Method for extracting pure natural graphite by using light metal halide salt at high temperature

Abstract

The invention relates to the technical field of graphite purification, and discloses a method for high-temperature purification of natural graphite by using light metal halide salt, which comprises the following steps: mixing a natural graphite raw material with composite light metal halide salt according to a mass ratio of 1: 0.1-0.3, the composite light metal halide salt comprising a mixture of chloride salt and fluoride salt; the method comprises the following steps: in a first stage, heating to 800-1000 DEG C under the protection of inert gas, and keeping the temperature for 1-2 hours; in the second stage, the atmosphere is switched into a reducing gas atmosphere, the temperature is increased to 1200-1400 DEG C, and heat preservation is conducted for 3-5 h; in the third stage, the temperature is raised to 1800-2000 DEG C by utilizing halogen gas generated by halogen salt pyrolysis, and heat preservation is carried out for 0.5-1 h; and performing acid pickling, water washing and vacuum drying on the pyrolysis product to obtain graphite. Compared with a traditional high-temperature purification method, the highest pyrolysis temperature is reduced by 500-800 DEG C, the energy consumption is reduced by 40-60%, the production cost is greatly reduced, and the economical efficiency and sustainability of the process are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of graphite purification, and particularly relates to a method for high-temperature purification of natural graphite using light metal halides. Background Art

[0002] In many industrial fields, especially in high-end industries such as semiconductors and new energy, the demand for high-purity graphite is increasing day by day, and its purity requirement usually reaches ≥99.9995%. However, there are many problems that cannot be ignored in the existing graphite purification methods.

[0003] Traditional high-temperature purification method: This method needs to be carried out at an extremely high temperature of 2500 - 3000 °C, which not only consumes a large amount of energy, but also has extremely high requirements for the high-temperature resistance performance of the heating equipment, resulting in high equipment costs and greatly limiting the promotion of large-scale industrial production.

[0004] Chemical pickling method: Common pickling solutions such as HF / HNO3 can remove some impurities to a certain extent, but it is easy to leave acidic impurities in the graphite. These residual impurities will have a serious impact on the product quality in subsequent processes such as semiconductor manufacturing, and using strong acids also faces great environmental protection risks. Improper treatment will cause environmental pollution.

[0005] Purification by fluorination method: This method may introduce fluoride pollution during the purification process, and the residual fluoride will affect the conductivity of the graphite. For graphite materials used in semiconductors, good conductivity is one of the key performance indicators, so the application of the fluorination method is greatly limited.

[0006] Existing halogen-assisted processes: When removing metal impurities (such as Fe, Al, Ca, etc.) in the existing halogen-assisted purification processes, the selective removal efficiency is relatively low, and it is difficult to meet the strict impurity control requirements of the semiconductor industry for high-purity graphite. At the same time, halogen residues will also cause problems in subsequent applications.

[0007] In summary, there is an urgent need for a graphite purification method that can simultaneously meet the requirements of low energy consumption, efficient impurity removal, environmental protection of the process, and suitability for large-scale industrial production. Summary of the Invention

[0008] The purpose of the present invention is to propose a method for high-temperature purification of natural graphite using light metal halides in order to solve the above problems.

[0009] In order to achieve the above purpose, the present invention adopts the following technical solutions: A method for high-temperature purification of natural graphite using light metal halides, comprising the following steps: S1: Mix the natural graphite raw material with the composite light metal halide in a mass ratio of 1:0.1 - 0.3. The composite light metal halide contains a mixture of chloride salt and fluoride salt; S2: Conduct gradient pyrolysis under a dynamically regulated reaction atmosphere: First stage: Heat up to 800 - 1000 °C under the protection of inert gas and keep warm for 1 - 2 h; Second stage: Switch to a reducing gas atmosphere, heat up to 1200 - 1400 °C, and keep warm for 3 - 5 h; Third stage: Utilize the halogen gas generated by the pyrolysis of the halide salt, heat up to 1800 - 2000 °C, and keep warm for 0.5 - 1 h; S3: The pyrolysis product is subjected to pickling, water washing, and vacuum drying to obtain high-purity graphite with a purity ≥ 99.9995%.

[0010] Preferably, the chloride salt in the composite light metal halide is at least one of NaCl and KCl, the fluoride salt is at least one of KF and NaF, and the molar ratio of the chloride salt to the fluoride salt is 1:2 - 3.

[0011] Preferably, the temperature ranges of each stage of the gradient pyrolysis match the decomposition temperature of the halide salt: The temperature of the first stage corresponds to the volatilization temperature of the low-melting-point metal halide (≤ 1000 °C); The temperature of the second stage corresponds to the formation temperature of the transition metal halide (1200 - 1400 °C); The temperature of the third stage corresponds to the complete decomposition temperature of the refractory metal halide (≥ 1800 °C).

[0012] Preferably, the inert gas is argon or nitrogen, with a flow rate of 1 - 5 L / min, and the reducing gas is hydrogen, with a flow rate of 2 - 8 L / min.

[0013] Preferably, the total energy consumption of the gradient pyrolysis is 40 - 60% of the traditional high-temperature purification method, and the highest temperature in the third stage ≤ 2000 °C.

[0014] Preferably, the pickling process uses dilute hydrochloric acid or dilute sulfuric acid, the pickling time is 0.5 - 2 h, and the pickling temperature is 30 - 60 °C.

[0015] Preferably, the pressure of the vacuum drying is 1 - 10 Pa, the drying temperature is 80 - 120 °C, and the drying time is 2 - 6 h.

[0016] Preferably, the pyrolysis process is carried out in a vertical furnace or a horizontal furnace, and the furnace body material is a high-temperature resistant ceramic or graphite composite material.

[0017] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are: The Cl⁻ provided by the sodium chloride of the present invention preferentially reacts with transition metals to form volatile chlorides; the F⁻ provided by potassium fluoride has a high reactivity with alkaline earth metals and can form volatile fluorides. Through this ion exchange mechanism, directional and efficient removal of different types of impurities can be achieved. At the same time, the gradient temperature is precisely controlled to match the decomposition temperature of the halide, effectively avoiding halogen residues.

[0018] During the entire pyrolysis process of the present invention, precise control of the reaction is achieved by dynamically switching the protective atmospheres of argon and hydrogen. Argon is used to prevent the oxidation of graphite, hydrogen is used to reduce metal oxides, and the halogen gas generated by the decomposition of the halide salt at the high-temperature stage strengthens the impurity removal reaction. This innovative method not only simplifies the process but also significantly improves the impurity removal efficiency.

[0019] The maximum pyrolysis temperature of the present invention is reduced by 500 - 800 °C compared with the traditional high-temperature purification method, and the energy consumption is reduced by 40 - 60%, greatly reducing the production cost and improving the economy and sustainability of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Shows a schematic structural diagram of the process flow chart provided by an embodiment of the present invention; Figure 2 Shows a schematic structural diagram of the contrast curve of impurity removal efficiency in different temperature ranges provided by an embodiment of the present invention; Figure 3 Shows a schematic structural diagram of the SEM image of the final product provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] Example 1

[0023] Raw material preparation: Select graphite raw materials with a purity of 95% and mix them with a composite light metal halide salt in a mass ratio of 1:0.1. Among them, the composite light metal halide salt is composed of NaCl and KF, and the molar ratio of NaCl to KF is 1:2.

[0024] Gradient pyrolysis The first stage: Under the protection of argon (argon flow rate 1 L / min), heat up to 800 °C at a heating rate of 10 °C / min and hold for 1 h.

[0025] Second stage: Switch to a hydrogen atmosphere (hydrogen flow rate 2 L / min), and heat up to 1200 °C at a heating rate of 5 °C / min, then hold for 3 h.

[0026] Third stage: Utilize the halogen gas generated by the decomposition of the halogen salt at high temperature, quickly heat up to 1800 °C, hold for 0.5 h, to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9995%.

[0027] Example 2

[0028] Raw material preparation: Select graphite raw materials with a purity of 98%, and mix them with the composite light metal halide salt in a mass ratio of 1:0.3. The composite light metal halide salt consists of KCl and NaF, and the molar ratio of KCl to NaF is 1:3.

[0029] Gradient pyrolysis First stage: Under nitrogen protection (nitrogen flow rate 5 L / min), heat up to 1000 °C at a heating rate of 10 °C / min, then hold for 2 h.

[0030] Second stage: Switch to a hydrogen atmosphere (hydrogen flow rate 8 L / min), and heat up to 1400 °C at a heating rate of 5 °C / min, then hold for 5 h.

[0031] Third stage: Utilize the halogen gas generated by the decomposition of the halogen salt at high temperature, quickly heat up to 2000 °C, hold for 1 h, to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9998%.

[0032] Example 3

[0033] Raw material preparation: Use graphite raw materials with a purity of 96%, and mix them with the composite light metal halide salt in a mass ratio of 1:0.2. The composite light metal halide salt consists of NaCl and NaF, and the molar ratio of NaCl to NaF is 1:2.5.

[0034] Gradient pyrolysis First stage: Under argon protection (argon flow rate 3 L / min), heat up to 900 °C at a heating rate of 10 °C / min, then hold for 1.5 h.

[0035] Second stage: Switch to a hydrogen atmosphere (hydrogen flow rate 5 L / min), and heat up to 1300 °C at a heating rate of 5 °C / min, then hold for 4 h.

[0036] Third stage: Utilize the halogen gas generated by the decomposition of the halogen salt at high temperature, quickly heat up to 1900 °C, hold for 0.8 h, to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9996%.

[0037] Example 4

[0038] Raw material preparation: Take graphite raw materials with a purity of 97% and mix them with a composite light metal halide in a mass ratio of 1:0.25. The composite light metal halide consists of KCl and KF, and the molar ratio of KCl to KF is 1:2.2.

[0039] Gradient pyrolysis The first stage: Under the protection of argon (argon flow rate 2 L / min), heat up to 850 °C at a heating rate of 10 °C / min and hold for 1.2 h.

[0040] The second stage: Switch to a hydrogen atmosphere (hydrogen flow rate 3 L / min), heat up to 1250 °C at a heating rate of 5 °C / min and hold for 3.5 h.

[0041] The third stage: Utilize the halogen gas generated by the decomposition of the halide at high temperature, quickly heat up to 1850 °C and hold for 0.6 h to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9998%.

[0042] Example 5

[0043] Raw material preparation: Select graphite raw materials with a purity of 95.5% and mix them with a composite light metal halide in a mass ratio of 1:0.28. The composite light metal halide consists of NaCl and KF, and the molar ratio of NaCl to KF is 1:2.8.

[0044] Gradient pyrolysis The first stage: Under the protection of argon (argon flow rate 4 L / min), heat up to 950 °C at a heating rate of 10 °C / min and hold for 1.8 h.

[0045] The second stage: Switch to a hydrogen atmosphere (hydrogen flow rate 6 L / min), heat up to 1350 °C at a heating rate of 5 °C / min and hold for 4.5 h.

[0046] The third stage: Utilize the halogen gas generated by the decomposition of the halide at high temperature, quickly heat up to 1950 °C and hold for 0.9 h to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9996%.

[0047] Example 6

[0048] Raw material preparation: Use graphite raw materials with a purity of 99% and mix them with a composite light metal halide in a mass ratio of 1:0.22. The composite light metal halide consists of KCl and NaF, and the molar ratio of KCl to NaF is 1:2.4.

[0049] Gradient pyrolysis The first stage: Under the protection of nitrogen (nitrogen flow rate 3.5 L / min), heat up to 920 °C at a heating rate of 10 °C / min and hold for 1.6 h.

[0050] The second stage: Switch to a hydrogen atmosphere (hydrogen flow rate of 7 L / min), heat up to 1320 °C at a heating rate of 5 °C / min, and hold for 4.2 h.

[0051] The third stage: Utilize the halogen gas generated by the decomposition of the halogen salt at high temperature, quickly heat up to 1920 °C, hold for 0.7 h, to obtain high-purity graphite. After testing, the purity of the obtained high-purity graphite is 99.9997%.

[0052] Comparative Example 1 Raw material preparation: Select graphite raw materials with a purity of 95%, and mix them with the composite light metal halide in a mass ratio of 1:0.05 (lower than the scope of the present invention). The composite light metal halide consists of NaCl and KF, and the molar ratio of NaCl to KF is 1:2.

[0053] Gradient pyrolysis: Carry out pyrolysis under the same temperature and atmosphere conditions as in Example 1. After testing, the purity of the obtained graphite is 99.99%, which is lower than the purity of the product obtained in Example 1.

[0054] Comparative Example 2 Raw material preparation: Select graphite raw materials with a purity of 95%, and mix them with the composite light metal halide in a mass ratio of 1:0.4 (higher than the scope of the present invention). The composite light metal halide consists of NaCl and KF, and the molar ratio of NaCl to KF is 1:2.

[0055] Gradient pyrolysis: Carry out pyrolysis under the same temperature and atmosphere conditions as in Example 1. Although the purity of the obtained graphite reaches 99.9995%, it is found in the subsequent cost accounting that due to the increase in the amount of halide used, the cost has increased significantly, and it does not have an economic advantage.

[0056] Comparative Example 3 Raw material preparation: Select graphite raw materials with a purity of 95%, and mix them with a single halide (only NaCl) in a mass ratio of 1:0.2.

[0057] Gradient pyrolysis: Carry out pyrolysis under the same temperature and atmosphere conditions as in Example 1. The purity of the obtained graphite is 99.999%, and the impurity removal effect is not as good as that of Example 1, indicating that the synergistic effect of the composite halide has an important influence on the purification effect.

[0058] Comparative Example 4 Raw material preparation: Select graphite raw materials with a purity of 95%, and mix them with the composite light metal halide in a mass ratio of 1:0.2. The composite light metal halide consists of NaCl and KF, and the molar ratio of NaCl to KF is 1:1 (not within the molar ratio range of the present invention).

[0059] Gradient pyrolysis: Pyrolysis was carried out under the same temperature and atmosphere conditions as in Example 1. The purity of the obtained graphite was 99.9992%, and the impurity removal efficiency was lower than that in Example 1, proving the rationality of the molar ratio of the composite halogen salt of the present invention.

[0060] Through the method of the present invention, the purity of graphite can be increased to more than 99.9995%, and the total amount of metal impurities is ≤20 ppm, fully meeting the extremely strict purity requirements for high-purity graphite in high-end fields such as the semiconductor industry.

[0061] The production cycle of the present invention is shortened to 60% of the traditional method, and the comprehensive cost is reduced by 45%. This is mainly due to the significant reduction in energy consumption and equipment loss, significantly improving the economic benefits of the process.

[0062] The prepared high-purity graphite is suitable for high-end fields such as the manufacture of thermal field components for semiconductor single crystal furnaces with a diameter of more than 6 inches, providing high-quality raw materials for high-end manufacturing in the semiconductor industry and having broad application prospects.

[0063] The above description of the embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for purifying natural graphite at high temperature using light metal halide salts, characterized in that: The following steps are involved: S1: mixing a natural graphite raw material with a composite light metal halide salt in a mass ratio of 1:0.1-0.3, wherein the composite light metal halide salt comprises a mixture of a chloride salt and a fluoride salt; S2: Gradient pyrolysis under dynamically controlled reaction atmosphere: The first stage: heating to 800-1000℃ under inert gas protection and keeping warm for 1-2h; The second stage: switch to reducing gas atmosphere, raise the temperature to 1200-1400℃, and keep it for 3-5h; The third stage: using the halogen gas produced by the pyrolysis of the halide salt, the temperature is raised to 1800-2000℃ and kept warm for 0.5-1h; S3: The pyrolysis product is acid-washed, water-washed and vacuum-dried to obtain high-purity graphite with a purity of ≥99.9995%.

2. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The chloride salt in the composite light metal halide salt is at least one of NaCl and KCl, the fluoride salt is at least one of KF and NaF, and the molar ratio of the chloride salt to the fluoride salt is 1:2-3.

3. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The temperature range of each stage of the gradient pyrolysis matches the decomposition temperature of the halide salt: The temperature of the first stage corresponds to the volatilization temperature of low melting point metal halides (≤1000°C); The temperature of the second stage corresponds to the formation temperature of transition metal halides (1200-1400°C); The temperature of the third stage corresponds to the complete decomposition temperature of refractory metal halides (≥1800°C).

4. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The inert gas is argon or nitrogen with a flow rate of 1-5 L / min, and the reducing gas is hydrogen with a flow rate of 2-8 L / min.

5. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The total energy consumption of the gradient pyrolysis is 40-60% of that of the traditional high-temperature purification method, wherein the highest temperature of the third stage is ≤2000°C.

6. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The pickling process uses dilute hydrochloric acid or dilute sulfuric acid, the pickling time is 0.5-2h, and the pickling temperature is 30-60°C.

7. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The vacuum drying pressure is 1-10 Pa, the drying temperature is 80-120° C., and the drying time is 2-6 h.

8. The method for purifying natural graphite with light metal halide at high temperature according to claim 1, characterized in that: The pyrolysis process is carried out in a vertical furnace or a horizontal furnace, and the furnace body is made of high-temperature resistant ceramics or graphite composite materials.

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