Impurities inside graphite materials mainly include metallic elements such as iron, silicon, calcium, sodium and aluminum, as well as non-metallic impurities. These impurities come from the raw materials and may be introduced during the production process. For graphite used in high-temperature high-vacuum applications, the volatile metal impurities will evaporate at high temperature and contaminate the processed materials, reducing the product quality. Therefore, the purification of graphite is essential for high-end applications. The purification process removes the impurities to reduce the ash content of graphite to the required level.
High-temperature thermal purification is the most fundamental purification method. The graphite material is heated to a very high temperature, usually above 2500℃, in an inert or reducing atmosphere. At this temperature, most metal impurities have low vapor pressure and will volatilize and be carried away by the gas flow. The high-temperature purification can effectively remove volatile metal impurities, but the removal efficiency for some non-volatile impurities is limited. The thermal purification process is energy-intensive because of the high temperature required. It is usually combined with the graphitization process to improve the efficiency.
Halogen purification is a widely used method for producing ultra-high-purity graphite. In this process, a halogen gas such as chlorine or fluorine is introduced into the purification furnace at high temperature. The halogen reacts with the metal impurities to form volatile metal halides, which are then carried away by the gas flow. Halogen purification can effectively remove most metal impurities, including those that are difficult to remove by thermal purification. The process can achieve very low ash content, meeting the strict requirements of semiconductor and photovoltaic applications. The halogen purification process requires strict control of the gas atmosphere and temperature to ensure safety and efficiency.
Chemical purification is another method for graphite purification, mainly used for natural graphite. The chemical purification process uses acid or alkali solution to dissolve the mineral impurities in natural graphite. For example, hydrofluoric acid can dissolve the silicate impurities, and other acids can remove metallic impurities. The chemical purification is usually carried out after the flotation concentration of natural graphite. The purified graphite is washed and dried to remove the residual chemicals. Chemical purification can effectively reduce the ash content of natural graphite, but the process produces wastewater that needs proper treatment.
The purification process also affects the properties of graphite. The high-temperature purification removes the impurities, but it may also cause some changes in the microstructure and properties of graphite. The removal of impurities can improve the thermal and electrical conductivity by reducing the scattering of phonons and electrons. However, the excessive purification treatment may cause the loss of some beneficial components and increase the production cost. Therefore, the purification degree should be matched with the application requirements. The ash content of graphite for different applications ranges from several hundred ppm for industrial grade to several ppm for ultra-high-purity semiconductor grade.
The selection of the purification method depends on the type of graphite, the required purity and the production cost. For synthetic graphite, high-temperature thermal purification and halogen purification are the main methods. For natural graphite, chemical purification is commonly used, and it can be combined with high-temperature purification for higher purity. The purified graphite products need to be tested for impurity content by methods such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry or X-ray fluorescence analysis.
Jincheng Graphite has mature graphite purification technology. The company has high-temperature purification equipment and can carry out halogen purification treatment according to customer requirements. The purification process is strictly controlled to ensure the uniformity of the purification effect. After purification, the graphite products are tested for impurity content and physical properties. The company can provide high-purity graphite products with different purity grades for semiconductor, photovoltaic and vacuum equipment applications. The technical team of Jincheng Graphite assists customers in selecting the appropriate purification degree and graphite grade according to the application requirements.
The development of graphite purification technology focuses on improving the purification efficiency, reducing the energy consumption and achieving higher purity. The research on the purification mechanism and the optimization of process parameters continues. The demand for ultra-high-purity graphite grows with the development of the semiconductor and new energy industries. Jincheng Graphite will continue to improve its purification technology and provide high-purity graphite products for high-end applications.
