Preparation method of nutrient soil for gypsum crystallization induced by coal gangue and its application in saline-alkali land improvement

Abstract

This invention relates to a method for preparing nutrient soil based on coal gangue-induced gypsum crystallization and its application in saline-alkali land improvement. It belongs to the field of solid waste resource utilization technology and solves the technical problem of the lack of an integrated method for "coal gangue soil treatment, directional conversion of pyrite FeS2, and in-situ preparation of gypsum CaSO4·2H2O". The method includes the following steps: Step 1, crushing the coal gangue and activating the coal gangue powder; Step 2, mixing cow dung, straw, and FeS2 to obtain initial organic material, adjusting the carbon-nitrogen ratio, and performing aerobic fermentation on the initial organic material to obtain organic fermented material with peak sulfur-oxidizing bacteria abundance; Step 3, uniformly mixing the activated coal gangue powder and organic fermented material, and placing it in a fermentation tank for controlled maturation. The coal gangue-based nutrient soil produced by this method has excellent chemical properties, good porosity, and stable and abundant aggregates. Gypsum CaSO4·2H2O can be used as a core component for saline-alkali land improvement.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to the preparation method of nutrient soil for coal gangue-induced gypsum crystallization and its application in saline-alkali land improvement. Background Technology

[0002] In the field of coal gangue soil treatment, existing technologies mostly focus on preparing conventional soil amendment substrates, such as [Patent Publication No.: CN119775069A, Patent Title: A Soil Amendment Based on Coal Gangue and Its Application], [Patent Publication No.: CN119143553A, Patent Title: A Method for Preparing Solid Waste-Based Soil Conditioner Using Microbial Mineralization of Coal Gangue], and [Patent Publication No.: CN119242313A, Patent Title: An Earthworm Castor Biochar-Based Soil Amendment and Its Preparation Method and Application]. These technologies aim to increase the organic matter and nutrient content of coal gangue for general soil fertilization. However, they do not pay attention to the transformation of pyrite FeS2, making it difficult to avoid secondary environmental risks such as continuous soil and water acidification and heavy metal activation caused by FeS2 oxidation. Existing technologies, such as [Patent Publication No.: CN1876855A, Patent Title: A Method for Removing Sulfur from Pyrite in Coal Gangue Using Microorganisms], although promoting the removal of sulfur from pyrite FeS2, 1- To SO4 2- While the process has achieved some success, it has failed to fundamentally transform the pyrite into a stable and beneficial resource. Existing technologies, such as [Microbial-Driven Coal Gangue Sulfur Conversion and Soil Improvement Application, Modern Chemical Industry, 45(A2), 174-178], although promoting the conversion of pyrite FeS2 to SO4 through a combined process of fermentation maturation and bacterial activation, have not fundamentally transformed it into a stable and beneficial resource. 2- At the same time, coal gangue minerals release Ca 2+ This process generates CaSO4. However, it does not further transform CaSO4 into gypsum (CaSO4·2H2O), which has a greater effect on improving saline-alkali soils. CaSO4·2H2O, on the other hand, can dissolve Ca... 2+ Replace Na adsorbed by soil colloids in saline-alkali land + This reduces soil alkalinity and improves the physical structure of heavy clay soils. In particular, industrial by-product gypsum with gypsum as its main component, such as desulfurized gypsum and phosphogypsum, has been widely used for the improvement of saline-alkali land.

[0003] However, existing technologies lack a technical solution for the directional conversion of pyrite FeS2 in coal gangue to gypsum CaSO4·2H2O. Although the phase transition of CaSO4 under hydrothermal conditions has been reported, research on processes to promote the stable hydration of anhydrous or hemihydrate CaSO4 to gypsum CaSO4·2H2O under conventional fermentation conditions at low temperatures below 65℃ and a water content below 50% is particularly lacking in the field of coal gangue resource utilization. The regulation of temperature, humidity, and aeration in the coal gangue co-composting process with organic solid waste mainly serves the degradation of organic matter and microbial metabolism, rather than the precise control of the specific inorganic reaction of CaSO4 hydration and crystallization, as exemplified by the technology described in [Patent Publication No.: CN116477995A, Patent Title: A Method and Application for the Preparation of Humus by Anaerobic Composting of Coal Gangue and Organic Solid Waste].

[0004] There are currently two technical systems for using coal gangue to improve saline-alkali land. One system involves treating coal gangue to render it harmless or activate it before using it directly as a general soil conditioner, such as the technology described in patent publication number CN119219447A, entitled "A Method for Preparing Saline-Alkali Land Soil Conditioner by Activating Coal-Based Solid Waste with Microbial Agents." The other system involves mixing coal gangue with purchased gypsum products to prepare a saline-alkali land soil conditioner for soil improvement, such as the technologies described in patent publication numbers CN121249370A and CN120209854A, entitled "A Soil Conditioner for Saline-Alkali Land and Its Preparation Method and Application" and CN120209854A, entitled "A Solid Waste-Based Porous Material for Remediating Saline-Alkali Soil and Its Preparation Method and Application." The improvement effect of coal gangue in these technologies is not explicitly attributed to the in-situ generated gypsum, nor is the in-situ preparation of gypsum, the core component of saline-alkali land improvement, integrated into the design of the coal gangue soil treatment process. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing nutrient soil for coal gangue-induced gypsum crystallization and its application in improving saline-alkali land, thus solving the technical problems such as the lack of an integrated method for coal gangue soil treatment and in-situ gypsum preparation.

[0006] To solve the above problems, the technical solution of the present invention is: a method for preparing nutrient soil for gypsum crystallization induced by coal gangue, comprising the following steps: Step 1, Specific Activation: Crush coal gangue with a total sulfur content ≥3% to 8~50 mesh, and activate the coal gangue powder with Bacillus subtilis solution; Step 2, Directional Fermentation: Mix cow dung, straw and FeS2 to obtain initial organic material, adjust the carbon-nitrogen ratio of the initial organic material to 25~30:1, and the FeS2 content is 2%~4% of the total dry weight of the initial organic material; carry out aerobic fermentation on the initial organic material to obtain organic fermented material with sulfur oxidizing bacteria abundance reaching the peak. Step 3, Controlled Maturation: The activated coal gangue powder obtained in Step 1 and the organic fermentation material obtained in Step 2 are uniformly mixed at a mass ratio of 3:7 to 7:3 to obtain a mixture. The mixture is placed in a fermentation tank for controlled maturation treatment. Specifically, the controlled maturation step includes: when the internal temperature of the mixture is 50℃~60℃, air is introduced from the bottom of the fermentation tank at a rate of 100L / t / h~200L / t / h to control the internal moisture content to 30%~40%; when the internal temperature of the mixture drops to 35℃~45℃, air is introduced from the bottom of the fermentation tank at a rate of 200L / t / h~300L / t / h to control the internal moisture content to 35%~45% and the surface moisture content to 45%~50%; finally, gypsum crystallizes and precipitates in the surface and pores of the mixture to obtain coal gangue-based functional nutrient soil for saline-alkali land improvement.

[0007] Preferably, in step 1, the *Bacillus subtilis* refers to *Bacillus subtilis* LT1906, whose Latin name is... Paenibacillus mucilaginosus It is deposited at the China General Microbiological Culture Collection Center, with accession number CGMCC No. 21337.

[0008] Preferably, in step 1, the activation process of the coal gangue powder is as follows: 1×10 7 CFU / mL ~2.5×10 7 A CFU / mL Bacillus subtilis culture was diluted with sterile water to a concentration of 5 × 10⁻⁶. 5 CFU / mL ~ 1×10 6 CFU / mL, spray 150L~200L of diluted Bacillus subtilis solution per ton of coal gangue powder to achieve a bacterial concentration of 1×10⁻⁶ CFU / mL in the coal gangue. 9 CFU / ton~1.6×10 9 CFU / ton, activation treatment time is 24~72 hours.

[0009] Specialized pretreatment of coal gangue with bacterial activation solution is a crucial foundation for subsequent efficient reactions. The enzymes, polysaccharides, and organic acids secreted by Bacillus subtilis effectively disrupt the silicate mineral structure on the surface of the coal gangue. Simultaneous mechanical mixing during spraying ensures uniform contact between the bacterial solution and the coal gangue powder, guaranteeing sufficient erosion of the surface aluminosilicate minerals, facilitating the oxidation of pyrite (FeS2) and the formation of calcium-containing silicate minerals (Ca). 2+Dissolution creates a maximized reaction interface, thereby effectively removing physical constraints and promoting deep oxidation reactions.

[0010] Preferably, in step 2, the organic fermentation material refers to the material whose aerobic fermentation temperature reaches 55℃~65℃ and is maintained for 3~5 days, at which time the abundance of sulfur-oxidizing bacteria reaches its peak.

[0011] During the aerobic fermentation stage, FeS2 is strategically added. At a high temperature of 55℃~65℃, a highly efficient sulfur-oxidizing bacterial community, mainly composed of Thiobacillus, is selectively enriched and activated. This provides a strong initial oxidation driving force for the subsequent controlled maturation after mixing with coal gangue, efficiently promoting the oxidation of sulfur in lower valence states. 1- Towards stable SO4 2- The conversion process significantly enhances the efficiency and thoroughness of sulfur conversion.

[0012] Preferably, in step 3, the internal temperature of the mixture is maintained at 50℃~60℃ for 10~15 days, and the air is introduced intermittently, with a ventilation cycle of 10~30 minutes followed by a 20~50 minute stop of ventilation. Based on the real-time internal temperature of the mixture, the ventilation rate and intermittent parameters are automatically adjusted: when the real-time internal temperature of the mixture is 50℃~60℃, the ventilation rate is set to 140L / tŸh~160L / tŸh, the ventilation time is set to 15~25 minutes, and the ventilation stop time is set to 30~40 minutes; when the real-time internal temperature of the mixture is higher than 60℃, the ventilation rate is set to 100L / tŸh~140L / tŸh, the ventilation time is set to 10~15 minutes, and the ventilation stop time is set to 40~50 minutes; when the real-time internal temperature of the mixture is lower than 50℃, the ventilation rate is set to 160L / tŸh~200L / tŸh, the ventilation time is set to 25~30 minutes, and the ventilation stop time is set to 20~30 minutes.

[0013] Preferably, in step 3, when the internal moisture content of the mixture is less than 30%, the intermittent micro-spraying system is activated to spray atomized water into the mixture until the moisture content reaches 40% and then water replenishment stops.

[0014] Temperature and humidity sensors are installed inside the fermentation tank, and a spray humidification system is installed inside and on top. Heating / cooling coils are installed around the tank and linked with a variable frequency fan to form a feedback control system, which can dynamically stabilize the moisture content, temperature and aeration rate during the maturation stage within the above range.

[0015] Mixing activated coal gangue powder with high-temperature organic fermentation materials and controlling the maturation process is crucial for achieving efficient gypsum crystallization. Maintaining a high temperature period of 50℃~60℃ for 10~15 days is essential to ensure the harmlessness of composting and the completion of specific metabolic activities by target microorganisms. It also allows thermophilic sulfur-oxidizing bacteria to fully oxidize pyrite FeS2 in the coal gangue, releasing SO4. 2- Providing a necessary reaction time window is a prerequisite for inducing subsequent gypsum crystallization and also the necessary duration for the entire hydrothermal-gas synergistic control mechanism to function. Internal humidity control: Humidity is monitored in real-time using integrated sensors, maintaining a moisture content of 30%–40%, significantly lower than the 50%–60% moisture content of conventional compost. This is because: 1. Compared to other organic materials, coal gangue has lower water absorption; a 30%–40% moisture content is sufficient to ensure adequate contact between the coal gangue powder surface and water; 2. It minimizes SO4 production caused by excessive moisture. 2- Significant dissolution. Internal gas-heat synergistic regulation: By linking ventilation strategies with temperature, oxygen supply can be precisely controlled during high-temperature periods (50℃~60℃). This satisfies the metabolic needs of thermophilic bacteria such as sulfur-oxidizing bacteria to efficiently oxidize pyrite FeS2, while avoiding SO4 emissions caused by excessive ventilation. 2- It migrates upwards with the water vapor to the surface of the mixture.

[0016] Preferably, in step 3, the internal temperature of the mixture is maintained at 35℃~45℃ for 10~15 days, and the air is introduced intermittently, with a ventilation cycle of 20~40 minutes followed by a 30~90 minute cessation of ventilation. Based on the real-time internal temperature of the mixture, the ventilation rate and intermittent parameters are automatically adjusted: when the real-time internal temperature of the mixture is 35℃~45℃, the ventilation rate is set to 240L / tŸh~260L / tŸh, the ventilation time is set to 25~35 minutes, and the ventilation stop time is set to 50~70 minutes; when the real-time internal temperature of the mixture is higher than 45℃, the ventilation rate is set to 200L / tŸh~240L / tŸh, the ventilation time is set to 20~25 minutes, and the ventilation stop time is set to 70~90 minutes; when the real-time internal temperature of the mixture is lower than 35℃, the ventilation rate is set to 260L / tŸh~300L / tŸh, the ventilation time is set to 35~40 minutes, and the ventilation stop time is set to 30~50 minutes.

[0017] Preferably, in step 3, when the internal moisture content of the mixture is less than 35%, the intermittent micro-spraying system is activated to spray atomized water into the mixture until the moisture content reaches 45% and then water replenishment stops; when the moisture content of the surface layer of material with a thickness of 0-5cm is less than 45%, the intermittent micro-spraying system is activated to spray atomized water onto the surface of the mixture until the surface moisture content reaches 50% and then water replenishment stops.

[0018] Maintaining a medium-low temperature period of 35℃~45℃ for 10~15 days is crucial to ensure the oxidative decomposition of recalcitrant organic matter in organic materials. This also allows sulfur-oxidizing bacteria at medium and low temperatures to further oxidize pyrite (FeS2) and acid-producing bacteria to synthesize humic acid and other organic acids, thus complexing and dissolving more calcium. 2+ Water-air synergistic SO4 2- With Ca 2+ This provides the necessary reaction time window for the migration and enrichment of the material surface, as well as the crystallization and precipitation of the stable phase of gypsum (CaSO4·2H2O). Internal humidity control: a relatively high water content of 35%~45% is beneficial for SO4 production. 2- With Ca 2+ The ripening process requires suitable moisture conditions for dissolution and migration, but excessive moisture content may lead to anaerobic conditions due to insufficient oxygen supply. Therefore, internal gas-heat synergistic regulation is employed: increasing the ventilation rate, extending the ventilation time, or shortening the ventilation stop time increases oxygen supply, thereby maintaining the metabolic activity of microorganisms during the ripening stage; more importantly, strong airflow drives water vapor upwards, promoting the dissolution of SO4. 2- and Ca 2+ The solution migrates to the surface of the mixture. Surface moisture control: A higher moisture content of 45%~50% helps to form a thin liquid film on the surface, which neither makes the mixture too wet nor causes SO4 to escape. 2- With Ca 2+ The continuous enrichment and supersaturation of ions on the surface to provide an interface, and the increased supersaturation of liquid phase ions, are the key driving forces for promoting the crystallization of gypsum CaSO4·2H2O. Surface water-heat synergistic control: Due to the evaporation and cooling of surface moisture, a temperature difference is formed between the surface and the interior, i.e., the internal temperature is maintained at 35℃~45℃, which promotes microbial metabolism and ion dissolution and migration, while the relatively low surface temperature of 30℃~40℃ is more conducive to the crystallization and precipitation of the stable phase of gypsum CaSO4·2H2O.

[0019] Preferably, in step 3, the coal gangue-based functional nutrient soil contains gypsum CaSO4·2H2O, which can be detected by X-ray diffraction.

[0020] After 20-30 days of controlled maturation, white powdery or needle-like crystals will be observed precipitating on the surface and in the pores of the material, which is the target product gypsum CaSO4·2H2O. The overall reaction process is as follows: ① Sulfur-oxidizing bacteria accelerate the oxidation of pyrite (FeS2): FeS2 + 7 / 2O2 + H2O → Fe 2+ +2SO4 2- +2H + ; ② Neutralize H by NH3 produced from organic fermentation materials during the high-temperature period + NH3 + H2O → NH4 + +OH - OH- +H + →H2O; ③ Hydration and crystallization reaction: Ca 2+ +SO4 2- +2H2O→CaSO4Ÿ2H2O.

[0021] Through the synergistic transformation of microorganisms and hydrothermal gas in coal gangue, pyrite FeS2 fixes sulfur and calcium, avoiding the acidification risk of FeS2 during natural weathering. The acidification reaction process is as follows: ① Initial spontaneous oxidation: FeS2 + 7 / 2O2 + H2O → Fe 2+ +2SO4 2- +2H + ; ②Fe 2+ Further oxidation: Fe 2+ +1 / 4O2+2H + →Fe 3+ +1 / 2H2O; ③ When pH < 3.0, FeS2 is converted by Fe 3+ Oxidation: FeS2 + 14Fe 3+ +8H₂O→15Fe 2+ +2SO4 2- +16H + .

[0022] Another objective of this invention is to provide an application of coal gangue-based functional nutrient soil prepared by the above-mentioned method for preparing nutrient soil with gypsum crystallization induced by coal gangue in the improvement of saline-alkali land. Applying the coal gangue-based functional nutrient soil prepared by the above method to saline-alkali soil reduces alkalinity, improves physical structure, and replenishes organic matter, thereby achieving the effect of saline-alkali land improvement.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention relates to the specific selection of colloidal Bacillus for pulverizing coal gangue to 8-50 mesh, ensuring that the surface aluminosilicate minerals of the coal gangue powder can be fully eroded, so as to oxidize pyrite FeS2 and calcium-containing silicate minerals Ca. 2+ Dissolution creates a maximum reaction interface, which is the first step in the subsequent directional crystallization of gypsum.

[0024] (2) The quantitative addition of FeS2 during the directional fermentation process and the determination of the fermentation endpoint at the peak of sulfur-oxidizing bacteria, as involved in this invention, indicate that the bio-oxidation process with FeS2 as the substrate in the fermentation material is at its most vigorous and effective stage. This provides a strong initial oxidative driving force for the subsequent controlled maturation after mixing with coal gangue containing pyrite FeS2, and efficiently promotes the lower valence state S 1- Towards stable SO4 2-The conversion process significantly enhances the efficiency and thoroughness of sulfur conversion.

[0025] (3) The hydrothermal gas co-controlled curing method involved in this invention promotes the curing of coal gangue Ca by precisely controlling a set of specific and interrelated process parameters, namely, moisture content, temperature and aeration rate. 2+ Leaching and pyrite FeS2 to SO4 2- The transformation reduces the risk of acid leaching and promotes the directional hydration of CaSO4 into the soil amendment component gypsum CaSO4Ÿ2H2O, thereby enhancing its functionality.

[0026] (4) The organic matter such as humic acid produced by the aerobic fermentation of organic materials in this invention, as well as the microbial community established by full maturation, together with the coal gangue mineral particles, construct a stable aggregate structure of "mineral-organic matter-microorganism". This structure makes the final product, coal gangue-based nutrient soil, have excellent chemical properties, good porosity, and stable and abundant aggregates.

[0027] (5) The product of this invention has significant advantages in the improvement of saline-alkali land: ① It reduces alkalinity and pH value: the CaSO4·2H2O dissolved in gypsum reduces the alkalinity and pH value. 2+ Sodium that can replace soil colloids + The Na that was displaced + SO4 produced by the dissolution of gypsum (CaSO4·2H2O) 2- ① It can be removed from the topsoil by being leached downwards during irrigation or rainfall; ② It improves the physical structure of the soil: the stable aggregate structure formed between coal gangue mineral particles effectively improves the permeability of saline-alkali soil and solves the problem of compaction; ③ It replenishes nutrients: the rich organic matter can comprehensively enhance the fertility and ecological function of saline-alkali soil. The above product advantages solve the problems of high cost and unstable effect of simultaneous application of organic fertilizer and agricultural gypsum (CaSO4·2H2O) in traditional saline-alkali land improvement methods.

[0028] (6) The present invention uses waste materials such as coal gangue, cow dung, and straw as raw materials. It achieves the synergistic treatment of waste by “treating waste with waste” and is a green treatment technology that takes into account environmental benefits, economic benefits and resource utilization efficiency throughout the whole process. Attached Figure Description

[0029] Figure 1 This is a process flow diagram of the method for preparing nutrient soil for coal gangue-induced gypsum crystallization according to the present invention.

[0030] Figure 2 The image shows the surface microstructure of the coal gangue powder activated by Bacillus subtilis in Example 1.

[0031] Figure 3 The mineral crystal structure of the activated coal gangue powder by Bacillus subtilis in Example 1 is shown.

[0032] Figure 4 The relative abundance of Thiobacillus spp. on the last day of different fermentation stages of the organic material in Example 1.

[0033] Figure 5 The relative abundance of Thiobacillus spp. on the last day of the high-temperature period for different FeS2 addition levels.

[0034] Figure 6 The soluble Ca in the mixture during the maturation period in Example 1 2+ The content of CaSO4·2H2O in the surface gypsum.

[0035] Figure 7 The mineral crystal structures of the coal gangue-based functional nutrient soils in Example 1 and Comparative Examples 1-7 are shown.

[0036] Figure 8 The pH values ​​for the leaching test of the coal gangue-based functional nutrient soil in Example 1 and Comparative Examples 1-7 are given. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0038] Example 1: As Figure 1 As shown, this embodiment provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue, including the following steps: Step 1, Specific Activation: Crush coal gangue with a total sulfur content of 5.8%, pass it through a 25-mesh sieve, and collect the undersize material for later use. Add 1×10... 7 A CFU / mL solution of *Bacillus mucilaginosus* (strain LT1906, deposited at the China General Microbiological Culture Collection Center, registration number: CGMCC No. 21337) was diluted to 5 × 10⁻⁶ with sterile water that had been boiled. 5 CFU / mL, sprayed at a rate of 150L diluted bacterial solution per ton of coal gangue, to achieve a final spray volume of 1×10⁻⁶. 9 CFU / ton; and mechanical mixing is carried out simultaneously during the spraying process. The mechanical mixing steps are as follows: start the belt conveyor, and spray from the top and sides of the material at 45° along the three sets of spray systems arranged above the belt conveyor. The coal gangue powder sprayed with bacterial solution directly enters the drum mixer. The spiral guide plate inside the drum mixer pushes the material forward, and the lifting plate throws the material up and then sprinkles it evenly, so that the bacterial solution and the material can fully contact each other during the tumbling process. The mixing time is 10 minutes to ensure that the bacterial solution and the coal gangue can fully and evenly contact each other, ensuring that the surface of the coal gangue is eroded and more calcium is dissolved.

[0039] Step 2, Directional Fermentation: Cow manure, FeS2, and corn stalks crushed to less than 2cm are mixed to form the initial organic material. The carbon-to-nitrogen ratio (C / N) of the initial organic material is measured on a dry basis. The ratio of cow manure to corn stalks is adjusted to achieve a C / N ratio of 30:1. The FeS2 content is 2% of the total dry weight of the initial organic material. Fermentation lasts 31 days, with separate fermentation periods during the warming phase (7 days, temperature increasing from 26℃ to 50℃) and the high-temperature phase (15 days). Samples were taken at the end of the cooling period (50℃~65℃) and the cooling period (9 days in total, with temperatures ranging from 52℃ to 31℃), specifically on day 7 (50℃), day 22 (57℃), and day 31 (31℃) of fermentation. The abundance of sulfur-oxidizing bacteria was detected by 16S rRNA gene sequencing. The peak value was reached at the end of the high-temperature period, specifically on day 22 of fermentation, when the activity of sulfur-oxidizing bacteria was strongest and their oxidation capacity for pyrite FeS2 in coal gangue was also strongest. Organic fermentation material from this peak period was used for future reference.

[0040] Step 3, Controlled maturation: Mix the activated coal gangue powder obtained in Step 1 and the high-temperature organic fermentation material obtained in Step 2 at a mass ratio of 6:4, adjust the moisture content of the mixture to 40%, place it in a fermentation tank, at a temperature of 52℃, and maintain the internal temperature of the mixture at 50℃~60℃ for 15 days. Air is introduced from the bottom of the fermenter. The ventilation rate and intermittent parameters are automatically adjusted based on the real-time internal temperature of the mixture, as follows: When the real-time internal temperature of the mixture is 50℃~60℃, the ventilation rate is set to 140L / tŸh~160L / tŸh, the ventilation time is set to 15~25 minutes, and the ventilation stop time is set to 30~40 minutes; when the real-time internal temperature of the mixture is above 60℃, the ventilation rate is set to 100L / tŸh~140L / tŸh, the ventilation time is set to 10~15 minutes, and the ventilation stop time is set to 40~50 minutes; when the real-time internal temperature of the mixture is below 50℃, the ventilation rate is set to 160L / tŸh~200L / tŸh, the ventilation time is set to 25~30 minutes, and the ventilation stop time is set to 20~30 minutes. The internal moisture content of the mixture is maintained at 30%~40%. When it is below 30%, the intermittent micro-spraying system is activated to spray atomized water into the mixture until the moisture content reaches 40% and then water replenishment stops.

[0041] Continue to maintain the internal temperature of the mixture at 35℃~45℃ for 15 days. When the temperature drops to 45℃, adjust the internal moisture content to 45% and the surface moisture content to 50%. Air is introduced from the bottom of the fermenter. The ventilation rate and intermittent parameters are automatically adjusted based on the real-time internal temperature of the mixture, as follows: When the real-time internal temperature of the mixture is 35℃~45℃, the ventilation rate is set to 240L / tŸh~260L / tŸh, the ventilation time is set to 25~35 minutes, and the ventilation stop time is set to 50~70 minutes; when the real-time internal temperature of the mixture is above 45℃, the ventilation rate is set to 200L / tŸh~240L / tŸh, the ventilation time is set to 20~25 minutes, and the ventilation stop time is set to 70~90 minutes; when the real-time internal temperature of the mixture is below 35℃, the ventilation rate is set to 260L / tŸh~300L / tŸh, the ventilation time is set to 35~40 minutes, and the ventilation stop time is set to 30~50 minutes. The internal moisture content of the mixture was maintained at 35%–45%. When the internal moisture content was below 35%, an intermittent micro-sprinkler system was activated to spray atomized water into the mixture until the internal moisture content reached 45%, at which point water replenishment ceased. The surface moisture content of the mixture was maintained at 45%–50%. When the surface moisture content was below 45%, an intermittent micro-sprinkler system was activated to spray atomized water into the mixture until the surface moisture content reached 50%, at which point water replenishment ceased. Finally, a coal gangue-based nutrient soil with soil-like physical and chemical properties was obtained (Tables 1 and 2).

[0042] Table 1 Chemical properties of coal gangue-based nutrient soil

[0043] Table 2 Physical properties of coal gangue-based nutrient soil

[0044] Figure 2 The images show the surface microstructure of coal gangue powder activated by Bacillus megaterium in Example 1. (a) shows coal gangue without activation, (b) shows coal gangue activated by Bacillus megaterium, and (c) shows coal gangue activated by Bacillus megaterium in Example 1. The untreated coal gangue powder exhibits a lamellar structure with a relatively smooth surface. After treatment with Bacillus megaterium, the surface structure is significantly eroded. After treatment with Bacillus megaterium, the lamellar structure of the coal gangue surface is significantly broken, showing obvious porosity and fragmentation. This phenomenon indicates that microbial metabolites and enzymes have an erosive effect on the silicate mineral structure of the coal gangue surface, resulting in significant changes in its surface morphology.

[0045] Figure 3The mineral crystal structure of coal gangue powder activated by Bacillus megaterium in Example 1 is shown. The characteristic diffraction peak intensity of CaAl₂Si₂O₈ŷ₄H₂O in coal gangue from Example 1 is weaker compared to coal gangue without bacterial activation and coal gangue activated by Bacillus megaterium. This is because Bacillus megaterium and its metabolites disrupt the silicate mineral structure of the coal gangue surface. This fully exposes the pyrite FeS₂ encapsulated within, creating conditions for subsequent oxidation and other reactions, and also facilitating the activation of Ca during the bacterial solution activation process. 2+ The dissolution of these substances provides a basis for the formation of related products.

[0046] Figure 4 The figures show the relative abundance of *Thiobacillus* (red portion) on the last day of different fermentation stages of the organic material in Example 1: day 7 (last day of the warming phase), day 22 (last day of the high-temperature phase), and day 31 (last day of the cooling phase). Microbial community structure was analyzed using 16S rRNA gene sequencing. The relative abundance of *Thiobacillus* was significantly higher during the high-temperature phase than during the warming and cooling phases. Therefore, the fermentation products from the high-temperature phase are preferred for subsequent mixing and maturation treatment, which helps to promote the continuous sulfur oxidation process and improve the stability and treatment effect of the final product.

[0047] Figure 5 The figure shows the relative abundance of *Thiobacillus* (red portion) on the last day of the high-temperature period, i.e., day 22 of fermentation, when different amounts of FeS2 were added. As can be observed from the figure, the relative abundance of *Thiobacillus* increased with the FeS2 addition from 2% to 3% and then to 4%, reflecting the regulatory effect of FeS2 addition on the abundance of *Thiobacillus* community during the high-temperature period.

[0048] Figure 6 The soluble Ca content inside the mixture of materials with different maturation periods in Example 1 2+ The content of CaSO4·2H2O in the surface gypsum. During the curing period of 0-15 days, i.e., the high-temperature stage of 50℃-60℃, the content of soluble Ca in the interior... 2+ The content of CaSO4 and 2H2O in the internal gypsum is relatively low, similar to that in the surface gypsum. During the 16th to 25th day of the curing period, i.e., the medium-low temperature stage (35℃ to 45℃), the content of soluble Ca in the internal gypsum is also low. 2+ The concentration reached its highest point on day 20 and decreased to its lowest point on day 25, while the gypsum CaSO4·2H2O content rose to its highest point on day 25. The results indicate that Ca... 2+ The leaching mainly occurs during the medium and low temperature periods. During this stage, acid-producing bacteria synthesize large amounts of humic acid and other organic acids, which then complex and dissolve into the Ca in the coal gangue. 2+ Ca 2+ As water vapor migrates to the surface of the mixture, it reacts with SO4. 2-Gypsum CaSO4·2H2O crystals continuously accumulate and precipitate in the surface layer under supersaturation. This result confirms that the hydrothermal gas conditions in Example 1 play a dual role in thermodynamic driving and crystallization kinetic regulation, and are the optimal conditions for controllable aging of coal gangue, promoting the directional transformation of pyrite FeS2 and gypsum crystallization.

[0049] Comparative Example 1: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1, except that in step 3, the internal real-time temperature of the mixture is changed from "50℃~60℃" to "40℃~50℃" and maintained for 15 days.

[0050] Comparative Example 2: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1, except that in step 3, the internal real-time temperature of the mixture is maintained at 50℃~60℃ for 15 days, but is changed to 5 days.

[0051] Comparative Example 3: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1. The difference from Example 1 is that in step 3, when the internal real-time temperature of the mixture is 50℃~60℃, the internal moisture content changes from "30%~40%" to "40%~50%".

[0052] Comparative Example 4: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1, except that in step 3, the internal temperature of the mixture is changed from "35℃~45℃" to "45℃~50℃" and maintained for 15 days.

[0053] Comparative Example 5: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1, except that in step 3, the internal temperature of the mixture is maintained at 35℃~45℃ for 15 days instead of 7 days.

[0054] Comparative Example 6: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1, except that in step 3, when the internal temperature of the mixture is kept at 35℃~45℃, the internal moisture content changes from "35%~45%" to "45%~50%".

[0055] Comparative Example 7: This comparative example provides a method for preparing nutrient soil for gypsum crystallization induced by coal gangue. The specific steps are the same as in Example 1. The difference from Example 1 is that in step 3, when the internal temperature of the mixture is kept at 35℃~45℃, the surface moisture content changes from "45%~50%" to "35%~45%".

[0056] Figure 7 The mineral crystal structures of the coal gangue-based functional nutrient soils in Examples 1 and 1-7 are shown. Only in Example 1 were the characteristic diffraction peaks of gypsum (CaSO4·2H2O) clearly detected, indicating the obvious formation of gypsum (CaSO4·2H2O) crystals. In Comparative Example 1, a 15-day high-temperature ripening period of 40℃~50℃ was maintained. This temperature range is lower than the optimal temperature for thermophilic sulfur-oxidizing bacteria, making it difficult to maintain the abundance and activity of the sulfur-oxidizing bacterial community, which is detrimental to the bio-oxidation of pyrite (FeS2). In Comparative Example 2, a 5-day high-temperature ripening period of 5℃ was maintained... During the high-temperature aging period of 0℃ to 60℃, the short time window was insufficient for the complete oxidation of pyrite FeS2. In Comparative Example 3, during the high-temperature aging period of 50℃ to 60℃, the internal moisture content of the mixture was too high (40% to 50%), interfering with microbial metabolism and hindering the biological oxidation of pyrite FeS2. In Comparative Example 4, maintaining a medium-temperature aging period of 45℃ to 50℃ for 15 days exceeded the optimal range for the oxidation and decomposition of recalcitrant organic matter in the organic material, resulting in insufficient generation of humic acid and other organic acids, leading to insufficient Ca2+ oxidation. 2+ Insufficient dissolution; Comparative Example 5, maintaining a 7-day aging period at 35℃~45℃, the short time window was insufficient for Ca to dissolve. 2+ Sufficient dissolution; Comparative Example 6, maintaining a 35℃~45℃ maturation period for another 15 days, with a high moisture content of 45%~50% inside the mixture, resulted in an anaerobic environment, limiting the abundance and activity of acid-producing bacteria, making it difficult to dissolve sufficient Ca. 2+ Comparative Example 7: The mixture was further aged at 35℃~45℃ for 15 days during the medium-low temperature period. The surface layer of the mixture had a low moisture content of 35%~45%, and no moisture difference was formed between the surface and the interior. Therefore, a thin liquid film could not form on the surface, preventing SO4 from escaping. 2- With Ca 2+ The FeS2 in pyrite accumulates continuously on the surface and reaches supersaturation. Therefore, in comparative examples 1-3, FeS2 in coal gangue cannot be oxidized to SO4. 2- SO4 in coal gangue (Comparative Examples 4-7) 2- Difficult to be with Ca 2+ Gypsum (CaSO4·2H2O) is generated.

[0057] Figure 8The pH values ​​of the leachate from the coal gangue-based functional nutrient soils of Examples 1 and 1-7 are shown in the leaching test. According to GB / T34230-2017 "Leaching Test Method for Coal and Coal Gangue", the pH of the leachate from the raw coal gangue powder is 5.8, indicating weak acidity, which is caused by the acidification of pyrite FeS2 in the coal gangue. The pH of the leachate from the coal gangue-based functional nutrient soil of Example 1 is 7.6, indicating neutrality. This shows that Example 1 of the present invention, through hydrothermal gas-controlled ripening treatment, effectively transformed the pyrite FeS2 in the coal gangue, reducing acidic leaching. The pH of the leachate from the coal gangue products of Comparative Examples 1-3 is concentrated around 5. The pH of the leaching solution for the coal gangue was between 0.8 and 6.1, exhibiting a slightly acidic pH. This was because the temperature, duration, and humidity during the high-temperature aging period deviated from the synergistic optimization range of this invention, resulting in the ineffective conversion of pyrite FeS2 in the coal gangue and the unavoidable acidic leaching. For the coal gangue products of Comparative Examples 4-7, the pH of the leaching solution was concentrated between 7.1 and 7.6, exhibiting a neutral pH. This was because, although the process parameters deviated from the optimized range during the medium and low-temperature aging periods, preventing the formation of gypsum CaSO4·2H2O, the pyrite FeS2 could still be converted to SO4 during the high-temperature aging period. 2- It effectively alleviates acid leaching.

[0058] Example 2: This example provides the application of coal gangue-based functional nutrient soil prepared by the method of coal gangue-induced gypsum crystallization preparation in Example 1 in the improvement of saline-alkali land.

[0059] The pot experiment was used to evaluate the effect of the coal gangue-based nutrient soil obtained in Example 1 on improving saline-alkali soil. Coal gangue-based functional nutrient soil was mixed with saline-alkali soil collected from Daying Village, Ying County, Shuozhou City, Shanxi Province at weight percentages of 30%, 40%, 50%, and 60%, respectively. After thorough mixing, the mixture was placed in pots (pot diameter 28cm, depth 19cm) and the moisture content was adjusted to 70% of field capacity. Three corn seeds (Zhengdan 958 hybrid corn) were sown in each pot. After 18 days, thinning was done, and one plant was transplanted per pot. Watering was done once a month, and routine management was implemented. Corn growth was analyzed at the seedling stage, jointing stage, large trumpet stage, silking stage, and maturity stage. Yield was measured at maturity, and soil samples (0-12cm soil layer) were collected to determine soil physicochemical properties.

[0060] Functional nutrient soil based on coal gangue with different admixture amounts can effectively improve the salinization degree of saline-alkali soil (Table 3). With the increase of coal gangue-based functional nutrient soil admixture amount, the pH value of the saline-alkali soil continuously decreased from 9.43 to 7.6, and the percentage of exchangeable sodium significantly decreased from 51.2% to 8.3%. This change directly reflects the Ca2+ content in gypsum CaSO4·2H2O. 2+ For soil colloid Na + Displacement effect: Ca 2+ Na adsorbed on the colloid +The exchange process forms soluble Na₂SO₄, which is subsequently leached away by water, effectively reducing soil alkalinity and pH. Simultaneously, the total water-soluble salt content decreased from 10.9 g / kg to 1.2 g / kg, and the electrical conductivity decreased from 9.04 mS / cm to 2.26 mS / cm, further confirming the effect of Na₂SO₄. + The accompanying leaching and removal of anions indicates that coal gangue-based functional nutrient soil significantly alleviates sodium toxicity and alkalinity of saline-alkali soil through the chemical modification mechanism of "sodium-calcium substitution".

[0061] Table 3. Degree of salinization of saline-alkali soil with different proportions of coal gangue-based functional nutrient soil.

[0062] With the increase of coal gangue-based functional nutrient soil content, the physical structure of saline-alkali soil was significantly improved (Table 4). Specifically, the soil bulk density decreased continuously from 1.82 g / cm³ to 1.37 g / cm³, and the porosity increased from 31.3% to 48.7%, indicating that the loose and porous physical structure of the coal gangue-based functional nutrient soil effectively improved the problems of poor aeration and drainage commonly found in saline-alkali soil. The content of water-stable aggregates underwent structural optimization, with the proportions of large aggregates (0.5–1 mm and 0.25–0.5 mm) increasing from 2.88% and 3.52% to 12.39% and 8.82%, respectively, while the proportion of micro-aggregates (<0.25 mm) decreased from 90.24% to 59.07%. This indicates that the coal gangue-based functional nutrient soil effectively promoted the formation of water-stable large aggregates, enhancing the soil's resistance to erosion and water retention capacity. The average mass diameter of the aggregates also increased from 0.26 mm to 0.62 mm, further confirming that the overall soil structure is developing towards a more stable and looser direction. This indicates that coal gangue-based functional nutrient soil systematically optimizes the physical structure of saline-alkali soil through the cementation and aggregation of its particles, providing an effective way to solve soil compaction and improve permeability.

[0063] Table 4. Physical properties of saline-alkali soil with different proportions of coal gangue-based functional nutrient soil.

[0064] The functional nutrient soil based on coal gangue with different admixture amounts significantly improved the chemical properties of saline-alkali soil (Table 5). With increasing admixture amount, the soil organic matter content increased from 8.06 g / kg to 22.11 g / kg, providing sufficient carbon source for soil microbial activity and plant growth, directly enhancing soil fertility. Total nitrogen increased from 0.46 g / kg to 1.78 g / kg, and total phosphorus increased from 0.52 g / kg to 0.67 g / kg, indicating that the coal gangue nutrient soil effectively supplemented soil nitrogen and phosphorus nutrients. Regarding available nutrients, alkaline-available nitrogen increased from 28.0 mg / kg to 68.8 mg / kg, available phosphorus from 2.01 mg / kg to 28.01 mg / kg, and available potassium from 145.21 mg / kg to 167.46 mg / kg, showing a significant improvement in soil nutrient availability. These changes fully demonstrate that coal gangue-based functional nutrient soil, through its rich organic matter and nutrient content, comprehensively improves the chemical properties of saline-alkali soil, enhances soil fertility and ecological functions, and creates more favorable conditions for plant growth.

[0065] Table 5 Chemical properties of saline-alkali soils with different proportions of coal gangue-based functional nutrient soils

[0066] Table 6. Plant height of maize at different growth stages in saline-alkali soil with different proportions of coal gangue-based functional nutrient soil.

[0067] Table 7. Corn yield in saline-alkali soil with different proportions of coal gangue-based functional nutrient soil.

[0068] Different dosages of coal gangue-based functional nutrient soil not only improved the physicochemical properties of saline-alkali soil but also had a significant positive impact on maize growth. In saline-alkali soil without coal gangue-based functional nutrient soil, maize growth and development were severely inhibited, and all plants died before the seedling stage (Table 6). With increasing dosage, maize plant height showed a clear upward trend at each growth stage. At maturity, yield-related indicators of maize were also greatly improved (Table 7). This indicates that coal gangue-based functional nutrient soil can effectively improve the physicochemical properties of saline-alkali soil, provide a more suitable environment for maize growth, promote maize growth and development, and increase yield. Within a certain range, the higher the dosage, the more significant the improvement and yield-increasing effects.

Claims

1. A method for preparing nutrient soil for gypsum crystallization induced by coal gangue, characterized in that, Includes the following steps: Step 1, Specific Activation: Crush coal gangue with a total sulfur content ≥3% to 8~50 mesh, and activate the coal gangue powder with Bacillus subtilis solution; Step 2, Directional Fermentation: Mix cow dung, straw and FeS2 to obtain initial organic material, adjust the carbon-nitrogen ratio of the initial organic material to 25~30:1, and the FeS2 content is 2%~4% of the total dry weight of the initial organic material; carry out aerobic fermentation on the initial organic material to obtain organic fermented material with sulfur oxidizing bacteria abundance reaching the peak. Step 3, Controlled Maturation: The activated coal gangue powder obtained in Step 1 and the organic fermentation material obtained in Step 2 are uniformly mixed at a mass ratio of 3:7 to 7:3 to obtain a mixture. The mixture is placed in a fermentation tank for controlled maturation treatment. Specifically, the controlled maturation step includes: when the internal temperature of the mixture is 50℃~60℃, air is introduced from the bottom of the fermentation tank at a rate of 100L / t·h~200L / t·h to control the internal moisture content to 30%~40%; when the internal temperature of the mixture drops to 35℃~45℃, air is introduced from the bottom of the fermentation tank at a rate of 200L / t·h~300L / t·h to control the internal moisture content to 35%~45% and the surface moisture content to 45%~50%; finally, gypsum CaSO4·2H2O crystallizes and precipitates in the surface and pores of the mixture to obtain coal gangue-based functional nutrient soil for saline-alkali land improvement.

2. The method for preparing nutrient soil for induced gypsum crystallization by coal gangue according to claim 1, characterized in that, In step 1, the *Bacillus subtilis* refers to *Bacillus subtilis* LT1906, whose Latin name is... Paenibacillus mucilaginosus It is deposited at the China General Microbiological Culture Collection Center, with accession number CGMCC No. 21337.

3. The method for preparing nutrient soil for induced gypsum crystallization by coal gangue according to claim 1, characterized in that, In step 1, the activation process of coal gangue powder is as follows: 1×10 7 CFU / mL ~2.5×10 7 A CFU / mL Bacillus subtilis culture was diluted with sterile water to a concentration of 5 × 10⁻⁶. 5 CFU / mL ~ 1×10 6 CFU / mL, spray 150L~200L of diluted Bacillus subtilis solution per ton of coal gangue powder to achieve a bacterial concentration of 1×10⁻⁶ CFU / mL in the coal gangue powder. 9 CFU / ton~1.6×10 9 CFU / ton, activation treatment time is 24~72 hours.

4. The method for preparing nutrient soil for induced gypsum crystallization by coal gangue according to claim 1, characterized in that, In step 2, the organic fermentation material refers to the material whose aerobic fermentation temperature reaches 55℃~65℃ and is maintained for 3~5 days, at which time the abundance of sulfur-oxidizing bacteria reaches its peak.

5. The method for preparing nutrient soil for coal gangue-induced gypsum crystallization according to claim 1, characterized in that, In step 3, the internal temperature of the mixture is maintained at 50℃~60℃ for 10~15 days. Air is introduced intermittently, with a ventilation cycle of 10~30 minutes followed by a 20~50 minute pause. The ventilation rate and intermittent parameters are automatically adjusted based on the real-time internal temperature of the mixture: when the real-time internal temperature of the mixture is 50℃~60℃, the ventilation rate is set to 140L / t·h~160L / t·h, the ventilation time is set to 15~25 minutes, and then ventilation is stopped. The ventilation time is set to 30-40 minutes; when the real-time internal temperature of the mixture is above 60℃, the ventilation rate is set to 100L / tŸh-140L / tŸh, the ventilation time is set to 10-15 minutes, and the ventilation stop time is set to 40-50 minutes; when the real-time internal temperature of the mixture is below 50℃, the ventilation rate is set to 160L / tŸh-200L / tŸh, the ventilation time is set to 25-30 minutes, and the ventilation stop time is set to 20-30 minutes.

6. The method for preparing nutrient soil for coal gangue-induced gypsum crystallization according to claim 1, characterized in that, In step 3, when the internal moisture content of the mixture is less than 30%, the intermittent micro-spraying system is activated to spray atomized water into the mixture until the internal moisture content reaches 40%, at which point water replenishment stops.

7. The method for preparing nutrient soil for induced gypsum crystallization by coal gangue according to claim 1, characterized in that, In step 3, the internal temperature of the mixture is maintained at 35℃~45℃ for 10~15 days. Air is introduced intermittently, with a ventilation cycle of 20~40 minutes followed by a 30~90 minute pause. The ventilation rate and intermittent parameters are automatically adjusted based on the real-time internal temperature of the mixture: when the real-time internal temperature is 35℃~45℃, the ventilation rate is set to 240L / t·h~260L / t·h, the ventilation time is set to 25~35 minutes, and ventilation is stopped. The ventilation time is set to 50-70 minutes; when the real-time internal temperature of the mixture is above 45℃, the ventilation rate is set to 200L / t / h~240L / t / h, the ventilation time is set to 20-25 minutes, and the ventilation stop time is set to 70-90 minutes; when the real-time internal temperature of the mixture is below 35℃, the ventilation rate is set to 260L / t / h~300L / t / h, the ventilation time is set to 35-40 minutes, and the ventilation stop time is set to 30-50 minutes.

8. The method for preparing nutrient soil for coal gangue-induced gypsum crystallization according to claim 1, characterized in that, In step 3, when the internal moisture content of the mixture is less than 35%, the intermittent micro-spraying system is activated to spray atomized water into the mixture until the internal moisture content reaches 45% and then water replenishment stops; when the moisture content of the surface layer of material with a thickness of 0-5cm is less than 45%, the intermittent micro-spraying system is activated to spray atomized water onto the surface of the mixture until the surface moisture content reaches 50% and then water replenishment stops.

9. The method for preparing nutrient soil for induced gypsum crystallization by coal gangue according to claim 1, characterized in that, In step 3, the coal gangue-based functional nutrient soil contains gypsum CaSO4·2H2O, which can be detected by X-ray diffraction.

10. The application of a coal gangue-based functional nutrient soil prepared by the method for preparing nutrient soil by inducing gypsum crystallization from coal gangue according to any one of claims 1-9 in the improvement of saline-alkali land.

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