Carbon has two natural crystalline allotropic forms: graphite and diamond. Each has its own distinct crystal structure and properties.
Graphite derives its name from the Greek word "graphein", to write. The material is generally greyish-black, opaque and has a lustrous black sheen. It is unique in that it has properties of both a metal and a non-metal. It is flexible but not elastic, has a high thermal and electrical conductivity, and is highly refractory and chemically inert. Graphite has a low adsorption of X-rays and neutrons making it a particularly useful material in nuclear applications.
The unusual combination of properties is due its crystal structure. (Figure 1.) The carbon atoms are arranged hexagonally in a planar condensed ring system. The layers are stacked parallel to each other. The atoms within the rings are bonded covalently, whilst the layers are loosely bonded together by van der Waals forces. The high degree of anisotropy in graphite results from the two types of bonding acting in different crystallographic directions. For example, graphite's ability to form a solid film lubricant comes from these two contrasting chemical bonds. The fact that weak Van der Waals forces govern the bonding between individual layers permits the layers to slide over one another making it an ideal lubricant. World production of graphite was estimated to be about 602,000 tons in 2000, with China being the biggest producer followed by India, Brazil, Mexico and then the Czech Republic.
Figure 1. Crystal structure of graphite.
There are two main classifications of graphite, natural and synthetic.
Natural Graphite is a mineral consisting of graphitic carbon. It varies considerably in crystallinity. Most commercial (natural) graphites are mined and often contain other minerals. Subsequent to mining the graphite often requires a considerable amount of mineral processing such as froth flotation to concentrate the graphite. Natural graphite is an excellent conductor of heat and electricity. It is stable over a wide range of temperatures. Graphite is a highly refractory material with a high melting point (3650°C.)
Natural graphite is subdivided into three types of material:
• High Crystalline
Amorphous graphite is the least graphitic of the natural graphites. However, the term "amorphous" is a misnomer since the material is still crystalline. Amorphous graphite is found as minute particles in beds of mesomorphic rocks such as coal, slate or shale deposits. The graphite content ranges from 25% to 85% dependent on the geological conditions.
Amorphous graphite is extracted using conventional mining techniques and occurs primarily in Mexico, North Korea, South Korea and Austria.
Flake graphite is found in metamorphic rocks uniformly distributed through the body of the ore or in concentrated lens shaped pockets. Carbon concentrations vary between 5% and 40%. Graphite flake occurs as a scaly or lamella form in certain metamorphic rocks such as limestone, gneisses and schists.
Flake graphite is removed by froth flotation. "As floated" graphite contains between 80% and 90% graphite. Flake graphite is produced with >98% through chemical beneficiation processes. Flake graphite occurs in most parts of the world.
Crystalline vein graphite is believed to originate from crude oil deposits that through time, temperature and pressure have converted to graphite. Vein graphite fissures are typically between 1cm and 1 m thick, and are typically > 90% pure. Although this form of graphite is found all over the world, it is only commercially mined in Sri Lanka by conventional shaft or surface mining techniques.
Synthetic graphite can be produced from coke and pitch. It tends to be of higher purity though not as crystalline as natural graphite. There are essentially two types of synthetic graphite. The first is electrographite, which is pure carbon produced from calcined petroleum coke and coal tar pitch in an electric furnace. The second type of synthetic graphite is produced by heating calcined petroleum pitch to 2800°C. On the whole synthetic graphite tends to be of a lower density, higher porosity and higher electrical resistance. Its increased porosity makes it unsuitable for refractory applications
Synthetic Graphite consists mainly of graphitic carbon that has been obtained by graphitisation, heat treatment of non-graphitic carbon, or by chemical vapour deposition from hydrocarbons at temperatures above 2100K.