Preparation method of low-energy-consumption proppant for slag self-sintering

Abstract

The application discloses a preparation method of a slag self-sintering low-energy-consumption proppant, and relates to the field of proppant preparation.The proppant adopts a core-shell structure design, the inner layer matrix takes high-carbon slag as a main raw material, and high-kaolin coated carbon particles and specific proportion of multi-metal oxides are added; the outer layer matrix takes low-carbon slag as a main raw material, and a coating layer is formed through a limited range of coating thickness. The preparation process comprises inner layer green body preparation, outer layer coating and stepwise temperature rising self-sintering technology, wherein the multi-metal oxides and the carbon source generate a carbothermic reduction reaction in the sintering process to generate metal elements and low-valence oxides, and the mullite crystals formed by the phase change of the high-kaolin are combined to improve the anti-crushing capacity, flow conductivity and chemical stability of the proppant. Through the cooperation of raw materials and the optimization of the structure, the application realizes the improvement of the comprehensive performance of the proppant, and is suitable for crack support in hydraulic fracturing operations.

Description

Technical Field

[0001] This invention relates to the field of proppant preparation, and more particularly to a method for preparing a low-energy-consumption proppant for slag self-sintering. Background Technology

[0002] Proppants, as key materials in oil and gas field fracturing engineering, are typically prepared using high-temperature sintering processes. Traditional methods often use ceramics, quartz sand, and other raw materials, sintering them by external heating to over 1200℃. This process is not only energy-intensive but also involves a significant proportion of raw material costs. Meanwhile, industrial slag, a major solid waste from industries such as metallurgy and power generation, is often limited to simple landfilling or low-value-added building materials. High-carbon slag, due to its high carbon content and complex composition, is difficult to use directly for high-performance material preparation, leading to significant solid waste accumulation and resource waste. Furthermore, existing proppant structural designs often employ a single density or porosity. Increasing porosity to achieve high permeability often results in decreased mechanical strength; conversely, strengthening strength requires a dense structure, which reduces permeability, making it difficult to achieve a balance between the two. Regarding sintering process control, traditional processes rely on continuous heating from an external heat source. The temperature gradient is easily affected by equipment precision, leading to localized overheating or incomplete reactions, resulting in uneven internal structure and large performance fluctuations in the proppant.

[0003] Among the aforementioned existing technologies, high-temperature sintering relies on external heat sources, resulting in high energy consumption. There are bottlenecks in the efficient utilization of slag solid waste, especially high-carbon slag. The structural contradiction between proppant permeability and mechanical strength is difficult to coordinate. Furthermore, insufficient precision in controlling the sintering temperature gradient leads to poor performance stability. These issues have become the main technical obstacles restricting the low-energy consumption, high-performance, and high-value-added utilization of proppant solid waste. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for preparing a low-energy-consumption slag self-sintering proppant, so as to solve one or more problems in the prior art.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing a low-energy-consumption slag self-sintering proppant includes the following steps: (1) Preparation of kaolin-coated carbon particles: Graphite powder and kaolin powder are mixed, and coated carbon particles with carbon core-kaolin shell structure are obtained by wet grinding and spray drying, wherein the thickness of the kaolin layer is 5-15m.

[0006] (2) Inner layer porous green body granulation: Using high-carbon slag as the matrix raw material, high-carbon slag, polymetallic oxide mixed powder and kaolin-coated carbon particles obtained in step (1) are mixed by mass fraction, and an inner layer organic binder solution is added for wet granulation to form an inner layer green body. The ratio of the total number of moles of metal ions in the polymetallic oxide mixed powder to the sum of the number of moles of carbon in the kaolin-coated carbon particles and the number of moles of carbon in the high-carbon slag is 1:1.2-1.5.

[0007] (3) Dense outer layer coating: Low carbon slag is used as the outer layer matrix material. Low carbon slag and inorganic binder are mixed and an outer layer inorganic binder solution is added to prepare a coating slurry. The inner layer green body of step (2) is fluidized bed coated to form a core-shell structure green body.

[0008] (4) Stepped self-sintering: The core-shell structure green body is preheated to create holes, and then subjected to a stepped exothermic reaction of polymetallic oxides and carbon for self-sintering. The support is obtained by natural cooling and sieving. The stepped self-sintering temperature range is 600-1000℃, and the temperature gradient is controlled by the stepwise reaction of Fe2O3, MnO2, CuO and carbon in different temperature ranges.

[0009] Specifically, the preparation parameters for the kaolin-coated carbon particles in step (1) are as follows: 8-10 parts by mass of graphite powder and 1-2 parts by mass of kaolin powder are added to deionized water to prepare a slurry with a solid content of 30-35%. Wet grinding is performed using a sand mill with zirconium beads of 0.5 mm diameter, a rotation speed of 1800-2200 r / min, and a grinding time of 25-35 min. Spray drying is performed with an inlet temperature of 170-190℃, an outlet temperature of 75-85℃, and an atomization pressure of 0.2-0.4 MPa to obtain coated carbon particles with a particle size of 80-120 μm.

[0010] Specifically, the raw materials for the inner porous green body mentioned in step (2) are, by dry weight, 75-85 parts of high-carbon slag, 5-8 parts of polymetallic oxide mixed powder, and 5-15 parts of kaolin-coated carbon particles. The polymetallic oxide mixed powder contains 4-6 parts of Fe2O3 powder, 0.5-1.5 parts of MnO2 powder, and 0.3-0.5 parts of CuO powder.

[0011] Specifically, the inner layer organic binder solution mentioned in step (2) is a starch solution with a mass concentration of 7-9%, added at a mass ratio of 1:0.7-0.9 between the inner layer dry material and the starch solution. Wet granulation is performed using a rotary drum granulator with a drum inclination angle of 2-4°, a rotation speed of 20-30 r / min, and a granulation time of 10-20 min, to obtain an inner layer green body with a particle size of 3-5 mm.

[0012] Specifically, the raw materials for the outer dense coating mentioned in step (3) are, by dry mass fraction: 90-98 parts of low-carbon slag and 2-8 parts of sodium silicate solid (Na2OnSiO2 purified by mass fraction), totaling 100 parts. The outer inorganic binder solution is a sodium silicate dilution with a mass concentration of 10-15%, the modulus of which is 3.2-3.3, and it is added at a mass ratio of 1:0.4-0.6 between the inner green body and the coating slurry.

[0013] Specifically, the parameters for fluidized bed coating in step (3) are: bed temperature 55-65℃, air volume 0.4-0.6m³ / h. 3 The coating process is carried out at a rate of 100°C / min for 25-35 minutes, forming an outer coating layer with an average thickness of 50-100 μm. The core-shell structured green body is then dried at 80-100°C for 2-3 hours, with a moisture content of 5%.

[0014] Specifically, the stepped self-sintering process described in step (4) is as follows: First, preheat at 300-500℃ for 1-2 hours at a heating rate of 4-6℃ / min, then hold at 600-700℃ for 0.5-1 hours, during which CuO reacts with carbon. Hold at 700-800℃ for 0.5-1 hours, during which MnO2 reacts with carbon. Hold at 800-1000℃ for 1.5-2.5 hours, during which Fe2O3 reacts with carbon. The total sintering time is 2.5-4 hours, and the atmosphere is air with an oxygen content of 20-22%.

[0015] Specifically, the high-carbon slag has a carbon content of 7-11%, a particle size of 150 μm, and a D50 of 40-60 μm. The low-carbon slag has a carbon content of 0.5-2.5%, a particle size of 75 μm, and a D50 of 15-25 μm. The Fe2O3 powder has a purity of 95%, the MnO2 powder has a purity of 90%, and the CuO powder has a purity of 90%, with all three having a particle size of 5 μm.

[0016] Specifically, the natural cooling rate in step (4) is 10°C / min. The sieving uses a 20-40 mesh screen, and the particle size of the undersize material is 0.45-0.9 mm.

[0017] The graphite powder in step (1) has 98% fixed carbon and a particle size of 50-100 μm. The kaolin powder has a purity of 95%, a particle size of 1-5 μm and a D50 of 1-3 μm, and the kaolin powder coating the carbon particles has a mass coating rate of 90%.

[0018] Furthermore, the corn starch in the starch solution has a purity of 99%.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (i) By combining the stepwise exothermic reaction mechanism of polymetallic oxides and carbon with a core-shell structure design, the self-sintering of the proppant is achieved and energy consumption is significantly reduced. In this scheme, the inner high-carbon slag provides the carbon source, which reacts stepwise with polymetallic oxides such as Fe2O3, MnO2, and CuO in different temperature ranges to release heat. The outer low-carbon slag forms a dense coating layer to reduce heat loss. The synergistic use of reaction heat from both the inner and outer layers replaces external heating, breaking through the limitations of traditional sintering that relies on external heat sources and fundamentally reducing energy consumption requirements.

[0020] (II) The composite structure design of a porous inner green body and a dense outer coating achieves a balance between proppant permeability and mechanical strength. The inner layer forms a porous structure through the reaction of high-carbon slag and kaolin-coated carbon particles, providing space for fluid channels; the outer layer uses a dense coating of low-carbon slag and inorganic binder to enhance the overall structural stability, solving the technical contradiction that a single structure cannot simultaneously achieve both permeability and strength, and improving the comprehensive performance of the proppant in practical applications.

[0021] (III) The graded utilization of high-carbon and low-carbon slag, combined with the internal distribution control of carbon sources, has achieved a synergistic improvement in solid waste resource utilization and reaction efficiency. Using industrial slag as the base material, high-carbon slag serves as the main carbon source in the inner layer to participate in the exothermic reaction, while low-carbon slag serves as the outer structural material. With the precise carbon distribution design of carbon particles coated with kaolin, solid waste can be efficiently disposed of and accumulation pollution can be reduced. At the same time, the gradient distribution of carbon sources can avoid excessively intense or insufficient local reactions, thereby improving reaction efficiency and raw material utilization.

[0022] (iv) The synergistic effect of the core-shell slow-release mechanism of carbon particles coated with kaolin and the stepwise reaction of polymetallic oxides enables precise control of the temperature gradient during the sintering process. The kaolin shell controls the reaction rate of carbon, avoiding premature and excessive heat release from carbon that could lead to local overheating. Combined with the stepwise reactions of Fe2O3, MnO2, and CuO in different temperature ranges, a stable temperature gradient is formed, ensuring uniform structural development of the green body during the stepwise self-sintering process, reducing internal defects, and improving the batch stability and performance consistency of the proppant. Attached Figure Description

[0023] Figure 1 This is a schematic flowchart of the preparation method in this invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and exemplary descriptions. It should be understood that the structures, proportions, sizes, etc., illustrated in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and are not intended to limit the implementation of this invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0025] Application Overview The industry typically addresses issues such as energy consumption control, solid waste resource utilization, performance balancing, and sintering process control in proppant preparation as follows: Regarding energy consumption, proppant sintering largely relies on external high-temperature heating processes. For example, ceramic proppant is calcined in a kiln to above 1200℃ to achieve molding, with external heat sources providing the energy required for sintering. Regarding solid waste utilization, the resource utilization of industrial slag, especially high-carbon slag, is mostly limited to low-value-added applications, such as use as roadbed filler or direct landfilling. Due to its high carbon content and complex composition, it is difficult to directly use it for high-performance proppant preparation. Regarding performance balancing, traditional proppants often employ a single structural design, optimizing porosity and strength by adjusting raw material ratios or sintering temperature, making it difficult to simultaneously meet both requirements. Regarding sintering process control, it relies on continuous input of external heat sources, achieving sintering by controlling the overall temperature within the furnace, lacking utilization of reaction heat and precise control of temperature gradients.

[0026] However, the above conventional solutions have significant shortcomings: high-temperature sintering requires a large amount of external energy, resulting in high production energy consumption and costs; the high-value utilization of high-carbon slag is limited, which not only wastes resources but also exacerbates solid waste accumulation and pollution; a single structural design cannot simultaneously meet the application requirements of high permeability (requiring a porous structure) and high mechanical strength (requiring a dense structure), resulting in a prominent contradiction in performance balance; temperature control methods that rely on external heat sources are prone to local overheating or incomplete reaction, leading to uneven development of the internal structure of the green body and poor stability of the proppant performance.

[0027] Comprehensive explanation This scheme aims to provide a method for preparing a low-energy-consumption proppant for slag self-sintering. Through core-shell structure design, a stepwise exothermic reaction of polymetallic oxides and carbon, and the graded utilization of industrial slag, it achieves low-energy preparation, high-performance regulation, and solid waste resource utilization of the proppant. The specific process route includes four key steps: preparation of carbon particles coated with kaolin, granulation of the inner porous green body, dense outer coating, and stepwise self-sintering. These steps work synergistically to form a proppant with specific structure and properties.

[0028] I. Preparation of Kaolin-Coated Carbon Particles This method first prepares carbon-core-kaolin-shell coated carbon particles, using graphite powder as the core carbon source and kaolin powder as the coating shell. Composite particles are formed through wet grinding and spray drying. Specifically, graphite powder and kaolin powder are mixed, and deionized water is added to prepare a slurry. The slurry is then wet-ground to uniformly disperse the kaolin particles and coat them onto the graphite powder surface. Finally, it is spray-dried to solidify and form a core-shell structured green body. In this structure, graphite powder serves as the key carbon source for the subsequent self-sintering reaction, while the kaolin shell layer regulates the carbon release rate. Its dense, layered structure delays the contact between carbon and oxides, preventing premature and excessive carbon reaction at low temperatures that could lead to heat concentration. Simultaneously, the high-temperature resistance of kaolin maintains the particle morphology during sintering, providing skeletal support for the formation of the inner porous structure.

[0029] II. Inner layer porous green body granulation Using high-carbon slag as the matrix material, a mixture of multi-metal oxide powder (containing Fe2O3, MnO2, and CuO) and the aforementioned kaolin-coated carbon particles is added, and then wet-granulated with an organic binder solution to form the inner green body. The high-carbon slag not only serves as the matrix framework, but its carbon content also forms a composite carbon source system together with the kaolin-coated carbon particles. The multi-metal oxide powder acts as the reactive component, with its total molar number of metal ions and the molar number of carbon in the composite carbon source configured in a specific ratio to ensure that the heat released in the subsequent reaction is sufficient to meet the self-sintering requirements. The addition of the organic binder promotes the agglomeration and formation of the raw material particles. During sintering, the high-carbon slag and coated carbon particles form numerous pores through carbon oxidation or reaction consumption, providing fluid permeation channels for the proppant. This step, through the synergistic effect of multiple components—matrix material, carbon source, oxides, and binder—achieves the initial formation of the inner green body and lays the foundation for the subsequent porous structure and self-sintering reaction.

[0030] III. Dense outer layer Using low-carbon slag as the matrix material, an inorganic binder is mixed to form a coating slurry. This slurry is then applied to the inner green body using a fluidized bed coating process, creating a core-shell structure. Low-carbon slag, due to its low carbon content and purer composition, is suitable as the matrix for the dense outer layer. Its fine particle size (75 μm) allows it to form a uniform slurry with the inorganic binder, which is then uniformly coated onto the surface of the inner green body in a fluidized bed. The inorganic binder enhances the density and structural stability of the outer layer through chemical bonding. This dense outer coating reduces heat loss from the inner layer reaction, improving heat utilization efficiency. Furthermore, it significantly enhances the overall mechanical strength of the proppant. Its continuous, dense structure can resist external pressure during fracturing, preventing the porous inner layer from collapsing due to insufficient strength. This achieves a performance balance between the porous (high permeability) inner layer and the dense (high strength) outer layer of the proppant.

[0031] IV. Stepped self-sintering The core-shell structured green body undergoes a stepped self-sintering process, sequentially involving preheating to create pores, a stepwise exothermic reaction between polymetallic oxides and carbon, and natural cooling and sieving to obtain the final proppant. During sintering, the temperature is gradually increased from 600℃ to 1000℃. The selective reactions of Fe2O3, MnO2, CuO, and carbon with carbon in different temperature ranges achieve self-supplied heat and temperature gradient control: in the 600-700℃ range, CuO first reacts with carbon (2CuO+C=2Cu+CO2), releasing heat and completing the initial temperature rise; in the 700-800℃ range, MnO2 reacts with carbon (MnO2+C=MnO+CO), continuously providing heat; in the 800-1000℃ range, Fe2O3 undergoes a major exothermic reaction with carbon (Fe2O3+3C=2Fe+3CO), releasing a large amount of heat to maintain high-temperature sintering. This stepped reaction system does not require continuous external heating and can meet the sintering requirements through self-supply of reaction heat, significantly reducing energy consumption. At the same time, the stepwise reaction in different temperature ranges can form a stable temperature gradient, avoiding structural defects caused by local overheating. Combined with the pores generated by the decomposition of organic binder in the preheating stage, a support with uniform structure and stable performance is finally formed.

[0032] Controlling the cooling rate during natural cooling avoids internal stress cracking caused by rapid cooling; the proppant particles obtained after sieving have uniform particle size, and their core-shell structure and porous-dense composite characteristics can simultaneously meet the dual requirements of permeability and mechanical strength for oil and gas field fracturing. Furthermore, the graded utilization of high-carbon and low-carbon slag in the raw materials realizes the high-value-added transformation of industrial solid waste and reduces the dependence on natural raw materials.

[0033] To verify the regulatory effect of key parameters on proppant performance in this scheme, the oil and gas industry standards SY / T 5108-2014 "Test Method for Performance of Proppants for Hydraulic Fracturing and Gravel Packing Operations" and SY / T 6302-2019 "Test Method for Conductivity of Fracturing Proppants" were adopted. Comparative experiments were designed using the controlled variable method to systematically evaluate the technical significance of the parameter limitation range and the practical application value of this scheme.

[0034] I. Experimental Design Basis and Testing Methods 1. Testing Standards and Methods Breakage resistance: According to ISO 13503-2:2024 Clause 4.4.2, the breakage rate (%) was determined using a PropPaver stainless steel loading device under a closing pressure of 69MPa. The calculation formula is: Breakage rate = (mass of particles <425m after the experiment / mass of the initial sample) × 100%. The lower the value, the stronger the breakage resistance.

[0035] Conductivity: Referring to SY / T 6302-2019, the conductivity (m) of the proppant-filled layer was measured under the conditions of a closing pressure of 35MPa and a temperature of 25℃. 2 (cm), the higher the value, the better the fluid permeability.

[0036] Acid solubility: According to Chapter 8 of SY / T 5108-2014, the specific steps are as follows: Weigh 5.000g of the proppant sample that has been dried to constant weight at 105℃; Prepare the acid solution (hydrochloric acid: hydrofluoric acid = 12:3, 361mL 37% hydrochloric acid + 46.23g ammonium bifluoride, bring the volume to 1000mL). The sample was reacted with 100 mL of acid solution in a 66 ° C water bath for 30 min, and then vacuum filtered and washed with distilled water until neutral. Dry at 105℃ to constant weight, and calculate the acid solubility (%) = (mass before reaction - mass after reaction) / mass before reaction 100%. The lower the value, the better the chemical stability.

[0037] 2. Variable Design and Scope Limitation Variable 1: Amount of carbon particles coated with kaolin (as a percentage of the mass of the inner green body raw material), with a range of 5-15%; Variable 2: Multimetal oxide ratio (Fe2O3:MnO2:CuO, molar ratio), limited range: 5:3:2 to 3:4:3; Variable 3: Outer layer thickness (mm), range: 0.2-0.6mm.

[0038] II. Experimental Scheme 1. Division of experimental groups Conventional group (groups 1-5): All variables are within the specified range, and the preparation process of this scheme (core-shell structure + self-sintering) is adopted. Control group (groups 6-9): The variables exceeded the specified range, and the preparation process of this scheme was used; Blank control group (10 groups): Existing technology (traditional ceramic support, bauxite raw material + external sintering).

[0039] 2. Material and Environmental Parameters Inner matrix: High-carbon slag (fixed proportion 60%); Outer matrix: Low-carbon slag (fixed proportion 90%). Sintering regime: stepped heating (600-800-1000℃, each holding for 2 hours); All other parameters (granulation pressure, binder dosage, etc.) remain the same.

[0040] III. Experimental Results and Data (Table 1) Table 1. Experimental Group Data Table Serial Number Kaolin-coated carbon particle addition amount (%) <![CDATA[Multi-metal oxide ratio (Fe2O3:MnO2:CuO)]]> Outer layer thickness (mm) Crushing resistance (crush rate %) <![CDATA[Flow conductivity (m 2 cm)]]> Acid solubility (%) Overall weighted score 1 5.0 5:3:2 0.2 4.20 48.50 3.10 88.90 2 8.0 4:3:3 0.3 3.80 52.30 2.80 91.70 3 10.0 3:4:3 0.4 3.20 55.80 2.50 95.60 4 12.0 4:4:2 0.5 3.50 53.10 2.70 93.50 5 15.0 3:3:4 0.6 4.00 49.70 2.90 89.80 6 3.0（<5%） 5:3:2 0.4 5.80 45.20 3.80 80.50 7 18.0（>15%） 3:4:3 0.4 4.50 42.60 3.20 84.20 8 10.0 8:1:1 0.4 6.20 50.30 4.10 78.90 9 10.0 3:4:3 0.8 (>0.6mm) 3.00 38.90 2.60 86.30 10 0 (blank control) 0 (No polymetallic oxides) 0 (Uncovered) 8.50 32.10 5.50 63.70 IV. Weighted Scoring Mechanism Based on the first-grade standard of petroleum industry proppant (5% anti-breakage rate, 50m flow capacity) 2 (cm, acid solubility 4%), weighted comprehensive score: Breakage resistance (weight 40%): Score = 100 - (measured breakage rate / 5%) × 1000.4; Traffic diversion capacity (weight 30%): Score = (Measured traffic diversion capacity / 50) × 1000.3; Acid solubility (weight 30%): Score = 100 - (measured acid solubility / 4%) 1000.3.

[0041] V. Experimental Conclusions The overall performance of the conventional group (groups 1-5) was significantly better than that of the control group and the blank control group, which verified the rationality of the parameter limitation range; In the control group, variables exceeding the range led to at least one performance degradation (e.g., acid solubility in control group 6 increased to 3.8%, and conductivity in control group 9 decreased to 38.9 m). 2 cm); The highest overall score was in Group 3 (95.60 points), indicating a non-linear relationship between performance and variables, which confirms the complexity of the synergistic effect of multiple components.

[0042] Based on the above experimental results, the conventional groups (groups 1-5) showed significant advantages in the three core performance indicators of anti-fracture ability, conductivity, and acid solubility. Their comprehensive weighted scores were generally higher than those of the control group and the blank control group, and the best-performing group 3 exhibited nonlinear optimization characteristics. To further reveal the intrinsic relationship between parameter regulation and performance improvement, the following will systematically analyze the changing trends and underlying causes of each performance indicator by combining the material's microstructure and molecular-level reaction mechanisms.

[0043] I. Resistance to breakage: The synergistic effect of crystal structure integrity and interfacial bonding strength The core influencing factors of the proppant's resistance to breakage (breakage rate) are the microstructure density and crystal phase stability of the proppant. Its molecular mechanism can be analyzed from two aspects: the carbothermic reduction reaction products and the phase transformation of kaolin. The strengthening effect of carbothermic reduction products: During the stepwise heating process, polymetallic oxides (Fe₂O₃, MnO₂, CuO) undergo a stepwise reduction reaction with the carbon source. The resulting elemental metals (Fe, Mn, Cu) and low-valence oxides (MnO) can fill the lattice defects in the slag matrix, forming a solid solution-dispersion-strengthened phase composite structure. In the conventional group, when the polymetallic oxide ratio is 3:4:3 (Group 3), the reduction products of Fe₂O₃ (reduction at 550-700℃), MnO₂ (reduction at 650-800℃), and CuO (reduction at 400-500℃) form a gradient distribution in time and space. Metal atoms diffuse into the network structure of the silicate glass phase, enhancing the connection strength of Si-O and Al-O bonds, thus improving the integrity of the crystal structure and reducing the breakage rate to 3.20%. In contrast, the extreme ratio (8:1:1) in control group 8 resulted in an excess of Fe2O3. The Fe element generated by rapid reduction at high temperature aggregated into coarse particles, which became stress concentration points, and the breakage rate increased to 6.20%.

[0044] Interface regulation of carbon-coated kaolin: Kaolin (Al2Si2O5(OH)4) undergoes a dehydration phase transition at 600-1000℃ to generate metakaolin (Al2Si2O7) and mullite (3Al2O32SiO2). When the carbon-coated kaolin particles added are 10.0% (Group 3), the CO / CO2 gas slowly released from the carbon core forms uniformly distributed nanoscale pores inside the particles, while the mullite crystals generated by the phase transition of the kaolin shell form a framework-pore support structure around the pores. This avoids the strength reduction caused by excessive pores and enhances the interfacial bonding force through crystal interlocking. In control group 6 (addition amount 3.0%), due to insufficient carbon source, the mullite crystals did not grow sufficiently, resulting in reduced interfacial bonding strength and a breakage rate of 5.80%. In control group 7 (addition amount 18.0%), excessive carbon caused gas to escape, forming through-pores, which destroyed the structural integrity and increased the breakage rate to 4.50%.

[0045] II. Flow conductivity: Molecular-level regulation of pore structure and pore size distribution The conductivity depends on the number, size, and uniformity of the interconnected pores within the proppant, and its molecular mechanism is directly related to the carbon source decomposition behavior and the thickness of the outer coating. Pore ​​formation mechanism of carbon source decomposition: Graphite powder (layered sp) in carbon particles coated with kaolin. 2The hybrid carbon structure gradually decomposes at high temperatures through surface oxidation and interlayer exfoliation, releasing gases that form pores within the green body. In the conventional group, when the outer coating thickness is 0.4 mm (Group 3), the dense outer low-carbon slag shell layer can suppress excessively rapid gas escape, creating a gradient distribution of pores between the inner and outer layers. The inner layer (high-carbon slag matrix) is dominated by interconnected pores of 10-50 μm (facilitating fluid permeation), while the outer layer (low-carbon slag shell layer) is dominated by closed pores of <5 μm (maintaining structural strength), achieving a flow conductivity of 55.80 μm. 2 cm. In control group 9 (coating thickness 0.8 mm), the outer layer was too thick, making it difficult for gas to escape, resulting in a decrease in the porosity of the inner layer. The pore size distribution was concentrated in the <10 μm range, and the conductivity decreased to 38.90 μm. 2 cm; the blank control group (uncoated) had a conductivity of only 32.10m due to the lack of directional porosity in traditional sintering. 2 cm.

[0046] Synergistic effect of polymetallic oxides in pore regulation: The heat released by the reduction reaction of polymetallic oxides raises the local temperature of the matrix, promoting the flow and filling of silicate melt, and indirectly regulating pore morphology. When the ratio is 3:4:3 (Group 3), MnO (melting point 1650℃) generated by the reduction of MnO2 and Cu (melting point 1083℃) generated by the reduction of CuO form a low-melting-point eutectic phase (MnO-Cu2O, melting point about 950℃), which fills some of the ineffective micropores (<2m) during the sintering stage at 1000℃, while retaining effective permeable pores (10-50m), further optimizing the pore size distribution; while in control group 8 (extreme ratio 8:1:1), due to the excess of Fe2O3, the Fe3O4 (melting point 1597℃) generated by reduction is difficult to form a eutectic phase, the proportion of ineffective micropores increases, and the conductivity is only 50.30m. 2 cm.

[0047] III. Acid Solubility: Molecular-Level Optimization of Chemical Stability and Phase Composition Acid solubility reflects the proppant's ability to resist corrosion by acidic fracturing fluids. Its core lies in the content of easily soluble phases (such as free CaO and MgO) in the material and the chemical inertness of the crystalline phase. The molecular mechanism is closely related to the composition of the slag matrix and the sintering phase transformation. Composition control of slag matrix: The graded utilization of high-carbon slag (inner layer) and low-carbon slag (outer layer) can reduce the content of easily soluble phases. In the conventional group, after carbothermic reduction, free CaO in the high-carbon slag reacts with CO2 to form CaCO3 (acid solubility <1%), while SiO2 and Al2O3 in the low-carbon slag combine with mullite during sintering to form a stable aluminosilicate network (Si-O-Al bond energy is approximately 452 kJ / mol, far higher than the 346 kJ / mol of the Ca-O bond), and the acid solubility can be controlled at 2.50% (Group 3). In control group 6 (kaolin-coated carbon particle addition 3.0%), due to insufficient carbon source and incomplete CaO reduction, the free CaO residue increased to 3.2%, and the acid solubility increased to 3.80%. The blank control group (bauxite raw material) contained more Fe2O3 impurities (easily reacting with acid to form Fe... 3+ Its acid solubility is as high as 5.50%.

[0048] Chemical inertness of polymetallic oxide reduction products: In the conventional group, the reduced metallic elements (Fe, Mn, Cu) and low-valence oxides (MnO) of polymetallic oxides exhibit excellent chemical stability and are not easily dissolved or subjected to ion exchange reactions in acidic environments. When the ratio is 3:4:3 (Group 3), MnO and Al2O3 form a stable MnAl2O4 spinel phase (acid solubility <0.5%), further reducing the overall acid solubility; while in control group 8 (extreme ratio 8:1:1), due to the excess of Fe2O3, the reduced Fe3O4 readily reacts with hydrochloric acid to form FeCl2 (a soluble salt), increasing the acid solubility to 4.10%.

[0049] IV. Molecular Mechanism of Nonlinear Optimization of Comprehensive Performance (Taking Group 3 as an Example) Group 3 (kaolin-coated carbon particle addition of 10.0%, multi-metal oxide ratio of 3:4:3, outer coating thickness of 0.4 mm) exhibits the best overall performance, and its synergistic effect at the molecular level is reflected in: Carbon source release and phase change matching: 10.0% coated carbon particles slowly release carbon, forming a step-by-step exothermic process with the 3:4:3 ratio of polymetallic oxides, from low temperature (CuO reduction) to medium temperature (MnO2 reduction) to high temperature (Fe2O3 reduction). The heat release rate is highly matched with the kinetic requirements of kaolin phase change (600℃ dehydration, 900℃ mullite formation) and slag sintering (800-1000℃ glass phase formation), avoiding structural defects caused by local overheating or insufficient heat. Gradient synergy between interface and pores: The outer layer coating thickness of 0.4 mm creates a continuous transition interface between the inner porous structure (flow conduction) and the outer dense structure (strength). Mullite crystals grow directionally at the interface (length-to-diameter ratio of about 5:1), which enhances the interfacial bonding strength (fracture resistance) and maintains pore connectivity (flow conduction). Thermodynamic stability of phase composition: The reduction products of polymetallic oxides (Fe, Mn, Cu elements and MnO, MnAl2O4 phases) form a thermodynamically stable composite system with the aluminosilicate network, reducing the content of easily soluble phases (acid solubility), while the dispersed distribution of metal elements further strengthens the structure (resistance to breakage).

[0050] When the variables exceed the specified range, the aforementioned synergistic effect at the molecular level is disrupted: for example, excessive coating of carbon particles (18.0%) leads to the formation of a weak interface due to carbon residue, excessively thick outer coating (0.8 mm) hinders the formation of pores, and the imbalance of the multi-metal oxide ratio disrupts the stepwise exothermic process, ultimately resulting in a significant decrease in performance parameters, which confirms the scientific validity and necessity of limiting the parameter range.

[0051] Exemplary Description Example 1 1. Preparation of inner green body Weigh the following raw materials according to parts by weight: High-carbon slag: 80 parts; Kaolin-coated carbon particles: 5 parts; Polymetallic oxides: 10 parts (of which 5 parts Fe2O3, 3 parts MnO2, and 2 parts CuO, molar ratio 5:3:2); Adhesive (polyvinyl alcohol): 3 parts; Deionized water: 12 parts.

[0052] The above raw materials were added to a planetary ball mill and mixed at 300 r / min for 2 hours to obtain a uniform inner layer slurry. The slurry was placed in a mold and wet granulated under a granulation pressure of 20 MPa to obtain inner layer green particles with a diameter of 5 mm.

[0053] 2. Outer coating Weigh out the outer layer matrix raw material according to the following mass fractions: Low-carbon slag: 90 parts (accounting for 90% of the total mass of the outer matrix); Binder (methylcellulose): 2 parts; Deionized water: 8 parts.

[0054] After being mixed evenly, an outer coating slurry is prepared. The slurry is then uniformly coated onto the surface of the inner green body particles using a casting method. By controlling the number of coatings (2 times) and the drying time (drying at 60℃ for 30 minutes), the outer coating thickness reaches 0.2 mm, resulting in a core-shell structure green body.

[0055] 3. Sintering treatment The core-shell structured green body was placed in a box-type sintering furnace, and a stepped heating program was executed: At room temperature of 600℃: heating rate 5℃ / min, hold for 2 hours (to remove adhesive and moisture); 600℃ to 800℃: heating rate 3℃ / min, hold for 2 hours (carbothermic reduction reaction stage); 800℃-1000℃: Heating rate 2℃ / min, hold for 2 hours (crystal phase formation stage); After naturally cooling to room temperature, the final proppant product is obtained.

[0056] 4. Performance Test Results The test results, conducted according to SY / T 5108-2014 and SY / T 6302-2019 standards, are as follows: Crushing resistance: Crushing rate 4.20%; Diversion capacity: 48.50m 2 cm; Acid solubility: 3.10%; Overall weighted score: 88.90.

[0057] Example 2 The differences between this embodiment and Embodiment 1 are as follows: the amount of kaolin-coated carbon particles added is 8.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 4:3:3; and the outer coating thickness is 0.3 mm.

[0058] Example 3 The differences between this embodiment and Embodiment 1 are as follows: the amount of kaolin-coated carbon particles added is 10.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 3:4:3; and the outer coating thickness is 0.4 mm.

[0059] Example 4 The differences between this embodiment and Embodiment 1 are as follows: the amount of kaolin-coated carbon particles added is 12.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 4:4:2; and the outer coating thickness is 0.5 mm.

[0060] Example 5 The differences between this embodiment and Embodiment 1 are as follows: the amount of kaolin-coated carbon particles added is 15.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 3:3:4; and the outer coating thickness is 0.6 mm.

[0061] Example 6 The difference between this embodiment and Embodiment 1 is that the amount of kaolin-coated carbon particles added is 3.0 parts (lower than the lower limit of the specified range); the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 5:3:2; and the outer coating thickness is 0.4 mm.

[0062] Example 7 The difference between this embodiment and Embodiment 1 is that the amount of kaolin-coated carbon particles added is 18.0 parts (exceeding the upper limit of the specified range); the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 3:4:3; and the outer coating thickness is 0.4 mm.

[0063] Example 8 The differences between this embodiment and Embodiment 1 are as follows: the amount of kaolin-coated carbon particles added is 10.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 8:1:1 (extreme ratio, exceeding the limit); and the outer coating thickness is 0.4 mm.

[0064] Example 9 The difference between this embodiment and Embodiment 1 is that the amount of kaolin-coated carbon particles added is 10.0 parts; the ratio of multi-metal oxides (Fe2O3:MnO2:CuO) is 3:4:3; and the outer coating thickness is 0.8 mm (exceeding the upper limit of the specified range).

[0065] Example 10 (Blank Control Group) 1. Weighing and mixing raw materials Weigh out 100 parts by weight of existing conventional ceramic proppant raw materials: Bauxite (main component Al2O3, content 85%): 85 parts; Quartz sand (SiO2 content 98%): 10 parts (to adjust the silicon-aluminum ratio and promote the formation of mullite phase); Adhesive (polyvinyl alcohol): 3 parts; Deionized water: 12 parts.

[0066] The above raw materials were added to a double planetary mixer and dry-mixed at 250 r / min for 30 min, then deionized water was added and wet-mixed for 1.5 hours to obtain a uniform traditional ceramic proppant mixture slurry.

[0067] 2. Molding The mixed slurry is placed in a circular mold and cold-pressed under a granulation pressure of 15 MPa to obtain solid cylindrical green pellets with a diameter of 5 mm (without core-shell structure and no inner / outer layer distinction).

[0068] 3. Sintering treatment (external sintering) The green pellets are placed in a conventional box-type sintering furnace and an external sintering process is performed (without a self-sintering mechanism, densification is achieved solely through high-temperature solid-phase reaction): At room temperature of 600℃: heating rate 5℃ / min, hold for 1 hour (to remove adhesive and moisture); 600℃-1200℃: heating rate 4℃ / min, holding time 3 hours (main sintering stage, promoting the reaction of Al2O3 and SiO2 to form mullite phase); Naturally cooled to room temperature, the existing conventional ceramic support product is obtained.

[0069] 4. Performance Test Results The test results, conducted according to SY / T 5108-2014 and SY / T 6302-2019 standards, are as follows: Crushing resistance: Crushing rate 8.50%; Diversion capacity: 32.10m 2 cm; Acid solubility: 5.50%; Overall weighted score: 63.70.

[0070] Specific work process Please refer to Figure 1 The inner layer raw materials are mixed to form a uniform slurry, which is then molded to obtain inner layer green body particles. The outer layer raw materials are mixed to form a coating slurry, which is then coated onto the surface of the inner layer green body and dried to form a core-shell structure green body. During the stepped heating process, the multi-metal oxides and carbon source undergo a stepwise carbothermic reduction reaction to generate elemental metals and low-valence oxides. Metal atoms diffuse into the silicate glass phase network structure. Simultaneously, kaolin undergoes a dehydration phase transformation to generate metakaolinite and mullite. Mullite crystals form a skeletal support structure around the pores. At high temperature, the metal reduction products combine with the silicate melt to form a solid solution-dispersion-strengthened phase composite structure. Low-valence oxides react with the aluminosilicate network to generate a stable spinel phase. Finally, the proppant product is obtained through a cooling process.

[0071] The technical features described above can be combined in any way. For the sake of brevity, not all possible combinations of the technical features described above are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for preparing a low-energy-consumption slag self-sintering proppant, characterized in that, Includes the following steps: (1) Preparation of kaolin-coated carbon particles: Graphite powder and kaolin powder are mixed, and carbon particles with carbon core-kaolin shell structure are obtained by wet grinding and spray drying, wherein the thickness of the kaolin layer is 5-15m; (2) Inner layer porous green body granulation: using high carbon slag as the matrix raw material, high carbon slag, polymetallic oxide mixed powder and kaolin-coated carbon particles obtained in step (1) are mixed by mass fraction, and an inner layer organic binder solution is added for wet granulation to form an inner layer green body; the ratio of the total number of moles of metal ions in the polymetallic oxide mixed powder to the sum of the number of moles of carbon in the kaolin-coated carbon particles and the number of moles of carbon in the high carbon slag is 1:1.2-1.5; (3) Dense outer layer coating: Low carbon slag is used as the outer layer matrix material. Low carbon slag and inorganic binder are mixed and an outer layer inorganic binder solution is added to prepare a coating slurry. The inner layer green body of step (2) is fluidized bed coated to form a core-shell structure green body. (4) Stepped self-sintering: The core-shell structure green blank is preheated to create holes, and then self-sintered by a stepped exothermic reaction of polymetallic oxides and carbon. The support is obtained by natural cooling and sieving. The stepped self-sintering temperature range is 600-1000℃. Temperature gradient control is achieved by the stepwise reaction of Fe2O3, MnO2, CuO and carbon in different temperature ranges.

2. The preparation method of the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The preparation parameters for the kaolin-coated carbon particles in step (1) are as follows: by mass, 8-10 parts of graphite powder and 1-2 parts of kaolin powder are added to deionized water to prepare a slurry with a solid content of 30-35%; wet grinding is carried out using a sand mill with zirconium beads of 0.5 mm diameter, a rotation speed of 1800-2200 r / min, and a grinding time of 25-35 min; spray drying is carried out with an inlet temperature of 170-190℃, an outlet temperature of 75-85℃, and an atomization pressure of 0.2-0.4 MPa to obtain coated carbon particles with a particle size of 80-120 μm.

3. The preparation method of the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The raw materials for the inner porous green body mentioned in step (2) are as follows, in dry mass parts: 75-85 parts of high carbon slag, 5-8 parts of polymetallic oxide mixed powder, and 5-15 parts of kaolin-coated carbon particles; the polymetallic oxide mixed powder contains 4-6 parts of Fe2O3 powder, 0.5-1.5 parts of MnO2 powder, and 0.3-0.5 parts of CuO powder.

4. The preparation method of the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The inner layer organic binder solution mentioned in step (2) is a starch solution with a mass concentration of 7-9%, which is added at a mass ratio of 1:0.7-0.9 between the inner layer dry material and the starch solution. The wet granulation adopts a rotary drum granulator with a drum tilt angle of 2-4, a rotation speed of 20-30 r / min, and a granulation time of 10-20 min to obtain an inner layer green body with a particle size of 3-5 mm.

5. The method for preparing the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The raw materials for the outer dense coating mentioned in step (3) are as follows, based on dry material mass parts: 90-98 parts of low-carbon slag and 2-8 parts of sodium silicate solid (Na2OnSiO2 pure mass parts), with a total of 100 parts; the outer inorganic binder solution is a sodium silicate dilution with a mass concentration of 10-15%, the modulus of which is 3.2-3.3, and is added according to the mass ratio of inner green body to coating slurry of 1:0.4-0.

6.

6. The method for preparing the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The parameters for fluidized bed coating in step (3) are: bed temperature 55-65℃, air volume 0.4-0.6m³ / h. 3 The coating time is 25-35 min, forming an outer coating layer with an average thickness of 50-100 μm; the core-shell structure green body is dried at 80-100℃ for 2-3 h with a moisture content of 5%.

7. The method for preparing the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The step-by-step self-sintering process described in step (4) is as follows: first, preheating at 300-500℃ for 1-2 hours with a heating rate of 4-6℃ / min, then holding at 600-700℃ for 0.5-1 hours, during which CuO reacts with carbon; holding at 700-800℃ for 0.5-1 hours, during which MnO2 reacts with carbon; holding at 800-1000℃ for 1.5-2.5 hours, during which Fe2O3 reacts with carbon; the total sintering time is 2.5-4 hours, and the atmosphere is air with an oxygen content of 20-22%.

8. The method for preparing the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The high-carbon slag has a carbon content of 7-11%, a particle size of 150 μm, and a D50 of 40-60 μm; the low-carbon slag has a carbon content of 0.5-2.5%, a particle size of 75 μm, and a D50 of 15-25 μm; the Fe2O3 powder has a purity of 95%, the MnO2 powder has a purity of 90%, and the CuO powder has a purity of 90%, with all three having a particle size of 5 μm.

9. The preparation method of the slag self-sintering low-energy-consumption support as described in claim 1, characterized in that: The natural cooling rate in step (4) is 10℃ / min; the sieving uses a 20-40 mesh screen, and the particle size of the undersize material is 0.45-0.9mm; The graphite powder in step (1) has 98% fixed carbon and a particle size of 50-100 μm; the kaolin powder has a purity of 95%, a particle size of 1-5 μm and a D50 of 1-3 μm, and the kaolin powder coating carbon particles has a mass coating rate of 90%.

10. The preparation method of the slag self-sintering low-energy-consumption support as described in claim 4, characterized in that: The corn starch in the starch solution has a purity of 99%.

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