Formation and Classification of Natural Graphite
Natural graphite is a kind of non-metallic minerals, often symbiotic with quartz, feldspar, etc., forming graphite-bearing rocks, or generated in deeply metamorphosed coal seams. According to the crystallization form of natural graphite, it is divided into two categories, namely, apparent crystalline and cryptocrystalline, the former is coarse crystalline, scaly or massive, also known as flake graphite; the latter has small grains and inconspicuous crystalline shape, commonly known as earthy graphite or amorphous graphite.
Crystalline Graphite (Flake Graphite)
Graphite crystal diameter size of 1μm scale and block graphite is called crystalline graphite. According to the shape of the crystallization can be divided into two kinds: the shape of the dense crystal graphite called particles, known as massive crystalline graphite; the shape of the fishscale called flake graphite.
Flake graphite is black or silver gray in appearance, in the form of scales or blades, mainly found in feldspar, quartz or amphibole and other rocks, with a clear oriented crystal structure. The size of graphite flake is generally (1.0-2.0)mm X (0.5-1.0)mm, thickness 0.02-0.05mm, the largest flake is 4-5mm, the smallest is (0.5-1.0)mm X (0.25-0.5)mm. The graphite content of the flake graphite deposits is not too high, about 3%-5%, and the individual rich ores can reach 20%.Flake graphite is of high quality and has a wide range of uses, but because of the low grade of the ore, it needs to be beneficially processed before it can be utilized in industry, and the cost of production is also relatively high.
Flake graphite deposits mainly exist in the old metamorphic rocks of Taikooi Dynasty. There is a large amount of organic carbon deposited in the original rock, with the change of stratum, under high temperature and high pressure, after deep metamorphism, the carbonaceous gasification escapes under high temperature, and then cools and recrystallizes into high-quality scaled graphite crystals under the suitable geological conditions, with a grain size of 3mm or less in general. The thickness of flake graphite deposits can reach more than 10 meters.
There is also a type of dense crystalline graphite, also known as massive crystalline graphite, which is formed by gas action. High-temperature and high-pressure carbon-containing gases from the depths of the earth rise along the fissures and condense into coarse crystalline graphite in the rock crevices where the temperature and pressure are lower near the surface, and the ore body fills the open fissures of gneisses, with thicknesses ranging from a few centimeters to a few meters, with a high graphite content, generally up to 60% or more, and some as high as 80%-90%, which contains some graphite single crystals, typically Sri Lankan graphite.
China’s production of crystalline graphite is mainly flake graphite, the main places of origin are Laixi and Pingdu in Shandong province, Liumao in Heilongjiang province, Xinghe in Inner Mongolia and other places.
Cryptocrystalline Graphite (Amorphous Graphite)
The grains of this graphite are very small, with an average size of 0.01-0.1μm, and it is difficult to recognize the crystals with the naked eye, so it is called cryptocrystalline graphite, and the trade name is earthy graphite or amorphous graphite. This kind of graphite is made by the coalfield by thermal contact metamorphism, retaining this coal mine appearance and layered structure. The ore is black dense lumpy, characterized by the transition from anthracite to graphite, with a graphite content of 60%-80% and as low as 15%. Cryptocrystalline graphite has poor selectivity, generally the higher grade ore is finished by hand selection and grinding, but the lower grade ore needs flotation. The graphite mines in Chenzhou, Hunan Province and Panshi, Jilin Province, China belong to this category.
The main countries in the world that produce natural graphite are Sri Lanka, Madagascar, South Korea and the United States.
Beneficiation of natural graphite
The grade of natural graphite ore is generally not too high, and the ore has to go through beneficiation after it is mined.
The surface of flake graphite is not easy to be wetted by water and has good selectivity. Flotation process is the mined graphite ore is first crushed, ground to the appropriate size and adjusted to a certain concentration of the slurry, adding trapping agent and foaming agent and stirring, in order to improve the graphite surface is not wetted by water nature. When the slurry flotation equipment is inflated, it can produce a relatively stable foam. As the surface of graphite particles are not easily wetted by water and adhere to the bubbles rise, the formation of mineralized foam layer, scraped out by machinery or from the overflow to become graphite concentrate. And quartz, feldspar and other impurities in the ore powder, stay in the slurry to become tailings and discharge.
Flotation process of flake graphite: add lime in the ball mill to adjust the slurry into weak alkaline, add flotation agent in the mixer of flotation, and go out the tailings after one roughing and one sweeping. The concentrate obtained from roughing goes through four times of grinding and nine times of selecting to get the final graphite concentrate. The ore should not be ground too finely in order to protect the scales of the graphite.
After many times of grinding and concentrating, graphite concentrate with grade 89%-95% can be obtained, and the recovery rate of beneficiation reaches 80%-87%.
The selectivity of amorphous graphite is not as good as scale graphite, because amorphous graphite contains some organic matter, which inhibits flotation. However, its grade is high, containing 60%-80% graphite. Therefore, it can be a finished product with a little selection and crushing. Sometimes, amorphous graphite can also be flotation to concentrate.