Graphite, as an allotrope of carbon, possesses many special properties due to its unique crystal structure. Graphite crystals belong to the hexagonal system and have a layered arrangement. In each layer, carbon atoms are connected to each other in a unique arrangement, with six carbon atoms forming regular hexagonal rings on the same plane, and each carbon atom being surrounded by three adjacent carbon atoms. The two 2p electrons perpendicular to the carbon atom planes, not involved in hybridization, can move freely within the crystal lattice and overlap to form π bonds, thus endowing graphite with good electrical and thermal conductivity.
Graphite is an excellent conductor, possessing not only good thermal conductivity but also outstanding electrical conductivity. At the same time, it exhibits high regular stiffness and strength, maintaining its robust and stable characteristics even in high-temperature environments exceeding 3600°C. Graphite is also highly favored for its excellent lubricating properties, chemical inertness, and corrosion resistance, making it widely applicable in numerous fields [3].
Based on the different crystalline forms of natural graphite, it can be industrially classified into crystalline graphite and aphanitic (amorphous) graphite. Among these, crystalline graphite with better crystallinity can be further subdivided into lump graphite (massive crystalline graphite) and flake graphite.
Flake graphite typically occurs as individual crystals or as parallel crystal intergrowths in the form of thin sheets and flakes. Its crystalline form is usually well-developed and often occurs as aggregates, with crystal grain sizes generally exceeding 1 μm. Naturally mined flake graphite ore has a relatively low grade, but compared to other types of graphite, it exhibits superior floatability, lubricity, and plasticity, which generally gives flake graphite higher economic value. Lump graphite typically has a carbon content of around 60%-65%, with some reaching over 80%, but its plasticity and lubricity are inferior to those of flake graphite.
Due to its poor crystallinity, the crystal morphology of aphanitic graphite is only visible under an electron microscope, and it appears earthy to the naked eye; hence, it is also known as earthy, amorphous, or microcrystalline graphite. Aphanitic graphite has a relatively low luster, and its lubricity cannot compare with that of flake graphite. However, its grade is generally higher, with some even reaching over 90% [4].
2. Applications of Graphite
Natural graphite possesses excellent physicochemical properties, leading to a wide range of applications. It plays an irreplaceable and significant role in numerous industries, including metallurgy, lubrication, sealing, nuclear energy, and energy storage.
In the metallurgical industry, natural graphite is a core raw material for refractories and foundry applications. Leveraging its good electrical conductivity and high-temperature resistance, graphite electrodes serve as key components in electric arc furnace steelmaking, reliably adapting to high-temperature smelting conditions. Simultaneously, graphite-based refractory materials, with their reliable high-temperature resistance and erosion resistance, are widely used as lining structures in steel, non-ferrous metal smelting, and various high-temperature metallurgical furnaces, making them indispensable basic materials for the metallurgical industry [5].
In the field of lubrication, lubricants are indispensable in modern life, used in automobiles, ships, aircraft, and machine tools. Under certain special conditions, such as high temperatures, the performance of lubricating oils can be significantly diminished, often resulting in insufficient lubricity. The bonding force between graphite layers is weak, allowing easy sliding under external force; therefore, graphite exhibits excellent lubricating properties and is an ideal solid lubricant. Furthermore, due to its high-temperature resistance, graphite-based lubricants can function effectively in extreme environments like high temperatures, achieving good results [6].
In the sealing field, graphite sealing materials, owing to their outstanding corrosion resistance, high-temperature resistance, and structural stability, find widespread application in industries such as chemical engineering, machinery, and electronics. Based on its unique layered structure and good sealing properties, natural graphite can effectively adapt to sealing conditions in various mechanical equipment, maintaining stable performance in complex and harsh environments and significantly extending the service life of sealing components [5]. Additionally, compressing porous, high-surface-energy expanded graphite can produce flexible graphite sheets. Due to their high-temperature resistance, corrosion resistance, chemical stability, and certain mechanical strength, these sheets can function effectively under special conditions, earning flexible graphite the reputation of "sealing king" [6].
In the nuclear energy sector, high-purity natural graphite is the core raw material for manufacturing neutron moderators in nuclear reactors. Its excellent neutron moderation performance and high-temperature structural stability make it a key material for nuclear power plants and nuclear research facilities. In specific types of nuclear reactors, natural graphite can also serve as a structural material, leveraging its good mechanical strength, radiation resistance, and high-temperature resistance to maintain long-term, stable service under extreme conditions of high temperature and intense radiation, without readily deforming or failing, thus ensuring the safe operation of nuclear equipment [5].
In the energy storage field, with the rapid development of the new energy industry, the market demand for natural graphite continues to expand. The rapid growth of the lithium battery industry has significantly driven market demand for anode materials, and natural graphite, with its unique electrochemical properties, is a preferred raw material for manufacturing lithium battery anodes. Currently, numerous fields such as new energy vehicles, mobile communication base stations, and microgrids rely on lithium batteries as their core power source, which ensures that graphite lithium battery anode materials will continue to possess strong industry competitiveness in the future new energy market [7].
3. Graphite Purification Methods
Currently, natural graphite purification methods can be categorized into physical and chemical methods. Among them, physical purification methods mainly include flotation and high-temperature methods; chemical purification methods include the alkali-acid method, hydrofluoric acid method, and chlorination roasting method.
3.1 Flotation Method
The flotation method is a mineral processing process that separates solid minerals from an aqueous suspension based on differences in their surface physical and chemical properties. Graphite's surface is not easily wetted by water, giving it good floatability and allowing easy separation from impurities, making it suitable for purification via flotation [8].
The overall process flow of the flotation method mainly includes three core steps: grinding, flotation separation, and middlings recirculation. First, the raw natural graphite ore is crushed and ground. Through mechanical grinding, the preliminary liberation of graphite monomers from gangue impurities is achieved, laying the foundation for subsequent separation operations. After grinding, the graphite pulp is mixed and soaked with specific chemical reagents. Through reagent modification, the surface tension differences of the graphite are optimized, widening the floatability gap between graphite and impurity minerals and creating conditions for efficient impurity removal. Subsequently, a combined flotation process involving multiple stages of roughing, scavenging, and cleaning is employed to progressively remove impurity components from the pulp, effectively increasing the purity of the graphite concentrate. During the flotation process, appropriate flotation reagents are added to further improve the floatability of graphite and enhance separation efficiency. Commonly used industrial collectors include diesel oil, kerosene, pine oil, etc. [9].
The flotation method, as the first step in graphite purification, is primarily applicable to the pretreatment of low-carbon, large-flake graphite. This process has a mature technology system, simple supporting equipment, low overall energy consumption, and controllable production costs, offering significant economic advantages. It is a highly cost-effective graphite rough purification technology in industrial applications. However, the flotation method has notable limitations and is not suitable for the efficient purification of microcrystalline graphite. Compared to crystalline graphite, microcrystalline graphite generally has lower overall impurity content, but its unique impurity occurrence state significantly restricts the effectiveness of flotation impurity removal. Microcrystalline graphite particles are extremely fine and intimately associated with gangue minerals, often embedded within clay minerals. During grinding, it is difficult to achieve sufficient liberation of graphite monomers from impurities, leading to a significant reduction in flotation separation efficiency. Ultimately, it is difficult to obtain high-purity graphite concentrates, failing to meet the requirements of high-end industrial applications. In summary, the graphite flotation process can only serve as a primary purification method, mainly used to remove most of the easily separable impurities from raw graphite ore, effectively reducing reagent consumption and energy loss in subsequent deep purification processes and providing a high-quality raw material base for further refining [10-11].
3.2 High-Temperature Method
The high-temperature method utilizes the difference in boiling points between graphite (high melting point) and impurities (lower boiling points). Under the protection of an inert atmosphere, the temperature is controlled within a suitable range, causing lower-boiling-point impurities such as metal oxides and silicates to vaporize and escape first, while the high-melting-point carbon remains in solid form, thus achieving deep separation and purification [12].
The raw materials used in the high-temperature method are typically graphite concentrates obtained after flotation purification or high-carbon graphite obtained after chemical purification, with carbon contents generally ranging from 95% to 99% [13]. This method can achieve graphite purities of around 99.995%. Although it can effectively remove impurities embedded in the graphite lattice without destroying the graphite structure, efficiently improving product quality, it is often only applicable to high-end technology sectors such as defense, nuclear industry, and aerospace. This is because the method's drawbacks are significant: it requires demanding equipment, high energy consumption, and involves substantial investment and operational costs, making it unfavorable for large-scale production and primarily serving high-end graphite products [12].
3.3 Alkali-Acid Method
The alkali-acid method, which consists of two processes-alkali fusion and acid leaching-in sequence according to the industrial flow, is a relatively effective method for purifying natural graphite. Alkali fusion process: Utilizing graphite's resistance to acids and alkalis, graphite and alkali are mixed uniformly in a certain proportion and then calcined at high temperature. In the molten state, impurities like aluminosilicates, quartz, and silicates in the graphite react chemically with the alkali, forming acid-soluble or water-soluble silicates, which are then removed by water washing. Acid leaching process: The alkali-fused graphite and hydroxides are subjected to hot leaching with a hydrochloric acid solution of a specific concentration, converting metal oxides into soluble salts. Finally, through processes like washing and dewatering, purified graphite is obtained.
Currently, the alkali-acid method is the purification method commonly adopted by high-purity graphite producers in China. Its greatest advantages include low initial investment, simple reaction equipment, strong versatility, good process adaptability, and relatively high purity and grade of the obtained products. The disadvantages are the necessity of calcination at high temperatures, significant energy and equipment wear, relatively low purification efficiency, substantial loss of valuable minerals, and potential for environmental pollution [14].
3.4 Hydrofluoric Acid Method
Hydrofluoric acid (HF) is a strong acid that can react with most impurities in graphite. However, graphite typically has good acid resistance at room temperature and is chemically stable, particularly in that it does not react with hydrofluoric acid. By soaking graphite in hydrofluoric acid, allowing reaction over time, followed by washing and filtration, relatively high-purity graphite remains.
Purification using the hydrofluoric acid method can remove most impurities from graphite, yielding high-purity graphite at a low cost. It is one of the simpler and more effective methods for graphite purification. However, hydrofluoric acid is highly toxic, posing significant safety hazards during the production process, which prevents its widespread adoption and application in industrial production [15].
3.5 Chlorination Roasting Method
The chlorination roasting method involves adding a certain amount of reductant (chlorine gas, carbon tetrachloride) to graphite and roasting at high temperatures under a specific atmosphere. Oxide impurities react with chlorine gas in a chlorination reaction, converting them into chlorides. Impurities such as silicates, aluminosilicates, and quartz in the graphite decompose into oxides like silicon dioxide, aluminum oxide, iron oxide, and calcium oxide under high-temperature heating. Since the boiling points of these chlorides are relatively low, they vaporize and escape during the high-temperature roasting process, thereby achieving impurity removal and purification.
The chlorination roasting method removes impurities by adding reducing gases like chlorine to convert impurities into chloride forms. This method offers high purification efficiency and good recovery rates. However, chlorine gas is a hazardous gas harmful to the environment, and the process is characterized by instability, which limits its development [16].