Graphite is widely used in new energy, electronics, aviation, aerospace, nuclear energy, and military industries, and is hailed as a crucial raw material supporting the development of my country's strategic emerging industries in the 21st century. It is listed as a key resource by countries and regions such as the EU, the US, and Australia. In the semiconductor industry, graphite products are widely used in high-temperature heat treatment equipment such as crystal growth furnaces, carbonization furnaces, and graphitization furnaces. High-purity graphite products used in third-generation semiconductor silicon carbide include graphite crucibles for crystal growth, graphite heaters, and graphite powder; their purity plays a decisive role in the quality of silicon carbide crystals.
Through technological development and iteration, graphite purification technologies are currently classified into five categories: flotation, alkaline-acid method, hydrofluoric acid method, chlorination roasting method, and high-temperature purification method.
Flotation method: The Flotation method is mainly used for the initial purification of graphite ore. Its basic principle is to use the hydrophobicity on the surface of the mineral particles themselves, or to generate or enhance hydrophobicity after treatment with flotation agents. The surface of graphite is not easily wetted by water and therefore has good floatability, making it easy to separate from impurity minerals. The flotation method has the advantages of low energy consumption and the reagents can be partially reused, but the purity of graphite is limited, and it is difficult to purify again. There is a loss of graphite powder during the purification process, and the recovery rate is very low.
Alkaline-acid method: Graphite and sodium hydroxide are mixed and reacted at 650°C to form water-insoluble hydroxide compounds and partially water-soluble products, which are washed with water to remove some impurities; then the alkali-melted product is mixed with a certain concentration of hydrochloric acid solution and reacted at 60-90°C to turn the impurities into soluble chlorides, then washed with water, and finally dried to obtain a high-purity graphite product. The advantages of this process are that the equipment is simple and easy to implement, the one-time investment is small, and the product obtained is of a higher grade; the disadvantages are large energy consumption, long reaction time, serious equipment corrosion, large graphite loss, and serious water pollution.
Hydrofluoric acid method: The impurities in the raw ore are reacted with hydrofluoric acid to generate fluoride and fluorosilic acid, which are easily soluble in water. The impurities in the raw ore are removed by washing with water to obtain high-grade graphite. This process has high impurity removal efficiency, high-grade products, little impact on the performance of graphite products, and low energy consumption. However, hydrofluoric acid is highly toxic and corrosive, and a strict safety wastewater treatment system must be installed during the production process, which requires a large investment in environmental protection.
Chlorination roasting method: Mix graphite with a certain amount of reducing agent, roast it at 1000°C in a specific atmosphere, and introduce chlorine gas for reaction, so that the valuable metals in the material are converted into gas phase or condensed phase chloride and complex with a lower melting boiling point and escape, thereby separating from the remaining components to achieve the purpose of purifying graphite. This method has low energy consumption, high purification efficiency, high recovery rate, and low cost. However, chlorine is toxic and corrosive to metal products. If it leaks, it will cause serious pollution to the environment.
High temperature purification method: Taking advantage of the fact that the melting point of graphite (3850°C) is much higher than the boiling point of the impurities contained in it, the graphite is heated to above 2700°C, so that the impurities in the graphite are vaporized first and escape to achieve the purpose of purification. High-purity graphite products can be obtained through this process, but high-temperature purification equipment is expensive, consumes a lot of power, has a limited production scale, and has low output.
In addition to the above five well-known graphite purification methods, in recent years, with the rapid development of third-generation semiconductor technology, a new type of graphite purification method has been born, namely the physical and chemical purification method.
The physical and chemical purification method is to place the graphite products that need to be purified in a vacuum furnace and heat them. By increasing the vacuum in the furnace, the impurities in the graphite products will automatically volatilize when they reach their saturated vapor pressure. In addition, halogen gas is used to convert the oxides with high melting and boiling points in the graphite impurities into halides with low melting and boiling points to achieve the purification effect.