With the improvement of human living standards, the demand for energy has increased sharply, and countries are paying more and more attention to energy, especially new energy and sustainable energy. The solar energy industry has a strong demand for special graphite materials; the research, development and production of graphite materials for nuclear use are becoming increasingly urgent; the rise of electric vehicles has also greatly driven the development of carbon materials and high-performance additives for power batteries such as lithium-ion batteries; Traditional carbon materials such as electrodes, electric brushes, conductive sliders for high-speed railways, and carbon blocks continue to be developed and produced along with the development of high-performance products.
my country's large-size and super-size isotropic graphite are in great demand, but domestic production is very scarce. Isotropic graphite, high-purity graphite, etc. are indispensable supporting products in solar power generation, nuclear power, electromechanical equipment, etc.
Whether it is large-size isotropic graphite, small-size brushes, conductive sliders, or even a series of carbon-graphite products such as lithium-ion battery anode materials, graphitization is a key technical link in its industrial production process. . Graphitization process can be divided into direct method and indirect method according to heating mode, and can be divided into batch type and continuous type according to operation mode. Since E.G. Acheson invented the graphitization furnace (called the Acheson furnace) in 1895, the production of artificial graphite has a history of more than 100 years. With the development of industry, the structure of graphitization furnace has also been greatly developed. The Acheson furnace has a DC electric furnace and an AC electric furnace. In addition to the Acheson furnace, there are also internal tandem graphitization furnaces and continuous graphitization furnaces.
1 Intermittent graphitization
Intermittent graphitization refers to the discontinuous graphitization process in the production process. Its characteristic is that the material does not move after being loaded into the furnace during the graphitization process, and the furnace is powered off after the process of heating, graphitization, and cooling. Its production equipment mainly includes Acheson graphitization furnace and internal tandem graphitization furnace.
1.1 Atchison graphitization furnace
The development of Acheson graphitization furnaces has a long history. At present, Acheson furnaces are generally used in the domestic artificial graphite industry. This kind of furnace is simple in structure, sturdy and durable, and easy to maintain. It has always been the most popular graphitization equipment in my country's carbon industry. Acheson graphitization furnace is classified as a direct heating furnace according to the heating method. The so-called direct heating means that the product itself is an electrical conductor, and the product is graphitized through resistance heating. The early Acheson graphitization furnace used alternating current, but alternating current has shortcomings such as low power, low current density, low power factor, and long power delivery time. After the 1960s, with the rapid development of rectifier equipment, Acheson graphitization furnaces began to use DC power supply, which greatly reduced energy consumption.
Graphitization furnace is a thermal equipment that consumes a lot of power. Therefore, how to reduce power consumption under the premise of ensuring product quality is the main topic of graphitization furnace energy saving, and it is also the only way to reduce graphitization production costs. The resistance in the graphitization furnace is mainly provided by the resistance material. It can be seen that during the entire graphitization process, the heat is mainly transferred into the product from the resistance material, and the current rate into the product is very small.
If the resistance of the resistor material is not uniform, the temperature of the furnace core will be uneven, which will cause cracks in the graphitized product and become a waste product. At the beginning of energization, if the power is too large and the power rises too fast, the temperature of the furnace core will rise too fast, which is very easy to produce cracks for large-size and fine-structure graphite products. The best situation of graphitization is to maintain the cracks produced in the first few processes of the product no longer increase, but usually the cracks continue to increase.
Li Gang proposed some technical improvements for the energy saving and production increase of the Acheson graphitization furnace, including the bricklaying method at the bottom of the furnace, the internal cooling of the conductive electrode, and the use of alumina powder as the end wall filler. At the same time, more reasonable power transmission was adopted. The process greatly improves the yield of the graphitization furnace and reduces the energy consumption of the monomer.
Gu Yiyun and others established a graphitization furnace thermal-electric coupling mathematical model using mathematical modeling, and through large-scale simulation calculations, they studied the influence of various parameters on the temperature distribution and evolution in the graphitization furnace, which is useful for controlling Acheson graphite The production process of the chemical furnace and the improvement of the product yield have a great influence.
Liu Ming studied the resistance of the Acheson graphitization furnace. Through the analysis of the composition of the furnace resistance and the change analysis of the furnace resistance during the graphitization process, he compared the difference between the ideal model and the reality, and established the model formula and correlated it with the reality. , Calculate the late furnace resistance formula for the specific target in the late stage of graphitization power transmission. Using this formula, by adjusting the furnace loading method and product specifications, the appropriate later furnace resistance can be selected, which has certain guiding significance for actual production.
By improving the graphitization process technology, enhancing the insulation effect of the insulation material, improving the structure of the graphitization furnace body, preventing the oxidation of the conductive electrode, and changing the connection method of the graphitization furnace and the power distribution transformer system, the graphitization of Acheson can be reduced. The furnace graphitization process consumes electricity to achieve the purpose of energy saving.
With the rapid development of modern industry, people are paying more and more attention to the environment, and climate change has become a hot spot and focus of the international community. The "Twelfth Five-Year" Comprehensive Work Plan for Energy Conservation and Emission Reduction formulated by China puts forward high requirements for energy conservation and emission reduction. Due to the characteristics of the Acheson graphitization furnace, only 30% of its electric energy is used for graphitization of products, and with the emission of harmful gases, expensive supporting environmental protection facilities are required. If the Acheson graphitization furnace is used to produce graphitized products with lower added value, its competitiveness is no longer as good as the internal tandem graphitization furnace. However, because the Acheson graphitization furnace has the characteristics of high graphitization temperature, it can meet the graphitization of some products with high graphitization requirements, especially the carbon graphite lithium-ion battery anode material, so it is still widely used.
Graphitization process is an important process for the production of carbon graphite anode materials for lithium-ion batteries, and the degree of graphitization is an important indicator of anode materials for lithium-ion batteries. While graphitizing, high temperature can also achieve the purpose of purification and impurity removal. When the graphitization temperature is increased to close to 2200°C, the impurities in the negative electrode material of the lithium-ion battery have basically been eliminated.
Gas-thermal purification is the use of reaction gas (Cl2) to convert impurities into chlorides of various elements with very low boiling points. For example, ferric chloride has a boiling point of 319°C. This process guarantees the high purity of the product, and at the same time avoids the decondensation of certain elements, such as silicon, when the temperature is lowered.
Graphite lithium-ion battery anode materials have high requirements for graphitization temperature. Modification of the Acheson graphitization equipment to pass in reaction gas can achieve simultaneous purification while graphitizing, achieving two birds with one stone. Therefore, the graphitization process of carbon graphite lithium-ion battery materials is the main development direction of the Acheson graphitization furnace in the future.
1.2 Internal tandem graphitization furnace
The internal string type graphitization furnace is an internally heated series connection graphitization furnace, invented by the American Castner, and is divided into horizontal internal series and three-dimensional internal series. The main difference between the internal tandem graphitization process and the Acheson graphitization process is that the product is heated directly through the electrode itself, without the need for resistive materials to generate heat. This is also the main feature of the internal tandem graphitization process compared to the Acheson graphitization process. Since the internal tandem graphitization process has no filler, it can reduce the heat carried out, which can reduce the heat carried out by 10% and reduce the power consumption by 20%~35%. The internal tandem graphitization furnace has the characteristics of high thermal efficiency and short power transmission time. It only takes 1 to 2 hours in the high temperature stage. When the product is directly heated, the resistance is uniform, and the yield of the product is high. Since the 1980s, most carbon plants in Germany, the United States, Japan and other countries have adopted the internal tandem graphitization process to produce large-scale ultra-high-power graphite electrodes. The furnace type of the internal tandem graphitization furnace can be divided into I type, U type, W type and plum blossom type. Among them, there are many U-shaped furnaces, including single-column and multi-column ones, including two-fold, four-fold and even multiple-fold ones.
In the 1980s, Lanzhou Carbon Factory and Jilin Carbon Factory began to study internal tandem graphitization furnaces. After more than 20 years of development, its process technology has been very mature and has formed a great productivity. The unit power consumption of the internal tandem graphitization process is maintained at 2800~3200 kW·h, the power transmission time is 14~18 h, the product qualification rate is high, and the product homogeneity is good.
Zhou Zhuoxian discussed and researched the power transmission technology of the internal tandem graphitization furnace, and summarized the power transmission characteristics of the energy-saving internal tandem graphitization furnace. The internal tandem graphitization furnace needs to heat up quickly in the early stage of power transmission, so as to achieve the purpose of energy saving. The specific heating rate should be controlled in accordance with the characteristics of the blanks of each manufacturer. Controlling the temperature at about 2000 ℃, that is, the temperature at which the graphitization process starts, is beneficial to accelerate the completion of graphitization by using the latent heat released by the graphitization process. The larger the transformer capacity and the higher the furnace core temperature, the faster the graphitization process will proceed, and the high-power operation time is relatively short, the heat loss is low, and the power transmission unit consumption is relatively low. The use of constant power output rectifier transformer can effectively increase the temperature of the furnace core. Adding a certain amount of graphite powder during the batching has certain benefits to reduce the unit consumption of graphitization, and the unit consumption of the long electrode is lower than that of the short electrode.
Zhao Jiesan proposed a new internal tandem graphitization furnace operation mode, that is, two furnaces at a time. This technology redesigned the furnace head and divided the rectifier unit into two. When the load is low, each unit is equipped with a furnace, and each follows its own power transmission curve; when the load is high, one furnace is stopped, and the two units are connected in parallel to impact one furnace to the final furnace, and then in parallel to impact the other furnace To the final furnace.
There are many factors that affect the indicators of internal tandem graphitization products, such as a faster heating rate and a relatively higher overall temperature compared with DC graphitization, the use of low-grade raw materials and excessive pursuit of lower electrical resistivity. The sublimation phenomenon of the product contact part and the electrode core at the graphitization temperature will also affect the graphitization product. In the production process, various factors are comprehensively considered, from the production cost to the index requirements of the product itself, combined with the process characteristics of the internal tandem graphitization furnace, and finally qualified products that meet the market requirements are obtained.
The production of internal tandem graphitization furnace is very important for the graphitization of carbon materials, especially in today's society where energy and environmental requirements are getting higher and higher. When producing carbon electrodes and other graphitized products, Acheson graphitization furnace This kind of process with high energy consumption and low energy efficiency has become increasingly unsuitable. However, internal string graphitization also has certain disadvantages, such as the furnace temperature is not as high as the Acheson graphitization furnace. It is believed that in the future, the graphitization of carbon products, especially the graphitization of traditional electrodes and isostatic products, will increasingly adopt the internal tandem graphitization process.
2 Continuous graphitization furnace
The continuous graphitization furnace is actually not a specific graphitization process furnace. The existing continuous graphitization furnace has many forms. The so-called continuous graphitization process is relative to the batch graphitization process. The so-called continuous graphitization process generally refers to a process in which there is no power failure during production. Graphitized products need to pass through a series of temperature zones to achieve continuous graphitization.
Dongxin Electric Carbon Plant began to study continuous graphitization schemes in the 1970s and designed a continuous graphitization furnace using the "external heating method". The graphite electrode channel used in the furnace is a heating channel. Carbon black is used as the external insulation material. The product adopts a mechanical propulsion method. The furnace door adopts a double-layer seal to prevent air from entering.
The continuous graphitization furnace also has another form, similar to the internal tandem graphitization method. The electrodes are connected end to end in the furnace and connected in series to form an electrode column. The current flows directly through the electrode column to generate Joule heat, thereby realizing graphitization. Different from the serial graphitization method, the electrode column is moving in the continuous graphitization furnace, and the temperature of different parts of the graphitization furnace is different. The temperature gradually rises from the feeding part to the highest temperature in the furnace body, and then gradually decreases from the furnace body to the discharge part.
When the graphitization furnace is operating normally, the temperature of each part of the furnace remains unchanged. The electrode to be graphitized is preheated in the feeding part with the movement of the electrode column, and the temperature is gradually increased, reaching the highest temperature in the furnace body, and then the temperature gradually decreases. , The temperature is lowered to the lowest temperature in the discharging part, and then out of the furnace. That is to say, the temperature of the electrode changes with the movement of the electrode column in the furnace, and the processes of loading the electrodes in different parts of the furnace, preheating, graphitizing and cooling are carried out at the same time. Since the electrode is self-heating and needs to be moved, this process is more complicated in operation and is not as mature as the continuous graphitization process of indirect heating that is external heating.
The following problems mainly affect the continuous graphitization process. The first is the problem of refractory layer. Because there needs to be a temperature zone with a temperature of 3000℃, which requires the refractory of the furnace body to have a higher refractoriness, but currently there is no refractory material with such a high refractoriness, that is, the furnace cannot reach such a high graphitization temperature. . The solution can only be through thickening the insulation layer, such as thickening insulation carbon black, so that less heat is transferred to the outer insulation layer, while using water to cool the outer layer of the furnace. Another problem is how to cool the product quickly so that the product is not oxidized. The solution is to use water cooling, while adding protective gas, such as nitrogen.
Carbon tube furnace, which is a small continuous graphitization furnace with external heating, which solves the above two problems. It mainly uses the principle of thermal purification for high-temperature purification of graphite, which may be a development direction for continuous graphitization in the future. It uses a cylindrical graphite boat as a vessel for materials, and uses mechanical propulsion to make the materials enter the high temperature zone. The process of the equipment is simple, and the personnel occupation is small, and the high-purity graphite products produced at the same time can reach more than 99.995%. Since the production of high-purity graphite is different from the graphitization of petroleum coke products, it will not produce a large amount of flue gas and there is no problem of flue gas emission.
The continuous graphitization production process is not widely used in China, especially the graphitization production of carbon graphite electrodes. The main reason is that the continuous graphitization process is not yet mature. For example, the graphitization temperature is low, the production volume is small, and it is difficult to produce large-scale graphitized electrodes. Thermal purification of natural graphite may be a development direction of continuous graphitization process. As the country attaches importance to the environment, acid purification has become increasingly difficult to gain a foothold. Generally, high-purity products only require the temperature to be above 2700℃, and the temperature is lowered, and the refractory problem of the continuous furnace is also solved.
3 Development trend
Whether it is a batch graphitization furnace or a continuous graphitization furnace, it has not actually achieved continuous industrial production, nor can it truly meet the demand for high-purity graphite materials in the domestic and foreign markets. In recent years, many companies and universities have developed continuous production equipment research and development, and have made great progress. Hunan Dingli Technology Co., Ltd. has developed a high-temperature continuous graphite purification furnace, which adopts resistance heating, continuous production, and can introduce inert gas or reactive gas. Zhuzhou Fulade Technology Co., Ltd. researched and developed graphite ultra-high temperature continuous purification equipment and successfully developed a high-temperature vertical continuous induction heating furnace, which can be used for continuous graphitization production of carbon graphite lithium ion battery anode materials. Compared with the Acheson graphitization furnace, the production capacity and efficiency of the continuous equipment have been greatly improved, the energy consumption is greatly reduced, and the emission of harmful gases is also greatly reduced. This is the inevitable trend of the future development of the graphitization furnace.
4 Conclusion
Graphitization process is the most complex and critical process in carbon graphite material products. my country is a big country of graphite products, as large as graphite electrodes, isostatic graphite, as small as electric brushes, etc., it is necessary to use stoneInking process. The development of graphitization process has also witnessed the development of my country's carbon industry. The most widely used domestic graphitization method is the batch graphitization method, but with the development and gradual maturity of the internal tandem graphitization furnace process technology, most domestic graphitization processes have adopted this new method.
With its own advantages, the Acheson graphitization furnace occupies most of the domestic carbon graphite anode material graphitization market. In particular, the Acheson graphitization furnace that has been improved for gas-thermal purification is favored by graphite anode material manufacturers. . With the development and continuous improvement of high-temperature continuous graphite purification equipment, continuous graphitization production will become the future development direction.