A method for reducing and decomposing citric acid gypsum using endogenous organic matter

Abstract

The application discloses a method for reducing and decomposing citric acid gypsum by using endogenous organic matter, and belongs to the technical field of chemical solid waste treatment and resource recycling. The method is characterized in that the organic impurities such as citric acid residue and mycelium in the citric acid gypsum are used as endogenous reducing agents to construct a binary reduction system together with added sulfur. By measuring the organic carbon content in the raw material, the proportion of sulfur incorporation is accurately controlled, the organic matter is used for preferential carbonization pore forming and in-situ reduction, the sulfur consumption is significantly reduced, and the reaction efficiency is improved. The application not only realizes the high-value utilization of citric acid gypsum, but also prepares high-purity calcium oxide and high-concentration sulfur dioxide, and compared with the traditional process, the operation cost is significantly reduced, and the economic benefit is remarkable.

Description

Technical Field

[0001] This invention relates to the field of chemical solid waste treatment and resource recycling technology, specifically to a method for reducing and decomposing citric acid gypsum using endogenous organic matter. Background Technology

[0002] Citric acid gypsum is the main solid waste generated during the industrial production of citric acid. In the process of producing citric acid from starchy raw materials such as corn and cassava through Aspergillus niger fermentation, approximately 1.5 to 1.8 tons of by-product gypsum (calculated as calcium sulfate dihydrate) are produced for every ton of citric acid produced. Unlike phosphogypsum, although citric acid gypsum has lower phosphorus and fluorine content among impurities, it contains a large amount of organic impurities originating from the fermentation process (such as unreacted sugars, proteins, mycelium, and residual citric acid liquid). The total organic carbon (TOC) content of these organic impurities is typically between 0.5% and 5.0%. Current technologies typically treat gypsum using high-temperature calcination or coke reduction methods.

[0003] In existing technologies, the resource utilization of industrial by-product gypsum mainly follows the following technical paths: (1) Building material application: production of paper-faced gypsum board, cement retarder, etc. However, the organic matter in citric acid gypsum will affect the setting time and strength of gypsum, and the organic matter in a humid state is prone to mold growth, which limits its application in building materials.

[0004] (2) High-temperature calcination to produce calcium oxide: Gypsum is directly calcined at high temperature (>1300℃), and calcium sulfate decomposes into calcium oxide and sulfur dioxide. This route has extremely high energy consumption and poor economic efficiency.

[0005] (3) Coke reduction method: Coke is added as a reducing agent to reduce calcium sulfate to calcium oxide and produce SO2 as a byproduct. This route has many problems. The ash content of coke is usually 10~15%, and the main components of the ash are SiO2 and Al2O3, which will contaminate the product. The whiteness of the prepared calcium oxide is usually <75, which cannot be used in high value-added fields. When coke and gypsum are not mixed evenly, local over-reduction is likely to occur, generating CaS odorous substances. Adding an external carbon source increases the cost of raw materials.

[0006] (4) Sulfur reduction method: Sulfur is used as a reducing agent. This route produces products with high purity, but the reaction is sensitive to the atmosphere.

[0007] Several existing patents relate to the process of sulfur reducing gypsum. For example, patent CN101708825B discloses a method for preparing calcium sulfide by reacting gaseous sulfur with gypsum under an inert atmosphere; however, it uses pure sulfur as the sole reducing agent, the target product is calcium sulfide rather than calcium oxide, and it does not involve any gypsum raw materials containing organic matter. Patents CN111574079A and CN111574080A disclose a process for preparing sulfoaluminate cement and co-producing sulfuric acid by decomposing gypsum using gaseous sulfur as a reducing agent and FeS2 from high-sulfur bauxite as an auxiliary reducing agent; however, the FeS2 in this process comes from added bauxite and is not an endogenous organic compound in gypsum, and the target product is cement rather than calcium oxide. Regarding the co-processing of gypsum with organic matter, patent CN119951855B discloses a process for co-processing arsenic-containing desulfurized gypsum slag with added organic solid waste and pyrite, with the goal of removing arsenic and recovering white arsenic. Although this patent involves organic matter participating in gypsum treatment, it uses added organic solid waste as an external carbon source, and the treatment target is arsenic-containing desulfurized gypsum.

[0008] In summary, existing sulfur reduction technologies do not consider utilizing the organic matter inherent in gypsum itself as a reducing agent, and existing co-treatment technologies for organic solid waste use added organic matter rather than the endogenous organic matter in gypsum. There are no reports in existing publicly available technologies of utilizing the organic matter in citric acid gypsum as a reducing agent; it is generally considered an undesirable impurity, believed to cause coking in kilns or difficulties in exhaust gas treatment. Therefore, attempts are often made to remove this organic matter through methods such as water washing, flotation, and high-temperature pre-calcination, but these methods are costly and generate secondary wastewater / exhaust gas pollution. Summary of the Invention

[0009] In order to overcome the deficiencies of the prior art described in the background art, the present invention provides a method for reducing and decomposing citric acid gypsum using endogenous organic matter.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is to provide a method for reducing and decomposing citric acid gypsum using endogenous organic matter, comprising the following steps: S1: The citric acid gypsum was dried to obtain dry-basis citric acid gypsum; then the calcium sulfate content and total organic carbon content M of the dry-basis citric acid gypsum were determined. TOC ; S2: Calculate the theoretical value M0 of sulfur required for reducing citric acid gypsum based on the calcium sulfate content, and then calculate the actual amount of sulfur required for reducing citric acid gypsum according to the formula shown in (I): M S =K×(M0-α×1000×M TOC (I) Among them, M SM0 represents the actual amount of sulfur required to reduce citric acid gypsum; α represents the theoretical value of sulfur required to reduce citric acid gypsum; α represents the substitution coefficient of organic carbon for sulfur, ranging from 0.8 to 1.2; K represents the excess coefficient, ranging from 1.05 to 1.2. S3: Preheat carbonize the dry-based citric acid gypsum to convert the endogenous organic matter in the dry-based citric acid gypsum into activated coke and form a porous structure inside the citric acid gypsum; then add the actual amount of sulfur powder, mix well and perform thermal reduction treatment to reduce and decompose the citric acid gypsum.

[0011] Based on the above technical solution, the present invention can be further improved as follows.

[0012] Furthermore, the drying method for citric acid gypsum in S1 is as follows: drying at 45~55℃ to constant weight.

[0013] Furthermore, the calcium sulfate content in the dry-basis citric acid gypsum is 90%~95%, and the total organic carbon content is M. TOC It ranges from 1% to 5%.

[0014] Furthermore, the theoretical value M0 of sulfur required for the reduction of citric acid gypsum in S2 was calculated using the following steps: S21: Calculate the amount of sulfur required to reduce pure calcium sulfate based on the reaction equation shown in (II); CaSO4 + 0.5S → CaO + 1.5SO2 (II) S22: Multiply the amount of sulfur calculated in S21 by the calcium sulfate content determined in S1 to obtain the theoretical value M0 of sulfur required for reducing citric acid gypsum.

[0015] Furthermore, in S2, α takes the value of 1.0; and K takes the value of 1.1.

[0016] Furthermore, the temperature for preheating and carbonization in S3 is 300~600℃, and the preheating and carbonization time is 10~20min.

[0017] Furthermore, the temperature of the thermal reduction treatment in S3 is 900~1100℃, and the time of the thermal reduction treatment is 30~60min.

[0018] Furthermore, the temperature of the preheating carbonization treatment in S3 is 500℃, and the preheating carbonization treatment time is 15min; the temperature of the thermal reduction treatment is 1000℃, and the thermal reduction treatment time is 50min.

[0019] The beneficial effects of this invention are: 1. This invention establishes a quantitative batching model based on organic carbon equivalent. Traditional sulfur reduction methods for citric acid gypsum do not consider the contribution of organic matter in the raw materials, leading to excessive sulfur addition or improper control of the reducing atmosphere. This invention, for the first time, establishes a quantitative relationship between the amount of sulfur added and the total organic carbon content of the citric acid gypsum raw materials (Formula I above). It utilizes the endogenous organic matter in citric acid gypsum to replace part of the expensive sulfur, saving approximately 10-50 kg of sulfur per ton of product (the specific saving depends on the total organic carbon content of the raw materials). This results in a saving of 187-372 yuan in sulfur costs per ton of gypsum, demonstrating significant economic benefits.

[0020] 2. The method for decomposing citric acid gypsum in this invention includes preheating carbonization and high-temperature reduction. The preheating carbonization temperature ranges from 300 to 600°C. At this temperature, endogenous organic matter such as residual citric acid and mycelium undergoes thermal decomposition. Unlike externally added coke, these organic matter exists in a molecularly dispersed state between gypsum particles. The reducing gases such as H2, CO, and CH4 generated by the decomposition first undergo a preliminary reduction reaction with calcium sulfate in the local microenvironment. Simultaneously, activated coke is generated in situ after the organic matter is carbonized, leaving a large number of micron-sized pores in the space originally occupied by the organic matter. This process forms a honeycomb-like porous structure, transforming the originally dense gypsum particles into a sponge-like porous aggregate. This porous structure provides ideal micro-reaction channels for the subsequent permeation and diffusion of sulfur vapor, significantly reducing the mass transfer resistance of the gas-solid reaction. The high-temperature reduction operates within a temperature range of 900–1100°C, where sulfur vaporizes into sulfur vapor. Unlike traditional sulfur reduction methods, the sulfur vapor does not react with dense gypsum particles. Instead, it rapidly penetrates the gypsum particles through porous channels formed by preheating and carbonization, reacting with CaSO4 on the vast internal surface area. Simultaneously, the activated coke generated during preheating and carbonization also participates in the reaction as a solid-phase reducing agent. The gaseous sulfur and solid activated coke form a synergistic enclosure in space, ensuring the complete reduction of CaSO4 to CaO. The key advantage of preheating and carbonization combined with high-temperature reduction lies in the porous structure formed by the carbonization of organic matter, which provides micro-reaction channels for sulfur vapor to flow from the particle surface to the interior, significantly reducing gas-solid mass transfer resistance. This results in a higher reaction rate at the same temperature, facilitating the expansion of the reaction from the particle surface to the interior. Compared to traditional sulfur reduction methods involving direct reaction of dense particles, the reaction time of this invention can be shortened by 10%–20%.

[0021] 3. Traditional coke reduction methods suffer from low whiteness and purity of calcium oxide due to the 10%–15% SiO2 and Al2O3 ash content in the coke, which contaminates the product and makes it unsuitable for food-grade or high-value-added applications. This invention uses sulfur as an external reducing agent. After complete carbonization of endogenous organic matter at high temperatures, very few residues remain. Therefore, the purity of the solid product, calcium oxide, depends solely on the purity of the raw material gypsum. Compared to phosphogypsum, citric acid gypsum has significantly lower phosphorus and fluorine content, allowing for the direct preparation of high-purity, high-whiteness calcium oxide products to meet the demands of the high-end market. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the method for reducing and decomposing citric acid gypsum using endogenous organic matter according to the present invention. Figure 2 The images show scanning electron microscope (SEM) images of citric acid gypsum before and after preheating and carbonization treatment; among them, Figure 2 a is a scanning electron microscope image before preheating and carbonization treatment. Figure 2 b is a scanning electron microscope image after preheating and carbonization treatment. Detailed Implementation

[0023] The specific embodiments of the present invention will be described in detail below with reference to the examples. The experimental data involved in the following examples are all measured values ​​by the applicant's laboratory, and the detection methods are performed in accordance with relevant national or industry standards. For example, the determination of the content of citric acid gypsum component refers to "GB / T 5484-2024 Chemical Analysis Methods for Gypsum".

[0024] Example 1

[0025] A method for the reductive decomposition of citric acid gypsum using endogenous organic matter, the process flow of which is as follows: Figure 1 As shown, the specific steps include: (1) Citric acid gypsum discharged from a citric acid plant was dried at 50°C to constant weight to obtain dry-basis citric acid gypsum; then the main components of the dry-basis citric acid gypsum were determined, and the results are as follows: CaSO4 content was 94%, total organic carbon (M TOC The content is 2.5%.

[0026] Theoretically, the required sulfur for the complete reduction of pure calcium sulfate to calcium oxide is as follows. It should be noted that the actual reaction pathway is more complex and may involve side reactions such as the carbothermic reduction of CaSO4, the formation of CaS, and its concerted decomposition: Overall reaction: CaSO4 (136.14 g / mol) + 0.5 S (32.06 g / mol) → CaO + 1.5 SO2; Theoretically, the amount of sulfur (M) required to reduce 1 ton of pure CaSO4 理论 )for: M理论 =(32.06÷2÷136.14)×1000=117.75kg; In this embodiment, the dry-basis citric acid gypsum has a CaSO4 content of 94%. Therefore, theoretically, the amount of sulfur (M0) required to reduce 1 ton of this citric acid gypsum is: M0 = 117.75 × 0.94 = 110.68 kg; (2) Based on the measured total organic carbon content M TOC And M0, calculate the actual amount of sulfur required to reduce 1 ton of citric acid gypsum according to the following formula (M S ): M S =K×(M0-α×1000×M TOC ); Where α is the substitution coefficient of organic carbon for sulfur, which is set to 1.0 in this embodiment (typical value for organic matter mainly composed of citric acid residue); K is the excess coefficient, which is set to 1.10 in this embodiment; then the actual amount of sulfur added (M) required to reduce 1 ton of citric acid gypsum in this embodiment is... S )for: M S =1.10×(110.68-1.0×1000×2.5%)=94.25kg; That is, 94.25 kg of sulfur is added per ton of dry basis citric acid gypsum; compared with the theoretical value of 110.68 × 1.10 = 121.75 kg / t without considering the contribution of organic matter, sulfur is saved by 27.5 kg / t, with a saving rate of 22.6%.

[0027] (3) Dry citric acid gypsum (its morphology and structure are as follows) Figure 2 (As shown in a) The gypsum is fed into a rotary kiln and preheated and carbonized at 500℃ for 15 minutes. At this temperature, the endogenous organic matter in the citric acid gypsum is converted into activated coke and forms a porous structure inside the citric acid gypsum (the morphology of the preheated and carbonized dry citric acid gypsum is shown in a figure). Figure 2 (As shown in b); then, sulfur powder was added at a rate of 94.25 kg of sulfur per ton of dry-based citric acid gypsum, mixed thoroughly, and heated to 1000℃ and held for 50 min. At this temperature, the sulfur completely vaporized, forming sulfur vapor, which permeated into the interior of the citric acid gypsum particles through the porous channels formed during the preheating and carbonization stage. The activated coke, sulfur vapor, and calcium sulfate underwent a deep reduction decomposition reaction, resulting in a significant increase in SO2 concentration in the flue gas. After the reaction was complete, the flue gas was discharged through the flue gas outlet, purified, and used to prepare sulfuric acid; the solid product was air-cooled to room temperature, collected, and its composition determined. From Figure 2 As can be seen, the citric acid gypsum particles before preheating and carbonization are mainly in the form of plate-like or flaky crystals, with a relatively smooth and flat surface, dense particle structure, and clear and complete edges; while Figure 2 b shows that after preheating and carbonization, the originally regular plate-like crystal structure was destroyed, the particle surface became rough and uneven, and obvious irregular holes and cracks appeared, and the overall appearance was loose and broken. This transformation from dense crystal to porous loose structure provides more contact channels for the subsequent penetration and diffusion of sulfur vapor, effectively reducing the mass transfer resistance of gas-solid reaction.

[0028] The analysis showed that the decomposition rate of citric acid gypsum reached 99.2%, the purity of the solid product calcium oxide was 97.5%, and the whiteness was 92.

[0029] Example 2

[0030] A method for the reductive decomposition of citric acid gypsum using endogenous organic matter, the process flow of which is as follows: Figure 1 As shown, the specific steps include: (1) Take citric acid gypsum discharged from a citric acid plant and dry it at 50°C to constant weight to obtain dry basis citric acid gypsum; then determine the main components of dry basis citric acid gypsum. The results are as follows: CaSO4 content is 92% and total organic carbon (TOC) content is 4.5%.

[0031] Theoretically, the required sulfur for the complete reduction of pure calcium sulfate to calcium oxide is as follows. It should be noted that the actual reaction pathway is more complex and may involve side reactions such as the carbothermic reduction of CaSO4, the formation of CaS, and its concerted decomposition: Overall reaction: CaSO4 (136.14 g / mol) + 0.5 S (32.06 g / mol) → CaO + 1.5 SO2; Theoretically, the amount of sulfur (M) required to reduce 1 ton of pure CaSO4 理论 )for: M 理论 =(32.06÷2÷136.14)×1000=117.75kg; In this embodiment, the dry-basis citric acid gypsum has a CaSO4 content of 92%. Therefore, theoretically, the amount of sulfur (M0) required to reduce 1 ton of this citric acid gypsum is: M0 = 117.75 × 0.92 = 108.3 kg; (2) Based on the measured total organic carbon content M TOC And M0, calculate the actual amount of sulfur required to reduce 1 ton of citric acid gypsum according to the following formula (M S ): M S =K×(M0-α×1000×M TOC ); Where α is the substitution coefficient of organic carbon for sulfur, which is set to 1.0 in this embodiment (typical value for organic matter mainly composed of citric acid residue); K is the excess coefficient, which is set to 1.10 in this embodiment; then the actual amount of sulfur added (M) required to reduce 1 ton of citric acid gypsum in this embodiment is... S )for: M S =1.10×(108.3-1.0×1000×4.5%)=69.63kg; That is, 69.63 kg of sulfur is added per ton of dry citric acid gypsum; compared with the theoretical value of 108.3 × 1.10 = 119.13 kg / t without considering the contribution of organic matter, sulfur is saved by 49.5 kg / t, with a saving rate of 41.6%.

[0032] (3) Dry citric acid gypsum is fed into a rotary kiln and preheated and carbonized at 400°C for 20 min. At this temperature, the endogenous organic matter in the citric acid gypsum is converted into activated coke and forms a porous structure inside the citric acid gypsum. Then, sulfur powder is added at a rate of 69.63 kg of sulfur per ton of dry citric acid gypsum. After mixing, the temperature is raised to 900°C and kept at this temperature for 60 min. At this temperature, the sulfur is completely vaporized and forms sulfur vapor. It penetrates into the interior of the citric acid gypsum particles through the porous channels formed during the preheating and carbonization stage. The activated coke, sulfur vapor and calcium sulfate undergo a deep reduction decomposition reaction. The SO2 concentration in the flue gas produced by the reaction increases significantly. After the reaction is completed, the flue gas is discharged through the flue gas outlet and purified for the preparation of sulfuric acid. The solid product is air-cooled to room temperature, collected and its composition is determined.

[0033] The analysis showed that the decomposition rate of citric acid gypsum reached 98.8%, the purity of the solid product calcium oxide was 97.0%, and the whiteness was 90.

[0034] Comparative Example 1 A method for decomposing citric acid gypsum includes the following steps: (1) Take the same batch of citric acid gypsum (TOC=2.5%) as in Example 1, and pre-calcine it at 900°C in air for 2 hours to completely oxidize and remove the organic matter in the citric acid gypsum; test the composition of the calcined citric acid gypsum, and its total organic carbon (M TOC The content is less than 0.1%.

[0035] (2) The calcined citric acid gypsum was sent into a rotary kiln and treated at 500℃ for 15 min. Then, sulfur powder was added at a rate of 94.25 kg of sulfur per ton of dry citric acid gypsum. After mixing, the temperature was raised to 1000℃ and kept at that temperature for 50 min. The sulfur was completely vaporized at this temperature to form sulfur vapor. The sulfur vapor reacted with calcium sulfate in a reduction decomposition reaction. After the reaction was completed, the flue gas was discharged through the flue gas outlet and purified for use in the preparation of sulfuric acid. The solid product was cooled to room temperature, collected, and its composition was determined.

[0036] The decomposition rate of citric acid gypsum was measured to be only 82.3%. With the same amount of sulfur added, the decomposition rate of Example 1 (99.2%) was significantly higher than that of Comparative Example 1 (82.3%). This result indicates that when organic matter is present, the porous structure formed by its carbonization provides micro-reaction channels for sulfur vapor to flow from the particle surface to the interior, allowing the same amount of sulfur to react more fully with CaSO4, thereby significantly improving sulfur utilization efficiency. If the organic matter were merely a harmful impurity, its removal should have resulted in at least a similar decomposition rate or even an increase due to reduced impurity interference, but in reality, the decomposition rate decreased significantly. This indirectly proves that the pore structure created by the organic matter plays a crucial role in promoting gas-solid mass transfer and is an important component of the "synergistic effect" of this invention.

[0037] Comparative Example 2 A method for decomposing citric acid gypsum includes the following steps: (1) Take the same batch of citric acid gypsum (TOC=2.5%) as in Example 1 and dry it at 50°C to constant weight.

[0038] (2) The dried citric acid gypsum was fed into a rotary kiln and preheated at 500°C for 15 min. At this temperature, the endogenous organic matter in the citric acid gypsum was converted into activated coke and formed a porous structure inside the citric acid gypsum. Then coke powder was added, and the amount of coke added was calculated according to the molar ratio of C:CaSO4 1:1. After mixing, the temperature was raised to 1000°C and kept for 50 min. The activated coke, coke and calcium sulfate underwent a reduction decomposition reaction. After the reaction was completed, the flue gas was discharged through the flue gas outlet and used to prepare sulfuric acid after purification. The solid product was air-cooled to room temperature, collected and its composition was determined.

[0039] Tests showed that the decomposition rate of citric acid gypsum reached 99%, but the calcium oxide product contained a large amount of SiO2 and Al2O3 ash, with a calcium oxide purity of only 80% and a whiteness of only 75, which could not meet the requirements for food grade or high added value and application.

[0040] Although specific embodiments of the present invention have been described in detail with reference to examples, they should not be construed as limiting the scope of protection of this patent. Various modifications and variations that can be made by those skilled in the art without inventive effort within the scope described in the claims are still within the scope of protection of this patent.

Claims

1. A method for the reduction and decomposition of citric acid gypsum using endogenous organic matter, characterized in that, Includes the following steps: S1: The citric acid gypsum was dried to obtain dry-basis citric acid gypsum; then the calcium sulfate content and total organic carbon content M of the dry-basis citric acid gypsum were determined. TOC ; S2: Calculate the theoretical value M0 of sulfur required for reducing citric acid gypsum based on the calcium sulfate content, and then calculate the actual amount of sulfur required for reducing citric acid gypsum according to the formula shown in (I): M S =K×(M0-α×1000×M TOC ) (I) Among them, M S M0 represents the actual amount of sulfur required to reduce citric acid gypsum; α represents the theoretical value of sulfur required to reduce citric acid gypsum; α represents the substitution coefficient of organic carbon for sulfur, ranging from 0.8 to 1.2; K represents the excess coefficient, ranging from 1.05 to 1.

2. S3: Preheat carbonize the dry-based citric acid gypsum to convert the endogenous organic matter in the dry-based citric acid gypsum into activated coke and form a porous structure inside the citric acid gypsum; then add the actual amount of sulfur powder, mix well and perform thermal reduction treatment to reduce and decompose the citric acid gypsum.

2. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 1, characterized in that, The drying method for citric acid gypsum in S1 is as follows: dry at 45~55℃ to constant weight.

3. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 1, characterized in that: The dry-based citric acid gypsum contains 90%~95% calcium sulfate and M total organic carbon content. TOC It ranges from 1% to 5%.

4. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 1, characterized in that, The theoretical value M0 of sulfur required for the reduction of citric acid gypsum in S2 was calculated using the following steps: S21: Calculate the amount of sulfur required to reduce pure calcium sulfate based on the reaction equation shown in (II); CaSO4 + 0.5S → CaO + 1.5SO2 (II) S22: Multiply the amount of sulfur calculated in S21 by the calcium sulfate content determined in S1 to obtain the theoretical value M0 of sulfur required for reducing citric acid gypsum.

5. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 1, characterized in that: In S2, α takes the value of 1.0; K takes the value of 1.

1.

6. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 1, characterized in that: The preheating and carbonization treatment temperature in S3 is 300~600℃, and the preheating and carbonization treatment time is 10~20min.

7. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 6, characterized in that: The temperature for thermal reduction treatment in S3 is 900~1100℃, and the time for thermal reduction treatment is 30~60min.

8. The method for reducing and decomposing citric acid gypsum using endogenous organic matter according to claim 7, characterized in that: In S3, the preheating carbonization treatment temperature is 500℃ and the preheating carbonization treatment time is 15min; the thermal reduction treatment temperature is 1000℃ and the thermal reduction treatment time is 50min.

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