Research progress in preparation of high purity graphite

Graphite has a high temperature, good thermal conductivity properties, good lubricity, plasticity, thermal shock resistance and chemical stability, and so, therefore, the graphite material is more widely used in atomic energy, automotive, aerospace technology, metallurgy, Chemical, machinery and other industries have become an indispensable non-metallic material in modern industrial technology. As an industrial raw material, graphite materials play a pivotal role in some special industries and high-tech technologies, such as atomic energy, automotive industry, aerospace technology, etc. The graphite used in these industries must be high-purity graphite with a carbon content of 99.9% or more. . Generally, the purity of graphite products cannot meet the requirements of the high-purity graphite industry. Therefore, the development and production of high-purity graphite materials has become one of the problems that need to be solved in the development of graphite materials in a wider and deeper field.

Since the 1990s, according to the stability of graphite chemical properties, high melting point and boiling point, under normal conditions, it does not interact with various strong acids, alkalis, strong oxidants and reducing agents, various organic and inorganic solvents. Characteristics, development of wet purification method (flotation method, acid-base method, hydrofluoric acid method) and fire purification method (chlorination roasting method, high temperature method) and other purification processes, improve the purity and quality of graphite products. This paper mainly introduces the principle and method of graphite purification, summarizes the research progress and achievements of graphite purification in recent years, and simultaneously analyzes the advantages and disadvantages of various high-purity graphite production methods, aiming at the research and production of graphite purification. Businesses provide reference and reference.

First, the principle and method of graphite purification

According to the physical and chemical principles of graphite purification process, the graphite purification methods are mainly divided into two categories: 1. Wet purification method, mainly including flotation method, acid-base method, hydrofluoric acid method; 2. Fire method purification method, mainly including Chlorination roasting method and high temperature method.

(1) Wet purification method

1. Flotation method

The flotation method utilizes the floatability of graphite to enrich and purify graphite, and is mainly adapted to natural flake graphite with good floatability.

After the flotation of graphite, the final concentrate grade is usually about 90%, sometimes up to 94~95%. Because some silicate minerals and compounds such as potassium, sodium, calcium, magnesium and aluminum are impregnated into the graphite scales in a very fine granular form, even if it is finely ground, it is difficult to remove the impurities. Therefore, it is difficult to remove such impurities by physical beneficiation . Therefore, the flotation method is mainly applied to the preliminary purification of graphite, and the purity after purification is not high. To further increase the grade of graphite, chemical or thermal methods must be used to further remove impurities in the graphite. Yue Chenglin's research confirmed this point. He found in the experimental study of the flake graphite rapid flotation process that the rapid flotation process can make the ore with a taste of 4.35%, and obtain the graphite fine with a grade of 91.89% and a recovery rate of 92.52%. mine. Compared with the ordinary flotation method, the graphite flotation grade of the fast flotation process is increased by 1.15%, the recovery rate is increased by 4.28%, and the graphite bulk yield is also significantly improved.

2, acid-base method

The acid-base method is also called the alkali acid method, and some people call it the alkali-alkali method or the sodium hydroxide method. Impurities in graphite such as

SiO ₂ , Al ₂ O ₃ , Fe ₂ O ₃ , MgO, CaO, etc. react with NaOH at a high temperature, and some impurities (SiO ₂ ) form a product soluble in water, which is removed by washing with water, and another part of the impurities does not react. Or a water-insoluble hydroxide is formed, which is then digested with hydrochloric acid to convert it into a soluble chloride, which is removed by washing with water. Therefore, the purification method of graphite is classified into an alkali fusion process and an acid hydrolysis process. In the alkali fusion process, a certain amount of graphite, sodium hydroxide and water are uniformly mixed, placed in a reactor, and heated and melted. After the reaction for a while, it is cooled to below 100 ° C and filtered until the pH of the washing liquid is 7-8. The reaction principle is as follows:

By using the pressure of the autoclave, the low-concentration alkali solution can be used instead of the molten NaOH or KOH to achieve the purpose of removing silicon. If the graphite raw material is treated with liquid ammonia, the Na ⁺ and Cl ^- content in the graphite can be reduced to less than 80 PPM.

In the acid hydrolysis process, the alkali-soluble (melted) graphite is mixed with a certain concentration of hydrochloric acid solution for hot-dip filtration, and after reacting for a certain period of time at a certain temperature, the mixture is filtered and washed to neutrality, dried, and purified. The reaction principle of the acid hydrolysis process is as follows:

High acidity graphite can be obtained after acid hydrolysis. Tang Xingming, a research institute of Panzhihua Refractory Company, used the acid-base method to purify 90% of flaky graphite produced by Panzhihua Jinjiang Graphite Mine, and obtained 99.5% or more of high-purity graphite with a maximum of 99.79%. Beijing University of Chemical Li Changqing the like using a similar procedure (alkali fusion process is added boric acid or sodium metaborate as a cosolvent, using the acid hydrolysis process also a certain concentration hydrofluoric acid) to carbon graphite flake graphite purified study stars When the alkali melting process reaction temperature is 600 ° C, time 60 min, the ratio of sodium hydroxide solution to graphite is 1:0.6, the NaOH concentration is 35%, and the amount of HCl in the acid hydrolysis process is 50% of the graphite mass, under the conditions The purity of graphite can reach 99.6%. XJLu of Shandong University used acid-base method to purify graphite with a carbon content of 87-88% to 99.4%, while reducing the sulfur content from 0.6% to 0.05%.

3. Hydrofluoric acid method

The hydrofluoric acid purification method utilizes hydrofluoric acid to react with almost all impurities in graphite to form water-soluble compounds and volatiles, and then rinses with water to remove impurity compounds, thereby purifying. After immersing graphite with a certain concentration of hydrofluoric acid, it is filtered to neutral, dried and purified to obtain high-purity graphite. The reaction principle is:

At the same time, hydrofluoric acid reacts with CaO, MgO, Fe ₂ O ₃ and the like to form a precipitate, and the reaction is as follows:

The coverage of the precipitates CaF ₂ , MgF ₂ , FeF _{3 ,} etc. hinders further progress of the reaction, and when a mixed acid is present, the following reaction is carried out:

From the above, the solubility of the insoluble fluoride is greatly increased when hydrochloric acid or dilute nitric acid is present. Hydrofluoric acid and mixed acid can dissolve almost all minerals under normal temperature and pressure, and it is an ideal chemical deliming agent for removing minerals. Mo Haibo reacts mixed acid solution (hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid solution and water) with impurities in graphite by steam heating, removes impurities in graphite by washing, stirring, drying and drying, and purifies The graphite can have a fixed carbon content of 99.99% to 99.995%. Qingdao Graphite Co., Ltd. has established the first automatic production line of high-purity graphite with an annual output of 500 tons in China using a mixed acid system of hydrofluoric acid, hydrochloric acid and sulfuric acid, which has created a precedent for large-scale production of high-purity graphite in China.

The hydrofluoric acid method has been industrialized at home and abroad for graphite production, and its use is more common. The research on purification of graphite by hydrofluoric acid method is also very active. The purity of high-purity graphite obtained by Zheng Dongming and other methods using hydrofluoric acid purification can reach 99.95%. Zhang Ranping used a sulfuric acid-hydrofluoric acid step-by-step purification method to purify 97% of high carbon graphite to a high purity level of 99.94% carbon. In addition, Zhang Qingyi et al. used a two-step process of hydrofluoric acid method combined with high-temperature alkali roasting to study the process route for producing high-purity microcrystalline graphite and the behavior of silicon in each process. The final purity of graphite is up to 99.56%. .

(2) Fire method purification method

1. Chlorination roasting method

The chlorination roasting method is to calcine the graphite at a certain high temperature and a specific atmosphere, and then pass chlorine gas to carry out a chemical reaction to chlorinate the impurities in the graphite to form a chloride or a complex of a gas phase or agglomerate. Low) escapes, thus achieving the purpose of purification. The reaction principle is as follows:

In order to improve the conversion of impurities, it is generally necessary to add a reducing agent C, and the reaction is as follows:

Li Jiye used chlorination roasting method to increase the carbon content of graphite from 86.09% to 98.76%, and proved that chlorination roasting method has the characteristics of high recovery rate and low cost. The patent is purified by graphitization of one or more of inert gas, chlorine gas, freon or tetrafluoroethane in a closed graphitization furnace to obtain high-purity graphite with an ash content of less than 50 ppm.

2, high temperature method

The high temperature purification method is developed on the basis of high temperature graphitization technology.

An important property of graphite is its high melting point and boiling point. Graphite is one of the most melting substances in nature. Its melting point and boiling point are much higher than the melting point and boiling point of the impurities contained. Therefore, it is theoretically considered that only graphite raw materials are heated. When the temperature is above 2700 °C, the low boiling point of the impurities can be utilized, so that they can be gasified and removed first. After a certain time of heat preservation, all the impurities can be removed. This is the theory of purifying graphite at high temperature.

The significant factors affecting the purification of graphite by high temperature method are as follows: First, the impurity content of graphite raw materials. The higher the raw material impurities, the higher the mass content of the obtained product, and the purity requirement of the raw material purified by the high temperature method is generally 99% or more. Second, the quality of graphite crucible. The ash content of graphite is higher than that of graphite ash, which is beneficial to the ash gasification in graphite. Otherwise, the ash escaping from the graphite cerium material will diffuse through the cerium to a lower concentration of the graphite material to be purified, resulting in poor graphite purification. Third, the temperature rise curve of the graphitization process. When the temperature is raised by a large current, the graphitization temperature is rapidly increased, which is favorable for the gasification of ash in the dispersed state graphite. Fourth, the particle size of the raw materials. In general, the smaller the particle size, the better the purification effect.

The graphite is purified by high temperature graphitization to make the carbon content of the graphite reach 99.99% or more.

In addition to the wet purification and pyrolysis purification methods, the researchers used a combination of fire and wet methods (ie, fire method - wet purification method) to purify graphite. The results of the Xiaoqi study show that the improved fire method - wet The method of purification can purify cryptocrystalline graphite with a fixed carbon content of 84.18% to a fixed carbon content of up to 99.25%.

Second, the advantages and disadvantages of graphite purification methods

(1) Flotation method

Flotation is a more common method. It is the one with the least energy consumption and reagent consumption and the lowest cost in the conventional purification scheme of minerals. This is the biggest advantage of flotation purification of graphite. However, the use of this method to purify graphite can only improve the grade of graphite, and it is very difficult to further improve the grade, and the recovery rate of graphite is very low, so the application range is limited. To obtain high carbon graphite with a carbon content of about 99%, chemically purified graphite must be used.

(two) acid-base method

The acid-base method is the most widely used method in China's high-purity graphite manufacturers. It has the advantages of low disposable investment, high product quality and strong adaptability. It also has the advantages of easy equipment realization and strong versatility. The disadvantages are high temperature sintering, melting, large energy consumption, long reaction time, severe equipment corrosion, large graphite loss and serious wastewater pollution.

(3) Hydrofluoric acid method

The main advantage of the hydrofluoric acid method is high impurity removal efficiency, high product quality, low impact on the performance of graphite products, and low energy consumption. The disadvantage is that hydrofluoric acid is highly toxic and highly corrosive. Strict safety protection measures must be taken in the production process. The strict requirements on equipment also lead to an increase in cost. There is a problem of fine scale graphite overflow and recovery rate. In addition, the wastewater produced by the hydrofluoric acid method is very toxic and corrosive, and it needs to be treated strictly before it can be discharged. The investment in environmental protection has greatly increased the cost of the hydrofluoric acid method.

(4) Chlorination roasting method

The advantage of the chlorination roasting method is that the low calcination temperature and the chlorine consumption amount make the production cost of graphite higher and the temperature method has a larger reduction, and the purification efficiency is higher (more than 98%), and the recovery rate is high. However, the chlorination roasting method is difficult to treat, and the pollution is serious. The disadvantages of serious corrosion of equipment and high cost of chlorine gas limit the popularization and application of the method.

(5) High temperature method

The biggest advantage of the high temperature method is that the carbon content of the product is extremely high, up to 99.995%. The disadvantage is that the purity of the raw material is high, and the high temperature furnace must be specially designed and constructed, the equipment is expensive, the investment is huge, and the electric heating technology is strict. In addition, the high electricity bill also makes the application of this method extremely limited. Only in the case of national defense, aerospace and other special requirements for the purity of graphite products, this method is considered to be used for small batch production of graphite.

Third, the conclusion

With the development of high-tech industries such as atomic energy industry, nuclear industry, aerospace technology, solar energy industry, ordinary purity graphite products can no longer meet the needs of use. The research and development and production of high-purity graphite products have become the hotspot and concern of people. Focus. The current graphite purification methods have their own advantages and disadvantages. When selecting the purification method, the properties of the raw materials, process characteristics, equipment investment, production energy consumption and environmental treatment should be considered comprehensively to obtain low-cost, high-performance high-purity graphite. product.

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