Slow-release saline-alkali soil conditioner based on phosphogypsum and preparation method of slow-release saline-alkali soil conditioner

Abstract

The invention belongs to the technical field of environmental protection and agricultural resource utilization, and particularly relates to a slow-release saline-alkali soil conditioner based on ardealite and a preparation method of the slow-release saline-alkali soil conditioner. Comprising the following steps: carrying out water washing, neutralization and physical modification pretreatment on phosphogypsum, removing impurities, effectively reducing the content of harmful impurities such as fluorine and phosphorus, and activating the reaction activity of the phosphogypsum; mixing the pretreated phosphogypsum with an organic material and a microbial agent, and carrying out aerobic composting fermentation; and after granulation, a layer of sustained-release film made of biodegradable polymer is coated on the surface of the granules. The product is of a core-shell structure, the core provides a calcium source, organic matters and beneficial bacteria, the shell slow-release film can control the release rate of calcium ions and other elements, the problem that the short-term salinity concentration is too high due to instantaneous dissolution of a traditional modifier is avoided, soil salt return can be effectively prevented and controlled, the lasting, stable and safe saline-alkali soil improvement effect is provided, and the saline-alkali soil improvement effect is good. Meanwhile, high-value resource utilization of the phosphogypsum is realized.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection and agricultural resource utilization technology, specifically relating to a slow-release saline-alkali soil conditioner based on phosphogypsum and its preparation method. Background Technology

[0002] Saline-alkali soils are one of the major obstacles to agricultural production worldwide. The core issues are high pH levels and excessive soluble salts, especially sodium (Na₂O₃). + This can lead to soil compaction, poor permeability, and reduced nutrient availability, which is detrimental to crop growth. Traditional soil amendment methods, such as irrigation to leach salts and the application of chemical amendments, while effective to some extent, suffer from high costs, slow results, and potential secondary pollution. Furthermore, existing technologies still face a potential challenge: after the amendments are applied to the soil, their effective components, especially calcium... 2+ If the release rate of soil amendments is too rapid, in areas with poor drainage or high evaporation, the large amount of Na2SO4 and other salts dissolved in a short period of time may not be leached out in time. This can temporarily exacerbate the salt concentration in the soil solution, causing salt stress damage to crop seedlings, or the salts may accumulate again in the topsoil as water evaporates, leading to a "salinization" phenomenon. Therefore, how to control the release rate of the active ingredients in soil amendments to match them with the patterns of soil moisture movement and crop nutrient requirements in order to achieve a stable, efficient, and long-lasting amendment effect is a technical problem that urgently needs to be solved.

[0003] Phosphogypsum is a large-scale industrial solid waste generated during the wet-process phosphoric acid production process, producing approximately 4.5 to 5.5 tons of phosphogypsum for every ton of phosphoric acid produced. Its main component is calcium sulfate dihydrate (CaSO4·2H2O), but it typically contains incompletely decomposed phosphate rock, free phosphoric acid, fluorides, heavy metals, and other harmful impurities. The stockpiling of large quantities of phosphogypsum not only occupies land, but its soluble impurities also leach out with rainwater, posing a serious risk to soil, groundwater, and the surrounding ecosystem. Because phosphogypsum is rich in calcium... 2+ Currently, there are studies on using phosphogypsum to improve saline-alkali soils. The Ca content in phosphogypsum... 2+ Replace Na on soil colloids through ion exchange. + The process involves the formation of water-soluble Na2SO4, which is then leached out with water, thereby reducing soil alkalinity and pH. However, direct application of untreated phosphogypsum poses significant risks: firstly, harmful impurities such as fluorine, phosphorus, and heavy metals can directly enter the soil and cause pollution; secondly, since phosphogypsum is typically acidic, it may trigger soil acidification; furthermore, the soil-improving effect of phosphogypsum is relatively limited, as it only provides a calcium source and cannot comprehensively improve saline-alkali soils.

[0004] Therefore, it is necessary to further transform and utilize industrial solid waste phosphogypsum to better improve saline-alkali soil and prevent secondary soil salinization, thereby increasing the environmental and economic value of phosphogypsum and saline-alkali soil. Summary of the Invention

[0005] One of the objectives of this invention is to overcome the shortcomings of the prior art and provide a soil conditioner for improving saline-alkali soil and preventing secondary soil salinization, while also addressing the environmental problems associated with phosphogypsum.

[0006] To achieve the above objectives, the present invention provides a method for preparing a slow-release saline-alkali soil conditioner based on phosphogypsum, comprising the following steps: S1. Pretreatment of phosphogypsum: Take the raw phosphogypsum and pretreat it using one or more of the following methods in combination, depending on the impurity content: Washing: Mix with water at a solid-liquid ratio of 1:(2-5), stir for 10-60 minutes, allow to settle, and then separate the upper suspension and lower precipitate. Washing can be repeated 1-3 times. The washing liquid can be recycled for preliminary pulp conditioning.

[0007] Neutralization: The pH value of phosphogypsum is adjusted to 5.0-7.0 using a neutralizing agent to fix soluble elements such as fluorine and phosphorus. The neutralizing agent includes at least one of quicklime, limestone powder, carbide slag, and red mud.

[0008] Composite modification: Under stirring conditions, treatment is combined with ultrasound or microwave. The ultrasound power is 300–800 W, and the treatment time is 2–30 min. The microwave power is 300–1000 W, and the treatment time is 2–20 min. The cavitation or thermal effects of ultrasound / microwave effectively break up particles, release and remove encapsulated impurities, change crystal morphology, increase specific surface area, and thus improve its reactivity.

[0009] The content of harmful substances in the pretreated phosphogypsum must meet or exceed the requirements for phosphogypsum used for soil amendment in GB / T 32124-2024 Standard for Treatment and Disposal of Phosphogypsum, especially water-soluble fluorine (calculated as F⁻, dry basis) ≤0.30% and water-soluble phosphorus pentoxide (P2O5, dry basis) ≤0.50%.

[0010] S2, Mixed Ingredients: The pretreated phosphogypsum is mixed with organic materials and microbial agents in the following mass percentages: pretreated phosphogypsum: 50% ~ 70%; organic materials: 25% ~ 40%; microbial agents: 2% ~ 5%; binder: 3% ~ 8%.

[0011] The organic material is one or more of the following: straw powder, livestock and poultry manure, aerobic fermentation products of kitchen waste, garden waste compost, distiller's grains, mushroom residue, biochar, and furfural residue. The organic material must be crushed to a particle size ≤2cm.

[0012] The microbial agent is a compound microbial agent with an effective viable count ≥ 200 million CFU / g. Preferred strains include, but are not limited to: Bacillus subtilis, Bacillus megaterium, Arbuscular mycobacterium, and nitrogen-fixing bacteria.

[0013] The binder is one or more of bentonite, sodium alginate, and lignin sulfonate, and is used for subsequent granulation.

[0014] S3, Aerobic composting fermentation: The uniformly mixed materials are piled into windrows or placed in fermentation tanks for aerobic fermentation. The pile parameters are controlled as follows: Composting cycle: 15-30 days; Pile temperature: During the first 1-5 days of fermentation, the pile temperature rapidly rises to 55-65℃ and is maintained for at least 3 days to kill pathogens and weed seeds and achieve harmlessness. In the later stage, the temperature gradually decreases to about 40℃; Moisture control: During fermentation, the moisture content of the material is maintained at 50%-60% by turning the pile or ventilating; Turning operation: When the temperature at the center of the pile exceeds 65℃ or every 2-3 days, turn the pile to ensure uniform oxygen supply.

[0015] During this process, microbial agents in the heap proliferate rapidly, decomposing organic materials to produce humic acid, amino acids, and other active substances. These substances synergistically interact with phosphogypsum to further passivate residual impurities and remove Ca... 2+ It combines with organic matter to form complexed calcium that is more readily utilized by the soil, while simultaneously generating a large number of beneficial metabolites. Specifically, the carboxyl and hydroxyl functional groups of humic acid combine with heavy metal ions or fluoride ions through complexation reactions to form more stable complexes, thereby reducing their bioavailability and mobility, and achieving impurity passivation. Meanwhile, the Ca in phosphogypsum... 2 ⁺ It gradually dissolves in an acidic composting environment and combines with organic acids such as humic acid and amino acids to form complex calcium, which is more easily absorbed by plants.

[0016] S4. Aging and Granulation: After composting, the material is aged for 5 to 10 days to stabilize its properties. Then, a disc granulator or extrusion granulator is used to granulate the aged material into pellets with a particle size of 2 to 5 mm, which are then dried, cooled, and screened.

[0017] S5. Encapsulated sustained-release membrane: The core particles obtained in step S4 are coated using a fluidized bed coating equipment or a roller coating machine to coat their surface with a slow-release membrane.

[0018] Sustained-release membrane material: The sustained-release membrane is composed of film-forming material, pore-forming agent and plasticizer.

[0019] Film-forming material: selected from one or more of environmentally friendly hydrophobic polymers, such as polylactic acid (PLA), polycaprolactone (PCL), ethyl cellulose (EC), or biodegradable starch-based polymers.

[0020] Pore-forming agents: selected from water-soluble substances, such as polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), NaCl, sucrose, etc., whose function is to form microporous channels in the membrane and control the release rate.

[0021] Plasticizers, such as glycerin and triethyl citrate, are used to increase the flexibility of film-forming materials.

[0022] Coating solution composition: Dissolve the above film-forming material in an appropriate amount of organic solvent (such as dichloromethane, acetone) or water (for aqueous dispersions), add pore-forming agent and plasticizer, and stir until homogeneous to form a coating solution. The mass concentration of the film-forming material in the coating solution is 2% to 10%.

[0023] Coating process: The coating solution is evenly sprayed onto the surface of the fluidized core particles using a spray method. Low-temperature hot air drying allows the solvent to evaporate, forming a dense microporous membrane. The drying temperature is controlled at 40-60℃ to avoid damaging microbial activity. The coating amount of the sustained-release membrane is 1% to 5% of the dry weight of the core particles.

[0024] The coated granules are cooled and sieved to remove any adhering particles, thus obtaining the final product.

[0025] This invention also provides a slow-release saline-alkali soil conditioner prepared by the above method. The conditioner has a core-shell structure: the core contains abundant calcium sulfate, organic matter (≥20%), humic acid (≥5%), functional microbial flora, and nutrients such as N, P, and K. The outer shell is a biodegradable microporous slow-release membrane.

[0026] The present invention also provides a method for using a slow-release saline-alkali soil conditioner based on phosphogypsum, comprising: applying the conditioner to saline-alkali soil or saline soil, spreading it evenly on the topsoil layer in one go or in several times, and tilling it to a depth of at least 20 cm. The amount used depends on the degree of soil salinity and alkali, and the application rate is 5t to 20t / hectare.

[0027] The technical solution of the present invention has the following beneficial effects: Low-cost treatment of solid waste pollution: Successfully transforming bulk solid waste phosphogypsum and various organic wastes into high-value-added products, realizing resource recycling, and reducing the cost of amendments and environmental pressure.

[0028] The product is safe and efficient: Through a composite pretreatment process of "water washing + neutralization + physical modification", harmful impurities such as fluorine and phosphorus are deeply removed, effectively reducing the risk of secondary pollution.

[0029] Controllable release rate and long-lasting effect: The sustained-release membrane effectively controls the Ca content in the core through a microporous diffusion mechanism. 2+ SO4 2- And the rate of nutrient dissolution. This makes the ion exchange process gradual, avoiding the rapid release of large amounts of Na+ in a short period of time. + The risk of a sharp increase in soil solution salinity due to leaching can be effectively suppressed by inhibiting the "salt return" phenomenon in the surface soil caused by strong evaporation.

[0030] Protective microorganisms: The encapsulation process takes place at low temperatures, and the physical barrier formed provides a relatively stable microenvironment for the internal microbial community, protecting it from harsh external conditions such as drastic pH changes and high-salt environments, which is conducive to its colonization and reproduction. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of the preparation method of the slow-release saline-alkali soil conditioner based on phosphogypsum according to the present invention.

[0032] Figure 2 This is a schematic diagram of the product structure of the slow-release saline-alkali soil conditioner based on phosphogypsum of the present invention. Detailed Implementation

[0033] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited thereto.

[0034] Example 1: like Figure 1 and Figure 2 As shown, this embodiment provides a method for preparing a slow-release saline-alkali soil conditioner based on phosphogypsum, including the following steps: Pretreatment: Take 100 kg of gypsum, add 300 kg of water, stir and wash for 20 minutes, let stand, and remove the supernatant. Add carbide slag to the washed phosphogypsum and adjust the pH to 6.5. Then treat with ultrasound at 500 W for 20 minutes. Tests showed that the water-soluble fluoride content decreased to 35 mg / kg, and the water-soluble P2O5 decreased to 28 mg / kg.

[0035] Ingredients: Take 60 kg of the above pretreated phosphogypsum, 25 kg of corn stalk powder (particle size <1 cm), 10 kg of well-rotted cow manure, 3 kg of compound microbial agent (containing Bacillus subtilis and Arbuscular mycobacterium, with a live count of 500 million CFU / g), and 2 kg of bentonite, and mix them evenly.

[0036] Composting: Pile the mixture together, controlling the initial moisture content to 55%. During fermentation, the temperature rises to 62℃ on the second day, remains at that level for four days, and then begins to decrease. Turn the pile every two days, with a total fermentation cycle of 20 days.

[0037] Granulation: After the compost has been aged for 7 days, it is granulated using a disc granulator (particle size 3-4mm) and dried at a low temperature of 80℃ until the moisture content is <15%. After cooling, it is sieved to obtain about 90kg of finished product amendment.

[0038] Coating with a sustained-release membrane: Take 5 kg of the granulated core particles and place them in a laboratory fluidized bed coating machine. Dissolve 25 g of polylactic acid (PLA) in 500 g of dichloromethane, add 5 g of polyvinylpyrrolidone (PVP K30) as a pore-forming agent and 2.5 g of triethyl citrate as a plasticizer, and stir until completely dissolved. Under conditions of an inlet temperature of 50℃ and a suitable fluidizing air volume, uniformly spray the coating solution onto the particle surface. After the solvent has completely evaporated, sustained-release particles with a smooth and dense microporous membrane coated on the surface are obtained, with a weight gain of approximately 3%.

[0039] Finished product: The coated granules are cooled to room temperature and then sieved to obtain the final product.

[0040] Application effect test: A pot experiment was conducted on saline-alkali land in northern Shaanxi. The initial pH of the saline-alkali land was 8.9, and the salt content was 0.4%. Three groups were set up: control group (no amendment applied), ordinary granule group (applied with uncoated amendment), and slow-release group (applied with the coated amendment of this invention). Each group was applied with an equal amount of amendment, which was 1% of the soil weight. Ten plump and uniformly sized Napier grass seeds were sown in each pot.

[0041] The results showed that on the 7th day after application, the surface electrical conductivity (EC value) of the soil in the ordinary granule group showed a brief peak, increasing by 15%, and then decreased; while the EC value of the soil in the slow-release group decreased steadily without any salt peak. After 60 days, the pH and ESP values ​​of both groups improved significantly, but the survival rate of Napier grass seedlings in the slow-release group was 20% higher than that in the ordinary granule group, the biomass was 15% higher, and the soil moisture evaporation test showed that its salt suppression effect was more lasting.

[0042] Example 2: Pretreatment: Take 100 kg of phosphogypsum, mix it with water at a solid-liquid ratio of 1:4, stir for 30 minutes, allow it to settle, separate the supernatant, and repeat the washing twice. Add limestone powder to the washed phosphogypsum to adjust the pH to 6.0. Then treat it under microwave conditions at 600W for 15 minutes. Tests showed that the water-soluble fluoride content decreased to 30 mg / kg and the water-soluble P2O5 decreased to 25 mg / kg, meeting the requirements of GB / T 32124-2024 standard.

[0043] Ingredients: Take 50 kg of the above pretreated phosphogypsum, 20 kg of rice straw powder (particle size < 1.5 cm), 8 kg of decomposed chicken manure, 2.5 kg of compound microbial agent (containing Bacillus megaterium and nitrogen-fixing bacteria, with a live count ≥ 300 million CFU / g), and 1.5 kg of sodium alginate, and mix them evenly.

[0044] Composting: Build a pile with the mixture, controlling the initial moisture content to 58%. During fermentation, the pile temperature rises to 60℃ on the 3rd day, remains at that temperature for 5 days, and then gradually decreases to 40℃. Turn the pile every 3 days, with a total fermentation cycle of 25 days.

[0045] Granulation: After the compost material has been aged for 8 days, granules with a particle size of 2–4 mm are prepared by extrusion granulation and dried at a low temperature of 75℃ until the moisture content is <12%. After cooling, the granules are sieved to obtain approximately 85 kg of finished product amendment.

[0046] Coating with a sustained-release membrane: Take 5 kg of the granulated core particles and place them in a fluidized bed coating machine. Dissolve 20 g of ethyl cellulose (EC) in 500 g of acetone, add 7.5 g of polyethylene glycol (PEG 4000) as a porogen and 2.5 g of glycerol as a plasticizer, and stir until completely dissolved. Under conditions of an inlet temperature of 45℃ and a suitable fluidizing air volume, uniformly spray the coating solution onto the particle surface. After the solvent has completely evaporated, sustained-release particles with a uniform microporous membrane coated on the surface are obtained, with a weight gain of approximately 2.5%.

[0047] Finished product: The coated granules are cooled to room temperature and then sieved to obtain the final product.

[0048] Application effect test: Pot experiments were conducted using alkaline soil from western Jilin Province, with an initial pH of 9.2 and a salt content of 0.6%. Three groups were set up: a control group (without amendment), a group treated with the amendment of this invention, and a group treated with gypsum. The amount of amendment applied to each group was 1% of the dry weight of the soil. After thorough mixing, alfalfa was planted.

[0049] The results showed that on the 10th day after application, the soil EC value in the gypsum group increased by 18%, while the soil EC value in the group treated with the amendment of this invention decreased steadily without any increase. After 90 days, compared with the control (CK), the soil pH in the group treated with the amendment of this invention decreased to 8.3, the ESP value decreased significantly by 24%, the alfalfa survival rate increased by 25%, the yield increased by 18%, and the soil salinization phenomenon was significantly reduced; the soil pH in the group treated with gypsum decreased to 8.9, the ESP value decreased by 18%, the alfalfa survival rate increased by 12%, the yield increased by 11%, but the soil EC value increased by 13%.

[0050] Other reagents with similar properties and the selection of similar reaction parameters can be determined by those skilled in the art based on common knowledge, and will not be elaborated further. The above descriptions are merely embodiments of the present invention; the specific structures, properties, and reactant ratios known in the schemes are not described in detail here. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, such as simply adjusting the parameter selection within or near a specified parameter range. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A slow-release saline-alkali soil conditioner based on phosphogypsum, characterized in that, The modifier has a core-shell structure, comprising a core and an outer shell. The core includes phosphogypsum, organic matter, humic acid, microorganisms, N, P, K, and calcium sulfate and complexed Ca generated by the reaction of humic acid, amino acids and phosphogypsum. The outer shell is a biodegradable microporous slow-release membrane.

2. The slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 1, characterized in that, The core contains ≥20% organic matter and ≥0.5 billion CFU / g of effective live bacteria.

3. A method for preparing a slow-release saline-alkali soil conditioner based on phosphogypsum as described in claim 1, characterized in that, Includes the following steps: The phosphogypsum was pretreated to achieve a pH value of 5.0-7.0, with water-soluble fluorine ≤0.30% and water-soluble phosphorus pentoxide ≤0.50%. The pretreated phosphogypsum, organic materials, microbial agents and binders are mixed in a set ratio; The mixed materials are subjected to aerobic composting fermentation, and the aerobic composting fermentation cycle is 15 to 30 days. The composted material is aged for 5 to 10 days and then granulated to obtain core particles. A slow-release film is coated on the surface of the core particles, and the modifier is obtained after low-temperature drying.

4. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, The pretreatment in step (a) includes one or more combinations of water washing, neutralization, ultrasonic treatment, and microwave treatment; the ultrasonic treatment has a power of 300-800W and a time of 2-30min; the microwave treatment has a power of 300-1000W and a time of 2-20min.

5. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, In step (b), the organic material is selected from one or more of straw, livestock and poultry manure, organic solid waste compost products, biochar, and furfural residue, and its particle size is less than 2 cm; the microbial agent is a compound microbial agent with an effective live bacteria count ≥ 200 million CFU / g; and the binder is bentonite, sodium alginate, or lignin sulfonate.

6. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, In step (b), the mixture comprises, by mass percentage: 50%–70% pretreated phosphogypsum, 25%–40% organic materials, 2%–5% microbial inoculant, and 3%–8% binder.

7. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, In step (c), ventilation and oxygen supply are provided by turning the pile. The turning frequency is once every 2 to 3 days or when the pile temperature exceeds 65°C.

8. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, The sustained-release membrane in step (e) is composed of a film-forming material, a pore-forming agent, and a plasticizer; the film-forming material is selected from one or more of polylactic acid, polycaprolactone, ethyl cellulose, and starch-based polymers; the pore-forming agent is selected from one or more of polyvinylpyrrolidone, polyethylene glycol, sodium chloride, and sucrose; and the plasticizer includes glycerol or triethyl citrate.

9. The method for preparing the slow-release saline-alkali soil conditioner based on phosphogypsum according to claim 3, characterized in that, In step (e), fluidized bed coating or roller coating is used for coating. The mass concentration of film-forming material in the coating solution is 2% to 10%, and the coating drying temperature is controlled at 40-60℃. The amount of the sustained-release membrane is 1% to 5% of the dry weight of the core particles.

10. A method for applying a slow-release saline-alkali soil conditioner based on phosphogypsum according to any one of claims 1-2, characterized in that: The soil conditioner is applied to saline-alkali soil or saline soil by spreading it evenly on the topsoil layer in one go or in several applications, and then tilling it to a depth of at least 20 cm. The application rate is 5 to 20 tons per hectare.

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