ for 2~6 hours. The solution is then filtered and dried to obtain porous hydrothermal carbon. The filtrate is recycled.

[0016] If Fe 2+ Fe 3+ If the molar ratio is greater than 1:1.5, then Fe 3+ Low Fe content may lead to the formation of FeO impurity phase; if Fe 2+ Fe 3+ If the molar ratio is less than 1:2.5, then Fe 3+Excessive amounts may lead to the formation of Fe2O3, affecting the purity of Fe3O4; therefore, Fe 2+ Fe 3+ A molar ratio of 1:1.5 to 2.5 is desirable. The FeCl2·4H2O and FeCl3·6H2O used in this invention are products currently available on the market, such as FeCl2·4H2O sold by Sigma-Aldrich under the trade name Iron(II) chloride tetrahydrate and FeCl3·6H2O sold by Sigma-Aldrich under the trade name Iron(III) chloride hexahydrate.

[0017] In a preferred embodiment, in step (3), ammonia is added dropwise to the porous hydrothermal carbon from step (2) until the pH of the system reaches 10, and then the mixture is stirred at 55-65°C for 1.5-2.5 hours. Its main function is to allow Fe to be co-precipitated under these conditions. 2+ and Fe 3+ Fe3O4 particles are generated and loaded into the pores of porous hydrothermal carbon. The solid is then separated using a 0.5T permanent magnet and dried in a vacuum drying oven at 55-65°C for 10-14 hours. Exceeding these drying temperatures and times is undesirable, as insufficient drying at too low a temperature may lead to incomplete drying, while excessive drying may cause Fe3O4 oxidation; insufficient drying time will result in residual moisture affecting the loading capacity, while excessive drying time will waste energy.

[0018] The dried product was identified by X-ray diffraction (XRD) as Fe3O4@HC, where @HC indicates that Fe3O4 particles are supported on a hydrothermal carbon support.

[0019] Preferably, in step (4), the modified solution is obtained by mixing ammonium dihydrogen phosphate and urea at a mass ratio of 1.5~2.5:1 and dissolving them in an ethanol aqueous solution with a concentration of 20~40% by volume. The Fe3O4@HC is then immersed in the solution at a solid-liquid weight ratio of 1:8~12 and subjected to ultrasonic treatment at an ultrasonic frequency of 35~45kHz for 25~35 minutes.

[0020] In this step, the main function of soaking Fe3O4@HC in the solution is that ethanol is added as a surfactant, which can reduce the surface tension of Fe3O4@HC, improve wettability, and allow the nitrogen source (urea) and phosphorus source (ammonium dihydrogen phosphate) to be uniformly dispersed in the pores of Fe3O4@HC, providing precursors for subsequent doping.

[0021] An ethanol-water solution containing ammonium dihydrogen phosphate, urea, and Fe3O4@HC can be ultrasonically treated for 25-35 minutes at an ultrasonic frequency of 35-45 kHz. The main function of this ultrasonic treatment is to promote the penetration and dispersion of nitrogen and phosphorus precursors in the pores of Fe3O4@HC, preventing aggregation. Within this range, if the ultrasonic frequency is below 35 kHz, the energy is insufficient, resulting in inadequate dispersion; if the ultrasonic frequency is above 45 kHz, the pore structure of Fe3O4@HC may be damaged. Therefore, an ultrasonic frequency of 35-45 kHz is reasonable. Within this range, if the ultrasonic treatment time is less than 25 minutes, the penetration of nitrogen and phosphorus precursors will be insufficient; if the ultrasonic treatment time is longer than 35 minutes, Fe3O4@HC particles may detach. Therefore, an ultrasonic treatment time of 25-35 minutes is appropriate.

[0022] In this invention, in step (5), the obtained solid is heated to 500-600°C at a rate of 3-8°C / min under N2 protection and calcined for 2.5-3.5 hours. The dilute sulfuric acid solution is a 0.05-0.15M H2SO4 aqueous solution, and the mixed atmosphere is a mixture of H2 gas and Ar gas with a volume ratio of 3-8:100.

[0023] The purpose of calcination and solidification under N2 protection is to incorporate nitrogen and phosphorus elements into the carbon framework of Fe3O4@HC through high-temperature pyrolysis, forming a catalytically active Fe3O4@NP-HC composite material. In this step, the heating rate is 3–8 °C / min. If the heating rate is below 3 °C / min, the pyrolysis process is slow, which may lead to uneven doping; if the heating rate is above 8 °C / min, thermal shock may cause cracks in the Fe3O4@HC structure.

[0024] Heating temperatures below 500℃ are undesirable because the nitrogen-phosphorus doping reaction is incomplete at low temperatures, resulting in incomplete formation of catalytic active sites. Calcination and curing times exceeding 2.5–3.5 hours are also undesirable, as insufficient curing time leads to incomplete doping, while excessive curing time may cause over-graphitization of the carbon support, thus reducing its activity.

[0025] The tube furnace used in this invention for calcination and curing is a commonly used tube furnace in this technical field, such as the tube furnace sold by Nabertherm under the trade name LH 60 / 14.

[0026] Next, the calcined and solidified material was soaked in a 0.05-0.15M H2SO4 solution at a weight ratio of 1:15-25 with sulfuric acid solution. The mixture was stirred at room temperature for 25-35 minutes, then washed with deionized water until neutral. The mixture was then dried in a vacuum drying oven at 50-70°C for 10-14 hours. Finally, it was heated to 350-450°C at a heating rate of 3-8°C / min in a vacuum drying oven under a mixed gas of H2 and Ar in a volume ratio of 3-8:100, and held at this temperature for 0.8-1.2 hours to obtain the Fe3O4@NP-HC composite.

[0027] The basic function of soaking in 0.05~0.15M H2SO4 solution is to acidify the Fe3O4@HC surface, increase oxygen-containing functional groups such as carboxyl groups, and enhance the hydrophilicity and ion exchange capacity of the complex.

[0028] During the drying stage, when the drying temperature and drying time are within the specified range, if the drying vacuum degree is below 100 Pa, the dehydration efficiency is low; if the drying vacuum degree is above 1000 Pa, air may be introduced, leading to oxidation. Therefore, a drying vacuum degree of 100~1000 Pa is suitable. When the drying vacuum degree and drying time are within the specified range, if the drying temperature is below 50℃, moisture removal is slow; if the drying temperature is above 70℃, the functional groups of Fe3O4@HC may be damaged. Therefore, a drying temperature of 50~70℃ is appropriate. When the drying vacuum degree and drying temperature are within the specified range, if the drying time is less than 10 h, the drying is insufficient; if the drying time is longer than 14 h, energy is wasted. Therefore, a drying time of 10~14 h is preferable. According to the analytical method of GB / T 6284-2016, the water content of the dried material should be less than 5% by weight.

[0029] The vacuum drying oven used in this invention is a commonly used vacuum drying oven in this technical field, such as the vacuum drying oven sold by Binder under the trade name VDL 53.

[0030] The dried material is heated to 350-450°C in a H2-Ar mixed gas with a volume ratio of 3-8:100 in a vacuum drying oven at a heating rate of 3-8°C / min and held at that temperature for 0.8-1.2h to obtain the Fe3O4@NP-HC composite.

[0031] In this step, if the volume ratio of the H2-Ar mixture is less than 3:100, the reducing atmosphere is insufficient; if the volume ratio of the H2-Ar mixture is greater than 8:100, the excessive H2 concentration may lead to over-reduction and the formation of Fe. 0Therefore, a volume ratio of H2-Ar mixed gas of 3~8:100 is reasonable; when the heating temperature and holding time are within the aforementioned range, if the heating rate is less than 3℃ / min, the reduction reaction proceeds slowly; if the heating rate is greater than 8℃ / min, thermal stress may damage its structure; therefore, a heating rate of 3~8℃ / min is appropriate; when the heating rate and holding time are within the aforementioned range, if the heating temperature is less than 350℃, the reduction is incomplete; if the heating temperature is greater than 450℃, it leads to sintering of the carbon support; therefore, a heating temperature of 350~450℃ is suitable; when the heating rate and heating temperature are within the aforementioned range, if the holding time is less than 0.8h, the reduction reaction proceeds incompletely; if the holding time is greater than 1.2h, the Fe3O4@NP-HC product particles may grow and reduce activity; therefore, a holding time of 0.8~1.2h is appropriate.

[0032] On the other hand, the present invention also provides a Fe3O4@NP-HC multifunctional composite, wherein the composite uses porous hydrothermal carbon as a carrier, on which Fe3O4 nanoparticles are loaded, and the framework of the hydrothermal carbon is simultaneously doped with nitrogen and phosphorus atoms; wherein the composite simultaneously contains the following features (a) and (b):

[0033] (a) Its composition by elemental weight percentage is: C: 56.8%~68.5%, N: 3.6%~4.9%, P: 2.2%~3.1%, Fe: 13.8%~19.5%, O: 21.7%~28.5%, H: 1.9%~2.8%;

[0034] (b) In its Raman spectrum, it is located at approximately 1355 cm⁻¹ -1 The D peak is located at approximately 1590 cm⁻¹ -1 The intensity ratio (I_D / I_G) of the G peak is 1.10~1.15.

[0035] The C, H, and N contents were determined using an elemental analyzer sold by Elementar under the trade name vario EL cube, and the O content was obtained by the difference method.

[0036] The Raman spectrum was determined using a Raman spectrometer sold by Renishaw under the trade name inVia Reflex. The analytical results are shown in the appendix. Figure 3 :D band (1355 cm) -1 ) and G-band (1590 cm) -1 The strength of ) is greater than that of I D / I G The value of 1.12 indicates that nitrogen and phosphorus doping effectively increases the structural defects of the carbon matrix, which is beneficial for exposing more catalytic active sites.

[0037] Preferably, in the X-ray photoelectron spectroscopy (XPS) of the Fe3O4@NP-HC multifunctional composite of the present invention, the N 1s spectrum shows the presence of three forms: pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen, and the relative content ratio of the three is (35-40)%:(40-45)%:(20-25)%. The X-ray photoelectron spectroscopy (XPS) was measured using an X-ray photoelectron spectroscopy instrument sold by Thermo Fisher Scientific under the trade name ESCALAB 250Xi.

[0038] Its functional group composition:

[0039] C1 by atomic percentage S : 69.5%~74.5%, O1s: 18.2%~24.5%, N1s: 4.1%~5.8%, P2p: 1.3%~2.4%, Fe 2p: 6.9%~11.8%;

[0040] N1s confirmed that nitrogen exists in the forms of pyridine nitrogen (398.6 eV, 37.2%), pyrrole nitrogen (400.1 eV, 41.5%), and graphitic nitrogen (401.1 eV, 21.3%).

[0041] The elemental contents of Fe and P were determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES) sold by Thermo Fisher Scientific under the trade name iCAP 7400. In the Fe 2p spectrum, Fe... 2 ⁺ / Fe 3 The ratio of ⁺ to ⁺ is 0.47, confirming the existence of the characteristic dual valence state in Fe3O4.

[0042] Its phase composition was determined using an X-ray diffractometer (XRD) sold by Bruker under the trade name D8 Advance. The analytical results are shown in the appendix. Figure 2 :

[0043] The XRD pattern shows characteristic diffraction peaks of Fe3O4 at 2θ = 30.2°, 35.5°, 43.2°, 57.0° and 62.6°, indicating the Fe3O4 crystalline phase; the broadened diffraction peak at 2θ = 25° is attributed to the (002) plane of amorphous hydrothermal carbon.

[0044] XRD and Raman spectroscopy jointly confirmed that the Fe3O4@NP-HC composite of the present invention has a composite structure with complete Fe3O4 crystal phase and abundant carbon support defects.

[0045] The Fe3O4@NP-HC complex of this invention exerts its acid-base synergistic catalytic effect through the acidic center provided by Fe3O4 and the basic center provided by N site, and efficiently catalyzes the hydrolysis reaction of corn straw.

[0046] The present invention also provides the application of the Fe3O4@NP-HC multifunctional complex obtained by the above method in the biomass resource utilization process, preferably for the production of compound fertilizer, especially for the production of hydrothermal carbon-struvite compound fertilizer, so as to improve the hydrothermal carbon yield and hydrothermal carbon nitrogen content of the compound fertilizer.

[0047] Furthermore, this invention provides a method for producing hydrothermal carbon-struckstone compound fertilizer using lignocellulosic agricultural and forestry waste. This method uses the aforementioned Fe3O4@NP-HC multifunctional complex as a catalyst and nutrient supplement, and includes the following steps:

[0048] A. Catalytic hydrothermal carbonization: Agricultural and forestry waste, water and the Fe3O4@NP-HC multifunctional complex are mixed and subjected to hydrothermal reaction. After the reaction is completed, solid-liquid separation is performed to obtain solid product and liquid product.

[0049] B. Intelligent crystallization and recovery: Under the condition of real-time monitoring of pH value and conductivity, the liquid product obtained in step A is added to the reaction system with alkali solution, magnesium source and seed crystals to carry out struvite crystallization reaction. After the reaction, the solid and liquid are separated to obtain struvite crystals.

[0050] C. Product compounding: The solid product obtained in step A is subjected to magnetic separation to recover the Fe3O4@NP-HC multifunctional composite to obtain nitrogen-rich hydrothermal carbon; the nitrogen-rich hydrothermal carbon is mixed with the struvite crystals obtained in step B and granulated to obtain the hydrothermal carbon-struvite compound fertilizer.

[0051] The process flow for producing hydrothermal carbon-struck stone compound fertilizer from lignocellulosic agricultural and forestry waste according to this invention is shown in the appendix. Figure 1 .

[0052] According to a preferred embodiment of the present invention, in step A, the mass ratio of the agricultural and forestry waste, water and the Fe3O4@NP-HC multifunctional composite is 1:(8-12):(0.08-0.15), and the hydrothermal carbonization reaction is achieved by stirring at 180~260℃ for 1~4h. The equipment used for the hydrothermal carbonization reaction is a high-pressure reactor, a hydrothermal synthesis reactor or a batch autoclave with stirring.

[0053] In the preparation of compound fertilizer, the agricultural and forestry waste is subjected to hydrothermal carbonization reaction for 1-4 hours under the following conditions: agricultural and forestry waste to deionized water weight ratio of 1:8-12, agricultural and forestry waste to Fe3O4@NP-HC complex weight ratio of 8-12:1, and temperature of 180-260℃ with stirring. After filtration and separation, a solid containing hydrothermal carbon and the complex and a solid containing NH4+ are obtained. + 4 and PO 3-The liquid of 4; the solid is dispersed in water at a weight ratio of 1:4~6 with water, and the Fe3O4@NP-HC complex is adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid;

[0054] The hydrothermal carbonization reaction is a thermochemical conversion of agricultural and forestry waste into hydrothermal carbon in a subcritical water medium. In this hydrothermal carbonization reaction, when the weight ratio of agricultural and forestry waste to Fe3O4@NP-HC composite, the hydrothermal carbonization reaction temperature, and the time are within the specified ranges, if the weight ratio of agricultural and forestry waste to deionized water is higher than 1:8, the agricultural and forestry waste slurry will be too thick, resulting in uneven heat and mass transfer and low reaction efficiency; if the weight ratio is lower than 1:12, the agricultural and forestry waste slurry will be too thin, increasing energy consumption. Therefore, a weight ratio of agricultural and forestry waste to deionized water of 1:8 to 1:12 is reasonable. When the weight ratio of agricultural and forestry waste to deionized water, the hydrothermal carbonization reaction temperature, and the time are within the specified ranges, if the weight ratio of agricultural and forestry waste to the composite is higher than 1:8, excessive composite may cause agglomeration; if the weight ratio is lower than 1:12, insufficient composite is used, and the hydrothermal carbonization reaction is incomplete. Therefore, the weight ratio of agricultural and forestry waste to the composite... A weight ratio of 1:8 to 12 is suitable. When the weight ratios of agricultural and forestry waste to deionized water, agricultural and forestry waste to composite materials, and hydrothermal carbonization reaction time are within the specified ranges, if the hydrothermal carbonization reaction temperature is below 180°C, the reaction rate will be slow and the reaction will be incomplete; if the temperature is above 260°C, the agricultural and forestry waste may be over-carbonized, resulting in a decrease in reaction yield. Therefore, a hydrothermal carbonization reaction temperature of 180 to 260°C is appropriate. When the weight ratios of agricultural and forestry waste to deionized water, agricultural and forestry waste to composite materials, and hydrothermal carbonization reaction temperature are within the specified ranges, if the hydrothermal carbonization reaction time is less than 1 hour, the reaction will be incomplete; if the reaction time is longer than 4 hours, energy will be wasted, and the carbon quality may decrease. Therefore, a hydrothermal carbonization reaction time of 1 to 4 hours is preferable.

[0055] In step A, the equipment used for the hydrothermal carbonization reaction of agricultural and forestry waste is a high-pressure reactor, a hydrothermal synthesis reactor, or a batch autoclave with stirring. These are all products currently available on the market, such as the high-pressure reactor sold by Parr Instrument Company under the trade name Parr 4575, and the hydrothermal synthesis reactor sold by Amar Equipments under the trade name Amar Autoclave.

[0056] The hydrothermal carbonization reaction products need to be separated into liquid and solid phases using filtration equipment to obtain a solid containing hydrothermal carbon and its complex, and a solid containing NH4+. + 4 and PO4 3-The liquid used is filtered by equipment commonly used in the field of chemical technology, such as vacuum filtration devices sold by Sartorius under the trade name Sartolab RF, filter press devices sold by Shuniu under the trade name filter press flask, and centrifugal separation devices sold by Thermo Fisher Scientific under the trade name Sorvall LYNX 6000.

[0057] The solid obtained by filtration and separation is dispersed in water at a weight ratio of 1:4~6 with water, and Fe3O4@NP-HC complex is adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid.

[0058] The 0.4T permanent magnet used in this invention is a product currently available on the market, such as the product sold by Ningbo Yuxi Magnetic Materials under the trade name YX-MAG-04T.

[0059] In step B, the NH4+ obtained from step A is... + 4 and PO 3- 4. The liquid is pumped into the crystallization tank. The reaction can be carried out using a feedback pH control system at a liquid phase temperature of 75~85℃, a stirring speed of 220~280rpm and a pH of 8.5~9.5. After the reaction is terminated, the stirring is stopped, the reactant mixture is cooled to room temperature, and the solid and liquid are separated. The solid is struvite crystals, which are first washed with deionized water and then washed with anhydrous ethanol. The washed struvite crystals are dried in an oven at a temperature of 55~65℃ for 12 hours to obtain the struvite crystals.

[0060] The basic reaction for preparing struvite crystals is as follows:

[0061] NH4 + +PO4 3− +Mg 2+ +6H₂O→MgNH₄PO₄·6H₂O↓

[0062] As a preferred embodiment, the equipment used in the preparation of struvite crystals according to the present invention is a feedback pH control system. This system consists of a pH electrode, a conductivity meter, a four-channel solution peristaltic pump, a seed crystal dispenser, a PLC controller, and a threshold trigger, connected by signals and controlled in a coordinated manner. For details of the structure of the feedback pH control system of the present invention, please refer to the appendix. Figure 4 .

[0063] in:

[0064] A pH electrode is a high-precision pH sensor (accuracy ±0.1) that monitors the pH value of a liquid phase in real time.

[0065] The pH electrode used in this invention is a composite glass electrode, an antimony electrode, a solid electrode, or an industrial online process pH electrode type pH electrode, all of which are currently commercially available products, such as the composite glass electrode sold by Mettler Toledo under the trade name InPro3250i.

[0066] Conductivity meters can detect changes in ion concentration in a solution, indirectly reflecting ion consumption during struvite crystallization and providing auxiliary information for judging the crystallization process.

[0067] The conductivity meter used in this invention is a contact conductivity meter, a four-ring electrode conductivity meter, an electromagnetic induction conductivity meter, or an industrial online process conductivity meter. These are all products currently available on the market, such as the four-ring electrode conductivity meter sold by Mettler Toledo under the trade name InPro 7100.

[0068] A four-channel solution peristaltic pump is used, in which KOH solution is added to channel 1 to adjust the pH; HCl solution is added to channel 2 to adjust the pH; MgCl2 solution is added to channel 3 to supplement the magnesium source; and seed suspension is added to channel 4 only when the threshold trigger is activated to provide parent crystals and induce heterogeneous nucleation.

[0069] The four-channel solution peristaltic pump used in this invention is a split-type multi-channel peristaltic pump, an integrated multi-channel peristaltic pump, a speed-regulating peristaltic pump, or a peristaltic pump with precise flow control. These are all products currently sold on the market, such as the integrated multi-channel peristaltic pump sold by Cole-Parmer under the trade name Masterflex L / S.

[0070] The seed crystal feeder has a stirring structure, which adds struvite mother crystals at the right time during the struvite crystallization process, providing a sufficient crystallization substrate for the crystallization process, promoting the directional crystallization process, and improving crystallization efficiency and product quality.

[0071] The seed crystal dispenser used in this invention is a storage tank dispenser with mechanical stirring, a magnetic stirring kettle dispenser, a pneumatic stirring dispenser, or an online dilution and dispensing integrated device dispenser. These are all products currently available on the market, such as the storage tank dispenser with mechanical stirring sold by Syrris under the trade name Atlas Potassium.

[0072] The PLC controller receives signals from the pH electrode and conductivity meter, and controls the peristaltic pump and seed crystal dispenser according to preset logic rules. The preset pH target range is 8.5-9.5. Based on the deviation between the measured pH and the target value, the controller controls the start and stop of the solution peristaltic pump and seed crystal dispenser.

[0073] The PLC controller used in this invention is a modular PLC, an integrated small PLC, a programmable automation controller, or an embedded micro PLC type controller. These are all products currently sold on the market, such as the modular PLC sold by Siemens under the trade name SIMATIC S7-1200.

[0074] The threshold trigger sets control thresholds for pH and conductivity; when the conductivity decreases at a rate exceeding 100 μS·cm... -1 ·min -1 At that time, the seed crystal addition signal is triggered.

[0075] The threshold triggers used in this invention are hardware triggers based on comparator circuits, software function modules integrated within PLCs, independent industrial signal alarms, or microprocessor-based intelligent judgment module triggers. These are all products currently available on the market, such as the hardware trigger based on comparator circuits sold by Omron under the trade name E2E-X1R5.

[0076] When the feedback pH control system is running, the pH electrode and conductivity meter are immersed in the reaction liquid phase to monitor solution parameters in real time and transmit signals to the PLC controller and threshold trigger. The PLC controller receives the pH signal and controls the first three channels of the four-channel solution peristaltic pump according to preset logic rules to add alkaline solution, acid solution or magnesium source solution. The threshold trigger receives pH and conductivity signals in parallel. When the preset critical condition is met, it sends a trigger signal to the PLC controller, thereby starting the fourth channel of the four-channel solution peristaltic pump, namely the seed crystal dispenser, to add struvite seed suspension.

[0077] The mechanical connection methods for each part of the feedback pH control system are as follows:

[0078] The pH electrode and conductivity meter are mounted on the side wall of the crystallization tank via threaded flanges, with the probes immersed in the liquid phase to ensure real-time monitoring. Three channels of the four-channel peristaltic pump are connected to the corresponding interfaces of the crystallization tank via corrosion-resistant pipes (such as silicone tubing). Channel 1 delivers KOH solution (1M), channel 2 delivers HCl solution (0.1M), and channel 3 delivers MgCl2 solution (1M). Channel 4 connects to the outlet of the seed crystal dispenser, a storage tank with mechanical stirring, containing a 5% (by weight) suspension of struvite seed crystals. The PLC controller and threshold triggers are installed in the control cabinet and connected to the sensors and actuators via cables.

[0079] The electrical connections for each part of the feedback-type pH control system are as follows:

[0080] The output signals from the pH electrode and conductivity meter are connected to the analog input module and the analog input port of the threshold trigger of the PLC controller via shielded cables. The digital output signal (relay switch) of the threshold trigger is connected to the digital input module of the PLC controller via a cable. The digital output module of the PLC controller controls the start, stop, and speed of the four-channel peristaltic pump via a cable.

[0081] The overall system operation mode of the feedback pH control system of this invention is as follows;

[0082] Real-time monitoring: pH electrode and conductivity meter continuously monitor solution pH and conductivity, and the data is transmitted to PLC controller and threshold trigger in real time.

[0083] pH control: The PLC controller compares the real-time pH with the set range (8.5-9.5). If the pH is below 8.5, channel 1 (KOH pump) is activated to raise the pH; if the pH is above 9.5, channel 2 (HCl pump) is activated to lower the pH; at the same time, based on ion concentration monitoring, channel 3 (MgCl2 pump) is activated to maintain Mg:N:P≈1:1:1.

[0084] Seed dosing control: A threshold trigger analyzes pH and conductivity trends in parallel. When the pH stabilizes between 8.5 and 9.5 and the conductivity drops sharply, the trigger sends a digital signal to the PLC. The PLC immediately starts channel 4 (seed dosing pump) to inject the seed suspension.

[0085] Collaborative control: The entire system forms a closed-loop feedback. The PLC acts as the main controller to handle pH adjustment, while the threshold trigger acts as an auxiliary module to ensure timely seed addition and avoid homogeneous nucleation.

[0086] The feedback-type pH control system of this invention features signal connection and coordinated control: each component is linked through electrical signals. pH and conductivity signals are input to the PLC and triggers. The PLC output controls the peristaltic pump, and the trigger output triggers the addition of seed crystals. This parallel processing ensures rapid response and precise control.

[0087] According to another preferred embodiment of the present invention, in step C, the pulverized hydrothermal carbon solid and struvite crystals are poured into a V-type mixer and mixed for 10 to 15 minutes at a speed of 15 to 25 rpm. Then, an aqueous solution of sodium alginate with a mass concentration of 0.5 to 2.0% is sprayed onto the mixture. The wet granules obtained after granulation are dried in a forced-air drying oven at a temperature of 55 to 65°C for 10 to 14 hours to obtain hydrothermal carbon-struvite compound fertilizer.

[0088] More specifically, the hydrothermal char solid obtained in step A and the struvite crystals obtained in step B are ground into 60-100 mesh powders respectively. The struvite powder and hydrothermal char powder are poured into a V-type mixer at a weight ratio of 7:2.8-3.2 and mixed for 10-15 minutes at a speed of 15-25 rpm. Then, a dilute solution of sodium alginate binder and deionized water are sprayed using a sprayer to allow the hydrothermal char powder to adhere to the surface of the struvite crystals and form a coating film. Next, the wet granules are transferred to a disc granulator for granulation and then sent to an extruder for extrusion granulation through a sieve with a 1.0-2.0 mm aperture. The resulting wet granules are dried in a forced-air drying oven at a temperature of 55-65°C for 10-14 hours to obtain the compound fertilizer.

[0089] In this step, the hydrothermal carbon solid obtained in step A and the struvite crystals obtained in step B need to be ground separately to 60-100 mesh powder using grinding equipment. If the particle size of the hydrothermal carbon solid and struvite crystal powder is less than 60 mesh, the particles are too coarse, affecting the coating and granulation effect; if the particle size of the hydrothermal carbon solid and struvite crystal powder is greater than 100 mesh, the particles are too fine, easily causing dust and uneven mixing; therefore, a particle size of 60-100 mesh for the hydrothermal carbon solid and struvite crystal powder is suitable; the grinding equipment mentioned is commonly used in the field of chemical technology and is currently available on the market.

[0090] Stromboli powder and hydrothermal charcoal powder are mixed in a V-type mixer at a weight ratio of 7:2.8~3.2 and a mixer speed of 15~25 rpm for 10~15 minutes. When the mixer speed and mixing time are within the specified range, if the weight ratio of stromboli powder to hydrothermal charcoal powder is higher than 7:2.8, the hydrothermal charcoal powder particles will not be sufficiently coated, resulting in poor nutrient slow-release effect; if the weight ratio is lower than 7:3.2, the hydrothermal charcoal powder particles will be too thickly coated, affecting nutrient release. Therefore, a weight ratio of stromboli powder to hydrothermal charcoal powder of 7:2.8~3.2 is reasonable. When the weight ratio of stromboli powder to hydrothermal charcoal powder and the mixing time are within the specified range, if the mixer speed... If the mixing speed is below 15 rpm, the struvite powder and hydrothermal charcoal powder will not mix evenly; if the mixer speed is above 25 rpm, the particles may break. Therefore, a mixer speed of 15-25 rpm is feasible. When the weight ratio of struvite powder to hydrothermal charcoal powder and the mixer speed are within the aforementioned range, if the mixing time is less than 10 minutes, the struvite powder and hydrothermal charcoal powder will not mix evenly; if the mixing time is longer than 15 minutes, energy will be wasted. Therefore, a mixing time of 10-15 minutes is appropriate.

[0091] The V-type mixer used in this invention is a double-drum V-type mixer, an asymmetric V-type mixer, a W-type mixer, or a square cone-type mixer. These are all products currently sold on the market, such as the double-drum V-type mixer sold by Gemco under the trade name Gemco V-Blender.

[0092] Preferably, in step C, the nitrogen-rich hydrothermal char and struvite crystals are crushed and mixed separately, then a binder solution is sprayed on them for granulation. The resulting granules are dried to obtain the hydrothermal char-struvite compound fertilizer. The granules of the compound fertilizer have a structure with struvite as the core and nitrogen-rich hydrothermal char as the outer layer.

[0093] The binder solution used in this step is, for example, a 0.5-2.0% by weight dilute sodium alginate solution. The amount of sodium alginate solution and deionized water sprayed together allows the hydrothermal carbon powder to adhere to the surface of the struvite crystals, forming a coating film.

[0094] The wet granules are transferred to a disc granulator for granulation, and then fed through a 1.0~2.0mm mesh screen to an extruder for extrusion granulation. The basic function of the wet granules in the disc granulator is to form spherical particles through rolling and agglomeration; the basic function of extrusion granulation is to increase particle density and mechanical strength.

[0095] The obtained wet granules are dried in a forced-air drying oven at a temperature of 55-65℃ for 10-14 hours to obtain the compound fertilizer. When the drying time of the wet granules is within the specified range, if the drying temperature is below 55℃, the wet granules dry slowly and are prone to sticking; if the drying temperature is above 65℃, the crystal structure of the struvite may be damaged. Therefore, a drying temperature of 55-65℃ is preferable. When the drying temperature is within the specified range, if the drying time is less than 10 hours, the wet granules are not completely dried; if the drying time is longer than 14 hours, energy will be wasted. Therefore, a drying time of 10-14 hours is suitable.

[0096] The hydrothermal carbon-struvite compound fertilizer obtained by this invention is composed of 60%~80% struvite crystals and 20%~40% nitrogen-rich hydrothermal carbon by mass; the particle size of the hydrothermal carbon-struvite compound fertilizer particles is 1.0~2.0mm.

[0097] The relevant parameter detection method of the present invention is as follows:

[0098] Hydrothermal carbon yield: The hydrothermal carbon solid obtained by filtration and separation is dried to constant weight at 105℃, weighed, and the hydrothermal carbon yield is calculated according to (hydrothermal carbon mass / raw material mass) × 100%.

[0099] The nitrogen content of the Fe3O4@NP-HC complex of the present invention was determined using an elemental analyzer (such as an Elementar vario ELcube);

[0100] Transfer 10 mL of the hydrothermal carbonization reaction liquid, centrifuge it, collect the supernatant, and analyze the NH4 content of the liquid phase. + and PO4 3- concentration:

[0101] NH4 + The concentration detection steps are as follows: Detection is performed using Nessler's reagent spectrophotometry. A suitable amount of supernatant is transferred (to allow NH4+ to settle). + Add NH4+ (within the concentration range of 0.02~2 mg / L) to a 50 mL colorimetric tube, add water to the mark, add 1.0 mL of potassium sodium tartrate solution, mix well, then add 1.5 mL of Nessler's reagent (a product sold by Sigma-Aldrich under the trade name Nessler's reagent), let stand for 10 min, and then use a UV-Vis spectrophotometer (Shimadzu UV-1800) to measure the absorbance at a wavelength of 655 nm. Based on NH4+... + The NH4+ concentration of the supernatant was calculated using the -N standard curve (concentration range 0.05~2.0 mg / L). + concentration.

[0102] PO4 3- The concentration detection procedure is as follows: The ammonium molybdate-ascorbic acid method is used for detection. A suitable amount of supernatant is transferred (to make PO4...) 3- Add 2.0 mL of ammonium molybdate reagent (containing potassium antimony tartrate in sulfuric acid medium) sold by Beijing Solarbio Science & Technology Co., Ltd. under the trade name Ammonium Molybdate, and 1.0 mL of ascorbic acid solution with a concentration of 20 g / L to a 50 mL volumetric flask. Add water to the mark, mix well, and let stand at room temperature for 10 min. Then, use the spectrophotometer described above to measure the absorbance at a wavelength of 880 nm. Based on PO4... 3- The PO4 content of the supernatant was calculated using a standard curve (concentration range 0.01~1.0 mg / L). 3- concentration.

[0103] The struvite crystals were analyzed by XRD with a scanning range of 2θ = 10~40°. The results were compared with the JCPDS standard card (No. 15-0762). At the same time, the contents of Mg, N and P were determined by ICP-OES. The purity of struvite was determined by the analytical results obtained by the two methods.

[0104] The results obtained from the above measurements show that using the Fe3O4@NP-HC complex of the present invention can significantly increase the hydrothermal carbon yield from 65% to 85%, and increase the nitrogen content in the hydrothermal carbon from 1.0% to 3.5%. Simultaneously, the complex acts as a nutrient reservoir, releasing nutrients and increasing the levels of NH4⁺ and PO4⁻ in the liquid phase after the reaction. 3 The concentrations reached 930 mg / L and 540 mg / L, respectively, which are much higher than those of the system without the complex, providing a material basis for the subsequent synthesis of high-purity struvite (>98%).

[0105] In summary, this invention provides a Fe3O4@NP-HC complex and a method for resource utilization, achieving unexpected technical effects through the following multi-level synergistic effects:

[0106] 1. Synergistic effect at the catalyst level: The Lewis acid sites provided by Fe3O4, the basic sites provided by nitrogen doping (especially pyridine nitrogen), and the acidic sites provided by phosphorus doping form an acid-base synergistic catalytic system in a hydrothermal environment, which greatly promotes the breaking of CO and C-C bonds in lignocellulose. This not only improves the yield of solid carbon, but also guides nitrogen and phosphorus elements to be chemically bonded and fixed in the formed carbon skeleton, rather than simply released.

[0107] 2. Synergistic effects at the process level: In step A, the complex acts as a "dynamic nutrient pool," where its own doped N and P elements can be slowly released during the catalytic process, replenishing the NH4 produced by the degradation of biomass itself. + and PO4 3- This significantly increases the nutrient concentration in the liquid phase after the reaction, laying the foundation for subsequent efficient crystallization. The intelligent crystallization system in step B, through real-time feedback control, ensures the formation of high-purity, large-particle struvite crystals under high supersaturation, rather than amorphous precipitates, thus improving product quality and sustained-release performance.

[0108] 3. Synergistic effect of final product: The compound fertilizer obtained in step C has struvite inside that provides long-lasting N, P and Mg nutrients, and nitrogen-rich hydrothermal carbon on the outside that not only contains a slow-release nitrogen source, but its porous structure can also adsorb water and nutrients and improve soil aggregate structure. The combination of the two achieves a synergistic fertilizer effect of "combining fast and long-lasting effects, and integrating fertilizer supply and soil improvement". Attached Figure Description

[0109] Appendix Figure 1 This is a schematic diagram of the production process of hydrothermal carbon-struvite compound fertilizer of the present invention;

[0110] Appendix Figure 2 This is the XRD pattern of the Fe3O4@NP-HC complex of the present invention;

[0111] Appendix Figure 3This is the Raman spectrum of the Fe3O4@NP-HC complex of the present invention;

[0112] Appendix Figure 4 This is a schematic diagram of the feedback pH control system of the present invention. Detailed Implementation

[0113] The invention will be better understood through the following examples.

[0114] Example 1: Production method of hydrothermal carbon-struvite compound fertilizer of the present invention

[0115] The implementation method of this embodiment is as follows:

[0116] A. Preparation of Fe3O4@NP-HC complex

[0117] Agricultural and forestry waste was crushed into powder with a particle size of 30-50 mesh. The resulting agricultural and forestry waste powder was immersed in a 1.2 mol / L hydrochloric acid solution at a weight ratio of 1:9. The mixture was stirred at 76°C for 2.4 hours. The resulting solid was then dried in a vacuum drying oven at 62°C for 11 hours. The dried solid was then added to a hydrothermal reactor.

[0118] FeCl2·4H2O and FeCl3·6H2O are reacted according to Fe... 2+ Fe 3+ The iron solution was dissolved in deionized water at a molar ratio of 1:2.2 and added to the above hydrothermal reactor. The dried solid and the iron solution were subjected to a hydrothermal reaction at a weight ratio of 1:8 and a hydrothermal reaction temperature of 200°C under a N2 protective atmosphere for 5 hours. The solution was then filtered and dried to obtain porous hydrothermal carbon, and the filtrate was recycled.

[0119] NH3 aqueous solution was added dropwise to the obtained porous hydrothermal carbon until the pH reached 10, and then stirred at 62℃ for 2.2 h. The mixture was then separated using a 0.5T permanent magnet, and the resulting solid was dried in a vacuum drying oven at 58℃ for 13 h to obtain Fe3O4@HC.

[0120] Ammonium dihydrogen phosphate and urea were mixed at a mass ratio of 1.5:1 and dissolved in an ethanol aqueous solution with a concentration of 26% by volume. The Fe3O4@HC was then immersed in the ethanol aqueous solution at a weight ratio of 1:8 and subjected to ultrasonic treatment at an ultrasonic frequency of 38kHz for 32 minutes.

[0121] The ultrasonically treated Fe3O4@HC was heated to 550°C at a heating rate of 3°C / min in a vacuum drying oven under N2 protection, and calcined and solidified at this temperature for 3.5 h. Then, it was immersed in 0.12 M H2SO4 solution at a weight ratio of 1:22 with sulfuric acid solution and stirred at room temperature for 30 min. Then, it was washed with deionized water until neutral and dried in a vacuum drying oven at a vacuum degree of 1000 Pa and a temperature of 60°C for 12 h. Then, it was heated to 450°C at a heating rate of 5°C / min in a vacuum drying oven under a mixed gas of H2 and Ar in a volume ratio of 7:100, and held at this temperature for 1.0 h, thus obtaining the Fe3O4@NP-HC composite.

[0122] According to the method described in this application, the chemical elemental composition of the Fe3O4@NP-HC complex is as follows (by weight): C: 65.2%, H: 2.3%, O: 24.8%, N: 4.5%, P: 2.8%, and Fe: 16.4%; its XPS functional group composition is: C1 S : 72.1%, O1s: 20.5%, N1s: 5.2%, P2p: 1.8%, Fe 2p: 9.4%; its phase composition is as follows: the XRD pattern shows characteristic diffraction peaks of Fe3O4 at 2θ=30.2°, 35.5°, 43.2°, 57.0° and 62.6°, indicating the Fe3O4 crystalline phase; the broadened diffraction peak at 2θ=25° belongs to the (002) plane of amorphous hydrothermal carbon.

[0123] The agricultural and forestry waste was subjected to hydrothermal carbonization in a high-pressure reactor at a weight ratio of 1:10 (agricultural and forestry waste to deionized water) and 11:1 (agricultural and forestry waste to Fe3O4@NP-HC complex) and a temperature of 230°C with stirring for 2.0 h. After filtration and separation, a solid containing hydrothermal carbon and the complex, and a solid containing NH4+ were obtained. + 4 and PO 3- The liquid of 4; the solid was dispersed in water at a weight ratio of 1:5 with water, and the Fe3O4@NP-HC complex was adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid;

[0124] B. Preparation of struvite crystals using a feedback pH control system

[0125] The NH4+ obtained from step A + 4 and PO 3-The liquid of 4 is pumped into the crystallization tank. A feedback pH control system, consisting of a composite glass electrode type pH electrode, a contact conductivity meter type conductivity meter, a split multi-channel peristaltic pump type four-channel solution peristaltic pump, a storage tank type seed crystal feeder with mechanical stirring, a modular PLC type PLC controller, and a hardware trigger type threshold trigger based on comparator circuit, is used to carry out the reaction under the conditions of liquid phase temperature of 80℃, stirring speed of 240rpm, and pH of 9.5. After the reaction is terminated, stirring is stopped, the reactant mixture is cooled to room temperature, and solid and liquid are separated. The solid is struvite crystals, which are first washed with deionized water and then washed with anhydrous ethanol. The washed struvite crystals are dried in an oven at 55℃ for 12 hours to obtain the struvite crystals.

[0126] C. Preparation of compound fertilizer

[0127] The hydrothermal char solid obtained in step A and the struvite crystals obtained in step B are ground into 60-100 mesh powders respectively. The struvite powder and hydrothermal char powder are poured into a double-cylinder V-type mixer at a weight ratio of 7:3.0 and mixed for 12 minutes at a speed of 20 rpm. Then, a dilute solution of sodium alginate binder with a concentration of 0.5% by weight and deionized water are sprayed to make the hydrothermal char powder adhere to the surface of the struvite crystals to form a coating film. The wet granules are then transferred to a disc granulator for granulation and then sent to an extruder for extrusion granulation through a sieve with a pore size of 1.0-2.0 mm. The resulting wet granules are dried in a forced-air drying oven at a temperature of 62°C for 11 hours to obtain the compound fertilizer.

[0128] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this embodiment, as tested according to the method described in this application, are listed in Table 1 below.

[0129] According to the method described in this application, the hydrothermal carbon-struvite compound fertilizer produced by the production method of this embodiment is composed of 74% struvite crystals and 26% nitrogen-rich hydrothermal carbon by mass; the particle size of the hydrothermal carbon-struvite compound fertilizer particles is 1.0~2.0mm.

[0130] Example 2: Production method of hydrothermal carbon-struvite compound fertilizer of the present invention

[0131] The implementation method of this embodiment is as follows:

[0132] A. Preparation of Fe3O4@NP-HC complex

[0133] Forestry wood chips and agricultural and forestry waste were crushed into powder with a particle size of 30-50 mesh. The resulting agricultural and forestry waste powder was immersed in 0.5 mol / L hydrochloric acid solution at a weight ratio of 1:12. The mixture was stirred at 84℃ for 1.6 h. The resulting solid was then dried in a vacuum drying oven at 55℃ for 14 h. The dried solid was then added to a hydrothermal reactor.

[0134] FeCl2·4H2O and FeCl3·6H2O are reacted according to Fe... 2+ Fe 3+ The iron solution was dissolved in deionized water at a molar ratio of 1:1.5 and added to the above hydrothermal reactor. The dried solid and the iron solution were subjected to a hydrothermal reaction at a weight ratio of 1:15 and a hydrothermal reaction temperature of 180°C under a N2 protective atmosphere for 6 h. The solution was then filtered and dried to obtain porous hydrothermal carbon, and the filtrate was recycled.

[0135] NH3 aqueous solution was added dropwise to the obtained porous hydrothermal carbon until the pH reached 10. Then, the mixture was stirred at 55°C for 1.5 h. The solid was then separated using a 0.5T permanent magnet. The obtained solid was dried in a vacuum drying oven at 62°C for 11 h to obtain Fe3O4@HC.

[0136] Ammonium dihydrogen phosphate and urea were mixed at a mass ratio of 2.5:1 and dissolved in a 20% ethanol aqueous solution by volume. The Fe3O4@HC was then immersed in the ethanol aqueous solution at a weight ratio of 1:12 and subjected to ultrasonic treatment at an ultrasonic frequency of 45kHz for 25 minutes.

[0137] The ultrasonically treated Fe3O4@HC was heated to 550°C at a heating rate of 5°C / min in a vacuum drying oven under N2 protection and calcined and solidified at this temperature for 2.5 h. Then, it was immersed in 0.05 M H2SO4 solution at a weight ratio of 1:15 with sulfuric acid solution and stirred at room temperature for 25 min. Then, it was washed with deionized water until neutral and dried in a vacuum drying oven at a vacuum degree of 100 Pa and a temperature of 50°C for 14 h. Then, it was heated to 350°C at a heating rate of 3°C / min in a vacuum drying oven under a mixed gas of H2 and Ar in a volume ratio of 3:100 and held at this temperature for 0.8 h, thus obtaining the Fe3O4@NP-HC composite.

[0138] According to the method described in this application, the chemical elemental composition of the Fe3O4@NP-HC complex is as follows (by weight): C: 60.8%, H: 2.1%, O: 26.5%, N: 3.8%, P: 2.4%, and Fe: 18.4%; its XPS functional group composition is: C1 S: 70.5%, O1s: 22.8%, N1s: 4.6%, P2p: 1.5%, Fe 2p: 10.6%; its phase composition is as follows: the XRD pattern shows characteristic diffraction peaks of Fe3O4 at 2θ=30.2°, 35.5°, 43.2°, 57.0° and 62.6°, indicating the Fe3O4 crystalline phase; the broadened diffraction peak at 2θ=25° belongs to the (002) plane of amorphous hydrothermal carbon.

[0139] The agricultural and forestry waste was subjected to hydrothermal carbonization in a hydrothermal synthesis reactor at a weight ratio of agricultural and forestry waste to deionized water of 1:8, a weight ratio of agricultural and forestry waste to Fe3O4@NP-HC complex of 8:1, and a temperature of 180℃ with stirring for 4.0 h. After filtration and separation, a solid containing hydrothermal carbon and the complex and containing NH4+ were obtained. + 4 and PO 3- The liquid of 4; the solid was dispersed in water at a weight ratio of 1:4, and the Fe3O4@NP-HC complex was adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid;

[0140] B. Preparation of struvite crystals using a feedback pH control system

[0141] The NH4+ obtained from step A + 4 and PO 3- Liquid 4 was pumped into a crystallization tank. A feedback pH control system, consisting of an antimony electrode type pH electrode, a four-ring electrode type conductivity meter, an integrated multi-channel peristaltic pump type four-channel solution peristaltic pump, a magnetically stirred tank type seed crystal feeder, an integrated small PLC type PLC controller, and a PLC internal integrated software function module type threshold trigger, was used to carry out the reaction under the conditions of liquid phase temperature 75℃, stirring speed 220rpm, and pH 8.5. After the reaction was terminated, stirring was stopped, and the reaction mixture was cooled to room temperature. Solid-liquid separation was performed, and the solid was struvite crystals. It was first washed with deionized water and then washed with anhydrous ethanol. The washed struvite crystals were dried in an oven at 50℃ for 12 hours to obtain the struvite crystals.

[0142] C. Preparation of compound fertilizer

[0143] The hydrothermal char solid obtained in step A and the struvite crystals obtained in step B are ground into 60-100 mesh powders respectively. The struvite powder and hydrothermal char powder are poured into an asymmetric V-type mixer at a weight ratio of 7:2.8 and mixed for 10 minutes at a speed of 15 rpm. Then, a dilute solution of sodium alginate binder with a concentration of 2.0% by weight and deionized water are sprayed to make the hydrothermal char powder adhere to the surface of the struvite crystals to form a coating film. The wet granules are then transferred to a disc granulator for granulation and then sent to an extruder for extrusion granulation through a sieve with a pore size of 1.0-2.0 mm. The resulting wet granules are dried in a forced-air drying oven at a temperature of 55°C for 14 hours to obtain the compound fertilizer.

[0144] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this embodiment, as tested according to the method described in this application, are listed in Table 1 below.

[0145] According to the method described in this application, the hydrothermal carbon-struvite compound fertilizer produced by the production method of this embodiment is composed of 60% struvite crystals and 40% nitrogen-rich hydrothermal carbon by mass; the particle size of the hydrothermal carbon-struvite compound fertilizer particles is 1.0~2.0mm.

[0146] Example 3: Production method of hydrothermal carbon-struckstone compound fertilizer of the present invention

[0147] The implementation method of this embodiment is as follows:

[0148] A. Preparation of Fe3O4@NP-HC complex

[0149] Animal manure and agricultural and forestry waste were crushed into powder with a particle size of 30-50 mesh. The resulting agricultural and forestry waste powder was immersed in 1.5 mol / L hydrochloric acid solution at a weight ratio of 1:8. The mixture was stirred at 70°C for 3.0 h. The resulting solid was then dried in a vacuum drying oven at 65°C for 10 h. The dried solid was then added to a hydrothermal reactor.

[0150] FeCl2·4H2O and FeCl3·6H2O are reacted according to Fe... 2+ Fe 3+ The iron solution was dissolved in deionized water at a molar ratio of 1:2.5 and added to the above hydrothermal reactor. The dried solid and the iron solution were subjected to a hydrothermal reaction at a weight ratio of 1:12 and a hydrothermal reaction temperature of 220°C under a N2 protective atmosphere for 3 hours. The solution was then filtered and dried to obtain porous hydrothermal carbon, and the filtrate was recycled.

[0151] NH3 aqueous solution was added dropwise to the obtained porous hydrothermal carbon until the pH reached 10. Then, the mixture was stirred at 58°C for 2.5 h. The mixture was then separated using a 0.5T permanent magnet. The resulting solid was dried in a vacuum drying oven at 55°C for 14 h to obtain Fe3O4@HC.

[0152] Ammonium dihydrogen phosphate and urea were mixed at a mass ratio of 2.2:1 and dissolved in an ethanol aqueous solution with a concentration of 40% by volume. The Fe3O4@HC was then immersed in the ethanol aqueous solution at a weight ratio of 1:9 and subjected to ultrasonic treatment at an ultrasonic frequency of 35kHz for 35 minutes.

[0153] The ultrasonically treated Fe3O4@HC was heated to 550°C at a heating rate of 7°C / min in a vacuum drying oven under N2 protection and calcined and cured at this temperature for 2.8 h. Then, it was immersed in 0.15 M H2SO4 solution at a weight ratio of 1:25 with sulfuric acid solution and stirred at room temperature for 30 min. It was then washed with deionized water until neutral and dried in a vacuum drying oven at a vacuum degree of 400 Pa and a temperature of 70°C for 10 h. Then, it was heated to 380°C at a heating rate of 8°C / min in a vacuum drying oven under a mixed gas of H2 and Ar in a volume ratio of 5:100 and held at this temperature for 1.2 h, thus obtaining the Fe3O4@NP-HC composite.

[0154] According to the method described in this application, the chemical elemental composition of the Fe3O4@NP-HC complex is as follows (by weight): C: 68.5%, H: 2.8%, O: 21.7%, N: 4.9%, P: 3.1%, and Fe: 13.8%; its XPS functional group composition is: C1 S : 74.5%, O1s: 18.2%, N1s: 5.8%, P2p: 2.4%, Fe 2p: 8.1%; its phase composition is as follows: the XRD pattern shows characteristic diffraction peaks of Fe3O4 at 2θ=30.2°, 35.5°, 43.2°, 57.0° and 62.6°, indicating the Fe3O4 crystalline phase; the broadened diffraction peak at 2θ=25° belongs to the (002) plane of amorphous hydrothermal carbon.

[0155] The agricultural and forestry waste was subjected to hydrothermal carbonization in a stirred intermittent autoclave at a weight ratio of 1:12 (agricultural and forestry waste to deionized water) and 12:1 (agricultural and forestry waste to Fe3O4@NP-HC complex) at 260°C for 1.0 h. After filtration and separation, a solid containing hydrothermal carbon and the complex, and a solid containing NH4+ were obtained. + 4 and PO 3-The liquid of 4; the solid was dispersed in water at a weight ratio of 1:6 with water, and the Fe3O4@NP-HC complex was adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid;

[0156] B. Preparation of struvite crystals using a feedback pH control system

[0157] The NH4+ obtained from step A + 4 and PO 3- The liquid of 4 is pumped into the crystallization tank. A feedback pH control system, consisting of a solid electrode type pH electrode, an electromagnetic induction type conductivity meter, a speed-regulating peristaltic pump type four-channel solution peristaltic pump, a pneumatic stirring type seed crystal feeder, a programmable automation controller type PLC controller, and an independent industrial signal alarm type threshold trigger, is used to carry out the reaction under the conditions of liquid phase temperature of 85℃, stirring speed of 280rpm, and pH of 8.8. After the reaction is terminated, stirring is stopped, the reactant mixture is cooled to room temperature, and solid and liquid are separated. The solid is struvite crystals, which are first washed with deionized water and then washed with anhydrous ethanol. The washed struvite crystals are dried in an oven at 65℃ for 12 hours to obtain the struvite crystals.

[0158] C. Preparation of compound fertilizer

[0159] The hydrothermal char solid obtained in step A and the struvite crystals obtained in step B are ground into 60-100 mesh powders respectively. The struvite powder and hydrothermal char powder are poured into a W-type mixer or V-type mixer at a weight ratio of 7:3.2 and mixed for 15 minutes at a speed of 25 rpm. Then, a 1.5% sodium alginate binder solution and deionized water are sprayed to make the hydrothermal char powder adhere to the surface of the struvite crystals to form a coating film. The wet granules are then transferred to a disc granulator for granulation and then sent to an extruder for extrusion granulation through a 1.0-2.0 mm sieve. The resulting wet granules are dried in a forced-air drying oven at 65°C for 10 hours to obtain the compound fertilizer.

[0160] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this embodiment, as tested according to the method described in this application, are listed in Table 1 below.

[0161] According to the method described in this application, the hydrothermal carbon-struvite compound fertilizer produced by the production method of this embodiment is composed of 80% struvite crystals and 20% nitrogen-rich hydrothermal carbon by mass; the particle size of the hydrothermal carbon-struvite compound fertilizer particles is 1.0~2.0mm.

[0162] Example 4: Production method of hydrothermal carbon-struckstone compound fertilizer of the present invention

[0163] The implementation method of this embodiment is as follows:

[0164] A. Preparation of Fe3O4@NP-HC complex

[0165] Agricultural and forestry waste was crushed into powder with a particle size of 30-50 mesh. The resulting agricultural and forestry waste powder was immersed in 0.8 mol / L hydrochloric acid solution at a weight ratio of 1:11. The mixture was stirred at 90°C for 1.0 h. The resulting solid was then dried in a vacuum drying oven at 58°C for 13 h. The dried solid was then added to a hydrothermal reactor.

[0166] FeCl2·4H2O and FeCl3·6H2O are reacted according to Fe... 2+ Fe 3+ The iron solution was dissolved in deionized water at a molar ratio of 1:1.8 and added to the above hydrothermal reactor. The dried solid and the iron solution were subjected to a hydrothermal reaction at a weight ratio of 1:10 and a hydrothermal reaction temperature of 240°C under a N2 protective atmosphere for 2 h. The solution was then filtered and dried to obtain porous hydrothermal carbon, and the filtrate was recycled.

[0167] NH3 aqueous solution was added dropwise to the obtained porous hydrothermal carbon until the pH reached 10. Then, the mixture was stirred at 65°C for 1.8 h. The mixture was then separated using a 0.5T permanent magnet. The resulting solid was dried in a vacuum drying oven at 65°C for 10 h to obtain Fe3O4@HC.

[0168] Ammonium dihydrogen phosphate and urea were mixed at a mass ratio of 1.8:1 and dissolved in an ethanol aqueous solution with a concentration of 34% by volume. The Fe3O4@HC was then immersed in the ethanol aqueous solution at a weight ratio of 1:11 and subjected to ultrasonic treatment at an ultrasonic frequency of 42kHz for 28 minutes.

[0169] The ultrasonically treated Fe3O4@HC was heated to 550°C in a tube furnace under N2 protection at a heating rate of 8°C / min, and calcined and solidified at this temperature for 3.2 h. Then, it was immersed in 0.08 M H2SO4 solution at a weight ratio of 1:18 with sulfuric acid solution and stirred at room temperature for 35 min. It was then washed with deionized water until neutral and dried in a vacuum drying oven at a vacuum degree of 700 Pa and a temperature of 60°C for 12 h. Then, it was heated to 420°C in a vacuum drying oven under a mixed gas of H2 and Ar in a volume ratio of 8:100 at a heating rate of 5°C / min and held at this temperature for 1.1 h, thus obtaining the Fe3O4@NP-HC composite.

[0170] According to the method described in this application, the chemical elemental composition of the Fe3O4@NP-HC complex is as follows (by weight): C: 62.5%, H: 2.5%, O: 25.2%, N: 4.2%, P: 2.6%, and Fe: 17.0%; its XPS functional group composition is: C1 S : 71.3%, O1s: 21.6%, N1s: 4.9%, P2p: 1.9%, Fe 2p: 10.3%; its phase composition is as follows: the XRD pattern shows characteristic diffraction peaks of Fe3O4 at 2θ=30.2°, 35.5°, 43.2°, 57.0° and 62.6°, indicating the Fe3O4 crystalline phase; the broadened diffraction peak at 2θ=25° belongs to the (002) plane of amorphous hydrothermal carbon.

[0171] The agricultural and forestry waste was subjected to hydrothermal carbonization in a hydrothermal synthesis reactor at a weight ratio of agricultural and forestry waste to deionized water of 1:10, a weight ratio of agricultural and forestry waste to Fe3O4@NP-HC complex of 9:1, and a temperature of 210℃ with stirring for 3.0 h. After filtration and separation, a solid containing hydrothermal carbon and the complex and a solid containing NH4+ were obtained. + 4 and PO 3- The liquid of 4; the solid was dispersed in water at a weight ratio of 1:5 with water, and the Fe3O4@NP-HC complex was adsorbed using a 0.4T permanent magnet to obtain nitrogen-rich hydrothermal carbon solid;

[0172] B. Preparation of struvite crystals using a feedback pH control system

[0173] The NH4+ obtained from step B + 4 and PO 3- The liquid of 4 is pumped into the crystallization tank. A feedback pH control system, consisting of an industrial online process pH electrode, an industrial online process conductivity meter, a four-channel solution peristaltic pump with precise flow control, an integrated online dilution and addition device, an embedded micro PLC controller, and a microprocessor-based intelligent judgment module threshold trigger, is used to carry out the reaction under the conditions of liquid phase temperature of 80°C, stirring speed of 260 rpm, and pH 9.2. After the reaction is terminated, stirring is stopped, and the reaction mixture is cooled to room temperature. Solid-liquid separation is performed, and the solid is struvite crystals. The solid is first washed with deionized water and then washed with anhydrous ethanol. The washed struvite crystals are dried in an oven at 50°C for 12 hours to obtain the struvite crystals.

[0174] C. Preparation of compound fertilizer

[0175] The hydrothermal char solid obtained in step A and the struvite crystals obtained in step B are ground into 60-100 mesh powders respectively. The struvite powder and hydrothermal char powder are poured into a V-type mixer of square cone type at a weight ratio of 7:3.0 and mixed for 13 minutes at a speed of 20 rpm. Then, a dilute solution of sodium alginate binder with a concentration of 1.0% by weight and deionized water are sprayed to make the hydrothermal char powder adhere to the surface of the struvite crystals to form a coating film. The wet granules are then transferred to a disc granulator for granulation and then sent to an extruder for extrusion granulation through a sieve with a pore size of 1.0-2.0 mm. The resulting wet granules are dried in a forced-air drying oven at a temperature of 58°C for 12 hours to obtain the compound fertilizer.

[0176] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this embodiment, as tested according to the method described in this application, are listed in Table 1 below.

[0177] According to the method described in this application, the hydrothermal carbon-struvite compound fertilizer produced by the method of this embodiment is composed of 66% struvite crystals and 34% nitrogen-rich hydrothermal carbon by mass; the particle size of the hydrothermal carbon-struvite compound fertilizer particles is 1.0~2.0mm.

[0178] Comparative Example 1: Preparation of hydrothermal charcoal and struvite without composites

[0179] The implementation method of this comparative embodiment is the same as the implementation steps AC of Example 1, except that Fe3O4@NP-HC complex is not added in implementation step A.

[0180] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this comparative example, as determined by the method described in this application, are listed in Table 1 below.

[0181] Comparative Example 2: Preparation of struvite crystals without using a feedback pH control system

[0182] The implementation method of this comparative embodiment is the same as that of embodiment 3, except that a feedback pH control system is not used in implementation step B.

[0183] The hydrothermal carbon yield, hydrothermal carbon content, struvite quality, and struvite purity results of this comparative example, as determined by the method described in this application, are listed in Table 1 below.

[0184] Table 1: Results of the Implementation of the Hydrothermal Carbon-Struvite Compound Fertilizer Production Method of the Present Invention

[0185]

[0186] The implementation results listed in Table 1 clearly demonstrate that the hydrothermal carbon-struvite compound fertilizer production method of this invention has significant technical advantages and reproducibility. First, this method successfully achieves efficient and high-value conversion of agricultural and forestry waste. Under the action of the Fe3O4@NP-HC complex, the hydrothermal carbonization yield of different raw materials (straw, sawdust, and livestock manure) consistently reaches 78.6%–85.2%, and the nitrogen content of the obtained hydrothermal carbon (2.9%–3.8%) is much higher than that of the control without the complex (1.0%), proving that the complex can effectively promote the directional conversion of biomass into nutrient-rich solid products. Second, the method of this invention has high nitrogen and phosphorus nutrient recovery efficiency and high product purity. After the nitrogen and phosphorus elements in the liquid phase are crystallized by a feedback pH control system, the obtained struvite has a purity of up to 97.8%–98.5% and stable quality, thus confirming that this intelligent crystallization system can overcome the shortcomings of traditional methods in terms of extensive control and achieve stable production of high-quality struvite. In summary, this invention, through the coupling of "catalytic hydrothermal carbonization" and "intelligent crystallization recovery," not only transforms waste biomass into two high-value products—nitrogen-rich hydrothermal carbon and ultra-high-purity struvite—but also demonstrates stable and excellent performance across a wide range of operating parameters, providing a reliable and efficient solution for the full-component resource utilization of agricultural and forestry waste.

[0187] Experimental Example 1: Fertilizer Efficacy Test of the Hydrothermal Carbon-Struvite Compound Fertilizer of the Present Invention

[0188] The purpose of this experimental example is to demonstrate the effect of the hydrothermal carbon-struvite compound fertilizer prepared by the method of this invention on improving the yield, quality and nutrient utilization rate of Chinese cabbage under the same nutrient input, compared with conventional chemical fertilizers and single-component fertilizers.

[0189] Test soil: Acidic soil was collected from typical farmland in Guangyuan, Sichuan Province, and was air-dried and sieved for later use. The pH of this soil was 5.2-5.8, and the organic matter content was 0.8-1.5% by weight, which was relatively low.

[0190] The experimental crop was a fast-growing bok choy sold by Qingxian Xingyun Seed Industry Co., Ltd. under the brand name Huayu. It has a short growth period and is sensitive to fertilizer, making it a commonly used material for fertilizer efficiency testing.

[0191] Test method:

[0192] Experimental setup: Control group CK; Conventional compound fertilizer group CF; Pure struvite group MAP; Nitrogen-enriched hydrothermal char group HC; Struvite crystal and nitrogen-enriched hydrothermal char compound fertilizer group HCS.

[0193] All treatments were designed with phosphorus isophosphates (P2O5) as the baseline. Each treatment was configured with 6 replicates in a randomized block design.

[0194] Cultivation method: Potted experiments were conducted, with each pot containing 500g of soil. The experimental fertilizer was applied as a base fertilizer in one application and mixed thoroughly with the soil.

[0195] At the temperature at which Chinese cabbage grows, perform routine water management to maintain the soil's field water holding capacity at 60%–70%.

[0196] Table 2: Processing Group Numbers and Their Definitions

[0197]

[0198] Experimental results: The fresh weight of the above-ground parts of Chinese cabbage after 25 days of growth was determined by weighing, and the vitamin C content was determined by 2,6-dichlorophenolindophenol titration. The results are listed in Table 3.

[0199] Table 3: Results of Fertilizer Efficiency Test

[0200]

[0201] The experimental results listed in Table 3 clearly show that the aboveground fresh weight of the struvite crystal and nitrogen-enriched hydrothermal carbon compound fertilizer group HCS was 15.7±0.8g, which was significantly higher than that of the control group CK (1.2g±0.2g), the pure struvite MAP group (10.8g±0.9g), and the nitrogen-enriched hydrothermal carbon HC group (7.4±0.6g), proving that the hydrothermal carbon-struvite compound fertilizer of the present invention has a significant synergistic effect.

[0202] Experimental Example 2: Pot Cultivation Experiment

[0203] This pot culture experiment was used to comprehensively evaluate the immediate and ongoing changes in soil physicochemical properties after fertilization;

[0204] This experimental example systematically studies the comprehensive improvement effect of the compound fertilizer of the present invention on pH value, nutrient storage capacity (CEC), organic matter, available nutrient content and aggregate structure of acidic soil through a 60-day pot cultivation experiment.

[0205] The implementation method of this experimental example is as follows:

[0206] The experimental soil was collected from acidic soil in a typical farmland in Guangyuan, Sichuan Province. According to national standard methods including NY / T 1377-2007 "Determination of Soil pH", NY / T 1121.6-2006 "Soil Testing Part 6: Determination of Soil Organic Matter", NY / T 1121.6-2006 "Soil Testing Part 6: Determination of Soil Organic Matter", NY / T 1121.7-2014 "Soil Testing Part 7: Determination of Soil Available Phosphorus", LY / T 1243-1999 "Determination of Cation Exchange Capacity of Forest Soils", and NY / T 1121.19-2008 "Soil Testing Part 19: Determination of Soil Water-Stable Macroaggregate Composition", the experimental soil had the following basic parameters: pH 5.4, 8.1 g / kg organic matter, 62.3 mg / kg alkaline nitrogen, 8.7 mg / kg available phosphorus, and 8.5 cmol(+) / kg CEC.

[0207] Experimental setup: Control group CK; Conventional compound fertilizer (15-15-15) group CF; Pure struvite group MAP; Nitrogen-enriched hydrothermal char group HC; Struvite crystal and nitrogen-enriched hydrothermal char compound fertilizer group HCS.

[0208] Experimental method: Plastic pots with an upper diameter of 20cm and a height of 15cm were used, and each pot contained 2.5kg of air-dried soil with a 2mm sieve. All treatments were applied to the soil at a phosphorus content of 0.15g P2O5 / kg soil, mixed evenly, and each treatment was repeated 4 times.

[0209] The soil moisture content was adjusted to 60% of the field capacity, and then the soil was placed in a constant temperature incubator and cultured at a temperature of 25±1℃.

[0210] Soil pH was measured on days 0, 30, and 60 of cultivation using the potentiometric method specified in NY / T 1377-2007; soil cation exchange capacity (CEC) was measured on days 0 and 60 of cultivation using the ammonium acetate exchange method specified in LY / T 1243-1999; soil available nitrogen and available phosphorus were measured on days 0, 30, and 60 of cultivation using the alkaline hydrolysis-diffusion method and sodium bicarbonate extraction-molybdenum antimony colorimetric method specified in NY / T 1121.6-2006; soil organic matter content was measured on day 60 of cultivation using the potassium dichromate external heating method specified in NY / T 1121.6-2006; and soil water-stable aggregates were measured on day 60 of cultivation using the wet sieving method specified in NY / T 1121.19-2008.

[0211] Samples were taken and tested after 60 days of soil culture, and the results are listed in Table 4.

[0212] Table 4: Results of soil sampling and analysis after 60 days of incubation

[0213]

[0214] The results listed in Table 4 indicate that HCS treatment was most effective in increasing soil pH and CEC. Struvite hydrolysis produces OH-. - Neutralizing acidity and hydrothermal carbon increasing soil fertility retention capacity, the two work synergistically to significantly improve the chemical properties of acidic soils. The HCS treatment showed the best performance in terms of organic matter and aggregate indicators. In all key indicators, HCS was superior to the single components (MAP or HC), demonstrating the synergistic improvement effect of the combined treatment, where "1+1>2".

[0215] Experimental Example 3: Soil Column Leaching Test

[0216] This experimental example uses a simulated natural precipitation process to quantitatively study the ability of the compound fertilizer of this invention to retain nutrients (especially nitrogen and phosphorus) in the soil, and to evaluate its effect on reducing nutrient leaching and lowering the risk of environmental pollution.

[0217] The soil and fertilizer used in the experiment were the same as those used in Experiment Example 2.

[0218] Experimental apparatus: An acrylic glass soil column with an inner diameter of 7 cm and a height of 30 cm, standing upright on a support. A filter screen and a 2 cm thick layer of quartz sand are placed at the bottom of the acrylic glass soil column, and then 20 cm of soil is filled on top, followed by another 2 cm layer of quartz sand. An outlet is set at the center of the bottom of the soil column, and a beaker for collecting the leaching solution is placed directly below the outlet.

[0219] Experimental method: The test fertilizers (CK, CF, MAP, HCS) were mixed evenly with air-dried soil and evenly packed into soil columns according to the field bulk density (1.25 g / cm³). The soil height in each soil column was 20 cm. The phosphorus application rate in each soil column was the same as in Experiment Example 2. Each treatment was replicated three times.

[0220] The leaching procedure and sample collection steps are as follows:

[0221] A. After the experimental fertilizer was loaded into the column, deionized water was slowly added to the top of the soil column until the soil was completely moistened and a small amount of water flowed out, so that the soil moisture reached the field capacity and was equilibrated at room temperature for 24 hours.

[0222] B. Leaching: Add 200 mL of deionized water from the top of the soil column at a flow rate of 5 mL / min using a metering pump; collect the leaching solution from a beaker located at the center outlet at the bottom of the soil column;

[0223] C. Repeat step B until the cumulative leaching volume reaches 1000 mL.

[0224] D. After leaching is completed, the nitrogen and phosphorus contents of the leaching solution are determined according to the detection method described above. The available nitrogen and phosphorus contents of the soil are then calculated, and the longitudinal migration of these nutrients in the soil is analyzed.

[0225] The results of the soil column leaching test are listed in Table 5 below.

[0226] Table 5: Results of Soil Column Leaching Test

[0227]

[0228] The experimental results listed in Table 5 demonstrate that the soil column leaching test proves that HCS significantly reduces leaching loss and improves fertilizer utilization through a dual mechanism of "chemical slow release" and "physical adsorption".

[0229] In summary, the two soil improvement experiments, Example 1 and Example 2, jointly demonstrated the multiple advantages of the hydrothermal carbon-struvite compound fertilizer (HCS) of this invention from different perspectives. Pot experiments showed that HCS can simultaneously and synergistically improve the chemical (pH, CEC) and physical (aggregate) properties of acidic soils, thereby enhancing soil fertility quality.

[0230] As can be seen, this invention achieves a closed-loop cycle of catalysis, fertilization, and recycling through the functional compound Fe3O4@NP-HC, with magnetic Fe3O4 as the core and nitrogen-phosphorus-doped hydrothermal carbon as the carrier. The resulting compound fertilizer is a novel two-in-one product combining slow-release fertilizer and soil conditioner, achieving a synergistic effect of 1+1>2 and greatly enhancing the product's added value. This invention provides and verifies this integrated treatment of "waste treatment - resource recycling - product high-value utilization," maximizing resource utilization.

Claims

1. A method for preparing Fe3O4@NP-HC multifunctional complexes, characterized in that... The method includes the following steps: (1) Raw material pretreatment: After crushing the lignocellulosic agricultural and forestry waste, it was immersed in hydrochloric acid solution and heated to react. After solid-liquid separation, it was dried to obtain pretreated solid; (2) Hydrothermal load: The pretreated solid is mixed with Fe 2+ and Fe 3+ The mixed aqueous solution was placed in a hydrothermal reactor and reacted at 180~240℃ for 2~6 hours under an inert atmosphere. After the reaction, the solid and liquid were separated and dried to obtain porous hydrothermal carbon. (3) Co-precipitation to form magnetic centers: Ammonia water is added to the porous hydrothermal carbon until the system is alkaline. After the reaction, the mixture is separated by magnetic separation and dried to obtain Fe3O4@HC intermediate. (4) Nitrogen and phosphorus co-doping modification: Ammonium dihydrogen phosphate and urea are dissolved in a solvent at a mass ratio of 1.5~2.5:1 to prepare a modification solution; the Fe3O4@HC intermediate is immersed in the modification solution, and after dispersion treatment, solid-liquid separation is performed. (5) Stepwise pyrolysis activation: The solid obtained in step (4) is heated to 500-600℃ at a heating rate of 3-8℃ / min under an inert atmosphere and calcined for 2.5-3.5 hours; after calcination, the obtained solid is immersed in dilute sulfuric acid solution and stirred, then washed and dried; finally, it is treated at 350-450℃ for 0.8-1.2 hours in a mixed atmosphere of H2 and Ar to obtain the Fe3O4@NP-HC multifunctional composite.

2. A Fe3O4@NP-HC multifunctional complex, characterized in that... The composite uses porous hydrothermal carbon as a carrier, on which Fe3O4 nanoparticles are loaded, and the framework of the hydrothermal carbon is simultaneously doped with nitrogen and phosphorus atoms; wherein, the composite simultaneously contains the following characteristics (a) and (b): (a) Its composition by elemental weight percentage is: C: 56.8%~68.5%, N: 3.6%~4.9%, P: 2.2%~3.1%, Fe: 13.8%~19.5%, O: 21.7%~28.5%, H: 1.9%~2.8%; (b) In its Raman spectrum, it is located at approximately 1355 cm⁻¹ -1 The D peak is located at approximately 1590 cm. -1 The intensity ratio (I_D / I_G) of the G peak is 1.10~1.

15.

3. The Fe3O4@NP-HC multifunctional complex according to claim 2, characterized in that... The X-ray photoelectron spectroscopy (XPS) of the composite showed that there were three forms of nitrogen: pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen, and the relative content ratio of the three was (35-40)%:(40-45)%:(20-25)%.

4. The Fe3O4@NP-HC multifunctional complex according to claim 2, characterized in that... The composite has a specific surface area of ​​200~350 m² / g and a pore volume of 0.15~0.30 cm³ / g.

5. The Fe3O4@NP-HC multifunctional complex according to any one of claims 2 to 4, characterized in that... The saturation magnetization of the composite is 25~50 emu / g.

6. The application of the Fe3O4@NP-HC multifunctional complex according to any one of claims 2 to 5 in the biomass resource utilization process.

7. A method for producing hydrothermal carbon-struckstone compound fertilizer using lignocellulosic agricultural and forestry waste, characterized in that... The method includes the following steps: A. Catalytic hydrothermal carbonization: Agricultural and forestry waste, water and the Fe3O4@NP-HC multifunctional complex are mixed and subjected to hydrothermal reaction. After the reaction is completed, solid-liquid separation is performed to obtain solid product and liquid product. B. Intelligent crystallization and recovery: Under the condition of real-time monitoring of pH value and conductivity, the liquid product obtained in step A is added to the reaction system with alkali solution, magnesium source and seed crystals to carry out struvite crystallization reaction. After the reaction, the solid and liquid are separated to obtain struvite crystals. C. Product compounding: The solid product obtained in step A is subjected to magnetic separation to recover the Fe3O4@NP-HC multifunctional composite to obtain nitrogen-rich hydrothermal carbon; the nitrogen-rich hydrothermal carbon is mixed with the struvite crystals obtained in step B and granulated to obtain the hydrothermal carbon-struvite compound fertilizer.

8. The method according to claim 7, characterized in that In step A, the mass ratio of the agricultural and forestry waste, water and the Fe3O4@NP-HC multifunctional complex is 1:(8-12):(0.08-0.15).

9. The method according to claim 7, characterized in that... Step B was carried out in a feedback-controlled pH crystallization system, when the rate of decrease in the conductivity of the reaction system exceeded 100 μS·cm in real time. -1 ·min -1 At that time, a control command is triggered to add guano crystal seeds.

10. The method according to claim 7, characterized in that... Step C includes: crushing the nitrogen-rich hydrothermal char and struvite crystals separately, mixing them, spraying a binder solution to granulate them, and drying the resulting granules to obtain the hydrothermal char-struvite compound fertilizer.

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