Natural graphite is a non-metallic mineral formed from carbon compounds in strata through pneumatic processes or deep metamorphism. Pneumatic processes refer to the process by which high-temperature, high-pressure gaseous carbon compounds deep within the Earth rise through cracks in the crust and decompose into high-purity, large-crystalline graphite veins near the surface, where pressure is lower.
Graphite produced by pneumatic processes typically appears as scaly crystals visible to the naked eye, hence the name "phaneritic graphite," also known as flake graphite. Flake graphite is black or steel-gray in appearance, has a metallic luster, and exhibits excellent electrical conductivity and lubricity. My country has abundant flake graphite resources in Liumao, Heilongjiang Province; Nanshu, Shandong Province; Xinghe, Inner Mongolia; and Yichang, Hubei Province. In addition to flake graphite ore, pneumatic processes can also produce a very small number of dense, granular, crystalline graphite blocks.
Deep metamorphism refers to the pyrolysis of coal or natural asphalt in strata under high pressure and abnormally high temperatures (e.g., large magma intrusions). Graphite crystals produced by deep metamorphism are very small, with an average particle size of only 0.01 to 0.1 μm. Even under an ordinary optical microscope, their crystal structure is difficult to discern, hence the name cryptocrystalline graphite, also known as earthy graphite.
Earthy graphite contains a high content of inorganic mineral impurities. It is dark black, lacks a metallic luster, and exhibits inferior conductivity and lubricity compared to flake graphite. Earthy graphite deposits are divided into two types: dispersed earthy graphite and dense massive earthy graphite. The former is of low grade, typically containing only 2% to 3% graphite, making it unsuitable for industrial mining. Natural graphite extracted from graphite mines generally contains considerable inorganic mineral impurities, sometimes exceeding 50%. Therefore, it must undergo flotation or magnetic separation to reduce the ash content to below 20% or 10% before use.